13 Representation Issues 1/3 {8652/0009} {AI95-00137-01} {AI05-0299-1} [This clause describes features for querying and controlling certain aspects of entities and for interfacing to hardware.] Wording Changes from Ada 83 1.a/3 {AI05-0299-1} The subclauses of this clause have been reorganized. This was necessary to preserve a logical order, given the new Ada 95 semantics given in this section. 13.1 Operational and Representation Aspects 0.1/3 {8652/0009} {AI95-00137-01} {AI05-0295-1} [Two kinds of aspects of entities can be specified: representation aspects and operational aspects. Representation aspects affect how the types and other entities of the language are to be mapped onto the underlying machine. Operational aspects determine other properties of entities.] 0.2/3 {AI05-0183-1} {AI05-0295-1} [Either kind of aspect of an entity may be specified by means of an aspect_specification (see 13.1.1), which is an optional element of most kinds of declarations and applies to the entity or entities being declared. Aspects may also be specified by certain other constructs occurring subsequent to the declaration of the affected entity: a representation aspect value may be specified by means of a representation item and an operational aspect value may be specified by means of an operational item.] 1/1 {8652/0009} {AI95-00137-01} There are six kinds of representation items: attribute_definition_clauses for representation attributes, enumeration_- representation_clauses, record_representation_clauses, at_clauses, component_clauses, and representation pragmas. [ They can be provided to give more efficient representation or to interface with features that are outside the domain of the language (for example, peripheral hardware). ] 1.1/1 {8652/0009} {AI95-00137-01} An operational item is an attribute_definition_clause for an operational attribute. 1.2/1 {8652/0009} {AI95-00137-01} [An operational item or a representation item applies to an entity identified by a local_name, which denotes an entity declared local to the current declarative region, or a library unit declared immediately preceding a representation pragma in a compilation.] Language Design Principles 1.a/3 {8652/0009} {AI95-00137-01} {AI05-0295-1} Representation aspects are intended to refer to properties that need to be known before the compiler can generate code to create or access an entity. For instance, the size of an object needs to be known before the object can be created. Conversely, operational aspects are those that only need to be known before they can be used. For instance, how an object is read from a stream only needs to be known when a stream read is executed. Thus, representation aspects have stricter rules as to when they can be specified. 1.a.1/3 {AI95-00291-02} {AI05-0295-1} Confirming the value of an aspect should never change the semantics of the aspect. Thus Size = 8 (for example) means the same thing whether it was specified with a representation item or whether the compiler chose this value by default. 1.a.2/3 Glossary entry: An aspect is a specifiable property of an entity. An aspect may be specified by an aspect_specification on the declaration of the entity. Some aspects may be queried via attributes. Syntax 2/1 {8652/0009} {AI95-00137-01} aspect_clause ::= attribute_definition_clause | enumeration_representation_clause | record_representation_clause | at_clause 3 local_name ::= direct_name | direct_name'attribute_designator | library_unit_name 4/1 {8652/0009} {AI95-00137-01} A representation pragma is allowed only at places where an aspect_clause or compilation_unit is allowed. Name Resolution Rules 5/1 {8652/0009} {AI95-00137-01} In an operational item or representation item, if the local_name is a direct_name, then it shall resolve to denote a declaration (or, in the case of a pragma, one or more declarations) that occurs immediately within the same declarative region as the item. If the local_name has an attribute_designator, then it shall resolve to denote an implementation-defined component (see 13.5.1) or a class-wide type implicitly declared immediately within the same declarative region as the item. A local_name that is a library_unit_name (only permitted in a representation pragma) shall resolve to denote the library_item that immediately precedes (except for other pragmas) the representation pragma. 5.a/1 Reason: {8652/0009} {AI95-00137-01} This is a Name Resolution Rule, because we don't want an operational or representation item for X to be ambiguous just because there's another X declared in an outer declarative region. It doesn't make much difference, since most operational or representation items are for types or subtypes, and type and subtype names can't be overloaded. 5.b/1 Ramification: {8652/0009} {AI95-00137-01} The visibility rules imply that the declaration has to occur before the operational or representation item. 5.c/1 {8652/0009} {AI95-00137-01} For objects, this implies that operational or representation items can be applied only to stand-alone objects. Legality Rules 6/1 {8652/0009} {AI95-00137-01} The local_name of an aspect_clause or representation pragma shall statically denote an entity (or, in the case of a pragma, one or more entities) declared immediately preceding it in a compilation, or within the same declarative_part, package_specification, task_- definition, protected_definition, or record_definition as the representation or operational item. If a local_name denotes a [local] callable entity, it may do so through a [local] subprogram_renaming_declaration [(as a way to resolve ambiguity in the presence of overloading)]; otherwise, the local_name shall not denote a renaming_declaration. 6.a Ramification: The "statically denote" part implies that it is impossible to specify the representation of an object that is not a stand-alone object, except in the case of a representation item like pragma Atomic that is allowed inside a component_list (in which case the representation item specifies the representation of components of all objects of the type). It also prevents the problem of renamings of things like "P.all" (where P is an access-to-subprogram value) or "E(I)" (where E is an entry family). 6.b The part about where the denoted entity has to have been declared appears twice - once as a Name Resolution Rule, and once as a Legality Rule. Suppose P renames Q, and we have a representation item in a declarative_part whose local_name is P. The fact that the representation item has to appear in the same declarative_part as P is a Name Resolution Rule, whereas the fact that the representation item has to appear in the same declarative_part as Q is a Legality Rule. This is subtle, but it seems like the least confusing set of rules. 6.c Discussion: A separate Legality Rule applies for component_clauses. See 13.5.1, "Record Representation Clauses". 7/2 {AI95-00291-02} The representation of an object consists of a certain number of bits (the size of the object). For an object of an elementary type, these are the bits that are normally read or updated by the machine code when loading, storing, or operating-on the value of the object. For an object of a composite type, these are the bits reserved for this object, and include bits occupied by subcomponents of the object. If the size of an object is greater than that of its subtype, the additional bits are padding bits. For an elementary object, these padding bits are normally read and updated along with the others. For a composite object, padding bits might not be read or updated in any given composite operation, depending on the implementation. 7.a/2 To be honest: {AI95-00291-02} Discontiguous representations are allowed, but the ones we're interested in here are generally contiguous sequences of bits. For a discontiguous representation, the size doesn't necessarily describe the "footprint" of the object in memory (that is, the amount of space taken in the address space for the object). 7.a.1/2 Discussion: {AI95-00291-02} In the case of composite objects, we want the implementation to have the flexibility to either do operations component-by-component, or with a block operation covering all of the bits. We carefully avoid giving a preference in the wording. There is no requirement for the choice to be documented, either, as the implementation can make that choice based on many factors, and could make a different choice for different operations on the same object. 7.a.2/2 {AI95-00291-02} In the case of a properly aligned, contiguous object whose size is a multiple of the storage unit size, no other bits should be read or updated as part of operating on the object. We don't say this normatively because it would be difficult to normatively define "properly aligned" or "contiguous". 7.b Ramification: Two objects with the same value do not necessarily have the same representation. For example, an implementation might represent False as zero and True as any odd value. Similarly, two objects (of the same type) with the same sequence of bits do not necessarily have the same value. For example, an implementation might use a biased representation in some cases but not others: 7.c/3 {AI05-0229-1} subtype S is Integer range 1..256; type A is array(Natural range 1..4) of S with Pack; X : S := 3; Y : A := (1, 2, 3, 4); 7.d The implementation might use a biased-by-1 representation for the array elements, but not for X. X and Y(3) have the same value, but different representation: the representation of X is a sequence of (say) 32 bits: 0...011, whereas the representation of Y(3) is a sequence of 8 bits: 00000010 (assuming a two's complement representation). 7.e Such tricks are not required, but are allowed. 7.f Discussion: The value of any padding bits is not specified by the language, though for a numeric type, it will be much harder to properly implement the predefined operations if the padding bits are not either all zero, or a sign extension. 7.g/3 Ramification: {AI05-0229-1} For example, suppose S'Size = 2, and an object X is of subtype S. If the machine code typically uses a 32-bit load instruction to load the value of X, then X'Size should be 32, even though 30 bits of the value are just zeros or sign-extension bits. On the other hand, if the machine code typically masks out those 30 bits, then X'Size should be 2. Usually, such masking only happens for components of a composite type for which Pack, Component_Size, or record layout is specified. 7.h Note, however, that the formal parameter of an instance of Unchecked_Conversion is a special case. Its Size is required to be the same as that of its subtype. 7.i Note that we don't generally talk about the representation of a value. A value is considered to be an amorphous blob without any particular representation. An object is considered to be more concrete. 8/3 {AI05-0112-1} {AI05-0295-1} A representation item directly specifies a representation aspect of the entity denoted by the local_name, except in the case of a type-related representation item, whose local_name shall denote a first subtype, and which directly specifies an aspect of the subtype's type. A representation item that names a subtype is either subtype-specific (Size and Alignment clauses) or type-related (all others). [Subtype-specific aspects may differ for different subtypes of the same type.] 8.a To be honest: Type-related and subtype-specific are defined likewise for the corresponding aspects of representation. 8.b To be honest: Some representation items directly specify more than one aspect. 8.c/3 Discussion: {AI05-0229-1} For example, a pragma Export (see J.15.5) specifies the convention of an entity, and also specifies that it is exported. Such items are obsolescent; directly specifying the associated aspects is preferred. 8.d Ramification: Each specifiable attribute constitutes a separate aspect. An enumeration_representation_clause specifies the coding aspect. A record_representation_clause (without the mod_clause) specifies the record layout aspect. Each representation pragma specifies a separate aspect. 8.e Reason: We don't need to say that an at_clause or a mod_clause specify separate aspects, because these are equivalent to attribute_definition_clauses. See J.7, "At Clauses", and J.8, " Mod Clauses". 8.e.1/3 {AI05-0112-1} We give a default naming for representation aspects of representation pragmas so we don't have to do that for every pragma. Operational and representation attributes are given a default naming in 13.3. We don't want any anonymous aspects; that would make other rules more difficult to write and understand. 8.f Ramification: The following representation items are type-related: 8.g * enumeration_representation_clause 8.h * record_representation_clause 8.i * Component_Size clause 8.j/1 * This paragraph was deleted.{8652/0009} {AI95-00137-01} 8.k * Small clause 8.l * Bit_Order clause 8.m * Storage_Pool clause 8.n * Storage_Size clause 8.n.1/2 * {AI95-00270-01} Stream_Size clause 8.o/1 * This paragraph was deleted.{8652/0009} {AI95-00137-01} 8.p/1 * This paragraph was deleted.{8652/0009} {AI95-00137-01} 8.q/1 * This paragraph was deleted.{8652/0009} {AI95-00137-01} 8.r/1 * This paragraph was deleted.{8652/0009} {AI95-00137-01} 8.s * Machine_Radix clause 8.t * pragma Pack 8.u * pragmas Import, Export, and Convention (when applied to a type) 8.v/3 * {AI05-0009-1} pragmas Atomic, Independent, and Volatile (when applied to a type) 8.w/3 * {AI05-0009-1} pragmas Atomic_Components, Independent_Components, and Volatile_Components (when applied to a type) 8.x * pragma Discard_Names (when applied to an enumeration or tagged type) 8.y The following representation items are subtype-specific: 8.z * Alignment clause (when applied to a first subtype) 8.aa * Size clause (when applied to a first subtype) 8.bb The following representation items do not apply to subtypes, so they are neither type-related nor subtype-specific: 8.cc * Address clause (applies to objects and program units) 8.dd * Alignment clause (when applied to an object) 8.ee * Size clause (when applied to an object) 8.ff * pragmas Import, Export, and Convention (when applied to anything other than a type) 8.gg * pragmas Atomic and Volatile (when applied to an object or a component) 8.hh/3 * {AI05-0009-1} pragmas Atomic_Components, Independent_Components, and Volatile_Components (when applied to an array object) 8.ii * pragma Discard_Names (when applied to an exception) 8.jj * pragma Asynchronous (applies to procedures) 8.kk/2 * {AI95-00414-01} pragma No_Return (applies to subprograms) 8.ll/3 {AI05-0229-1} While an aspect_specification is not a representation item, a similar categorization applies to the aspect that corresponds to each of these representation items (along with aspects that do not have associated representation items). 8.1/3 {8652/0009} {AI95-00137-01} {AI05-0183-1} An operational item directly specifies an operational aspect of the entity denoted by the local_name, except in the case of a type-related operational item, whose local_name shall denote a first subtype, and which directly specifies an aspect of the type of the subtype. 8.mm/1 Ramification: {8652/0009} {AI95-00137-01} The following operational items are type-related: 8.nn/1 * External_Tag clause 8.oo/1 * Read clause 8.pp/1 * Write clause 8.qq/1 * Input clause 8.rr/1 * Output clause 9/4 {AI05-0183-1} {AI12-0116-1} A representation item that directly specifies an aspect of a subtype or type shall appear after the type is completely defined (see 3.11.1), and before the subtype or type is frozen (see 13.14). 9.a/1 Ramification: {8652/0009} {AI95-00137-01} The fact that a representation item (or operational item, see next paragraph) that directly specifies an aspect of an entity is required to appear before the entity is frozen prevents changing the representation of an entity after using the entity in ways that require the representation to be known. 9.b/4 This paragraph was deleted.{AI05-0183-1} {AI12-0116-1} 9.1/4 {8652/0009} {AI95-00137-01} {AI05-0183-1} {AI12-0116-1} An operational item that directly specifies an aspect of an entity shall appear before the entity is frozen (see 13.14). 9.c/1 Ramification: Unlike representation items, operational items can be specified on partial views. Since they don't affect the representation, the full declaration need not be known to determine their legality. 9.2/4 {AI12-0116-1} If a representation item, operational item, or aspect_specification is given that directly specifies an aspect of an entity, then it is illegal to give another representation item, operational item, or aspect_specification that directly specifies the same aspect of the entity. 9.c.1/4 Ramification: {AI12-0116-1} This rule applies to all aspects, not just those that are operational aspects or representation aspects. For instance, it applies to subtype predicates and type invariants. 9.c.2/4 To be honest: {AI12-0116-1} This rule is also intended to cover other ways to specify representation aspects, such as obsolescent pragma Priority. Priority is not a representation pragma, and as such is neither a representation item nor an aspect_specification. Regardless, giving both a pragma Priority and an aspect_specification for Priority is illegal. We didn't want to complicate the wording solely to support obsolescent features. 9.3/4 {AI05-0106-1} {AI05-0295-1} {AI12-0116-1} Unless otherwise specified, it is illegal to specify an operational or representation aspect of a generic formal parameter. 9.d/3 Reason: Specifying an aspect on a generic formal parameter implies an added contract for a generic unit. That contract needs to be defined via generic parameter matching rules, and, as aspects vary widely, that has to be done for each such aspect. Since most aspects do not need this complexity (including all language-defined aspects as of this writing), we avoid the complexity by saying that such contract-forming aspect specifications are banned unless the rules defining them explicitly exist. Note that the method of specification does not matter: aspect_specifications, representation items, and operational items are all covered by this (and similar) rules. 10/4 {AI05-0295-1} {AI12-0109-1} For an untagged derived type, it is illegal to specify a type-related representation aspect if the parent type is a by-reference type, or has any user-defined primitive subprograms. Similarly, it is illegal to specify a nonconfirming type-related representation aspect for an untagged by-reference type after one or more types have been derived from it. 10.a/3 Ramification: {8652/0009} {AI95-00137-01} {AI05-0295-1} On the other hand, subtype-specific representation aspects may be specified for the first subtype of such a type, as can operational aspects. 10.b/4 Reason: {AI05-0229-1} {AI05-0295-1} {AI12-0109-1} The reason for forbidding specification of type-related representation aspects on untagged by-reference types is because a change of representation is impossible when passing by reference (to an inherited subprogram). (A by-reference object cannot be copied to change its representation.) The reason for forbidding specification of type-related representation aspects on untagged types with user-defined primitive subprograms was to prevent implicit change of representation for type-related aspects of representation upon calling inherited subprograms, because such changes of representation are likely to be expensive at run time. Changes of subtype-specific representation attributes, however, are likely to be cheap. This rule is not needed for tagged types, because other rules prevent a type-related representation aspect from changing the representation of the parent part; we want to allow specifying a type-related representation aspect on a type extension to specify aspects of the extension part. For example, specifying aspect Pack will cause packing of the extension part, but not of the parent part. 10.c/4 Discussion: {AI12-0109-1} "By-reference type" usually cannot be used in Legality Rules, as it is privacy breaking. Our use here is privacy breaking, but we're stuck with it for compatibility reasons. Since representation aspects cannot be specified on partial views, privacy violations only can happen when a type includes a component of a private type. In that case, whether these rules are triggered depends on the full type of the private type - which is clearly privacy breaking. 11/3 {8652/0009} {AI95-00137-01} {8652/0011} {AI95-00117-01} {AI95-00326-01} {AI05-0295-1} Operational and representation aspects of a generic formal parameter are the same as those of the actual. Operational and representation aspects are the same for all views of a type. Specification of a type-related representation aspect is not allowed for a descendant of a generic formal untagged type. 11.a/3 Ramification: {8652/0009} {AI95-00137-01} {AI05-0295-1} Specifying representation aspects is allowed for types whose subcomponent types or index subtypes are generic formal types. Specifying operational aspects and subtype-related representation aspects is allowed on descendants of generic formal types. 11.b/3 Reason: {AI05-0295-1} Since it is not known whether a formal type has user-defined primitive subprograms, specifying type-related representation aspects for them is not allowed, unless they are tagged (in which case only the extension part is affected in any case). 11.c/2 Ramification: {AI95-00326-01} All views of a type, including the incomplete and partial views, have the same operational and representation aspects. That's important so that the properties don't change when changing views. While most aspects are not available for an incomplete view, we don't want to leave any holes by not saying that they are the same. 11.d/3 {AI05-0083-1} However, this does not apply to objects. Different views of an object can have different representation aspects. For instance, an actual object passed by reference and the associated formal parameter may have different values for Alignment even though the formal parameter is merely a view of the actual object. This is necessary to maintain the language design principle that Alignments are always known at compile time. 12/3 {AI05-0295-1} The specification of the Size aspect for a given subtype, or the size or storage place for an object (including a component) of a given subtype, shall allow for enough storage space to accommodate any value of the subtype. 13/3 {8652/0009} {AI95-00137-01} {AI05-0295-1} If a specification of a representation or operational aspect is not supported by the implementation, it is illegal or raises an exception at run time. 13.1/3 {AI95-00251-01} {AI05-0295-1} A type_declaration is illegal if it has one or more progenitors, and a nonconfirming value was specified for a representation aspect of an ancestor, and this conflicts with the representation of some other ancestor. The cases that cause conflicts are implementation defined. 13.a/2 Implementation defined: The cases that cause conflicts between the representation of the ancestors of a type_declaration. 13.b/3 Reason: {AI05-0295-1} This rule is needed because it may be the case that only the combination of types in a type declaration causes a conflict. Thus it is not possible, in general, to reject the original representation item or aspect_specification. For instance: 13.c/2 package Pkg1 is type Ifc is interface; type T is tagged record Fld : Integer; end record; for T use record Fld at 0 range 0 .. Integer'Size - 1; end record; end Pkg1; 13.d/2 Assume the implementation uses a single tag with a default offset of zero, and that it allows the use of nondefault locations for the tag (and thus accepts representation items like the one above). The representation item will force a nondefault location for the tag (by putting a component other than the tag into the default location). Clearly, this package will be accepted by the implementation. However, other declarations could cause trouble. For instance, the implementation could reject: 13.e/2 with Pkg1; package Pkg2 is type NewT is new Pkg1.T and Pkg1.Ifc with null record; end Pkg2; 13.f/3 {AI05-0295-1} because the declarations of T and Ifc have a conflict in their representation items. This is clearly necessary (it's hard to imagine how Ifc'Class could work with the tag at a location other than the one it is expecting without introducing distributed overhead). 13.g/3 {AI05-0295-1} Conflicts will usually involve implementation-defined attributes (for specifying the location of the tag, for instance), although the example above shows that doesn't have to be the case. For this reason, we didn't try to specify exactly what causes a conflict; it will depend on the implementation's implementation model and what representation aspects it allows to be changed. 13.h/3 Implementation Note: {AI05-0295-1} An implementation can only use this rule to reject type_declarations where one of its ancestors had a nonconfirming representation value specified. An implementation must ensure that the default representations of ancestors cannot conflict. Static Semantics 14 If two subtypes statically match, then their subtype-specific aspects (Size and Alignment) are the same. 14.a/3 Reason: {AI05-0295-1} This is necessary because we allow (for example) conversion between access types whose designated subtypes statically match. Note that most aspects (including the subtype-specific aspects Size and Alignment) may not be specified for a nonfirst subtype. The only language-defined exceptions to this rule are the Static_Predicate and Dynamic_Predicate aspects. 14.b Consider, for example: 14.c/1 package P1 is subtype S1 is Integer range 0..2**16-1; for S1'Size use 16; -- Illegal! -- S1'Size would be 16 by default. type A1 is access all S1; X1: A1; end P1; 14.d/1 package P2 is subtype S2 is Integer range 0..2**16-1; for S2'Size use 32; -- Illegal! type A2 is access all S2; X2: A2; end P2; 14.e/3 {AI05-0229-1} procedure Q is use P1, P2; type Array1 is array(Integer range <>) of aliased S1 with Pack; Obj1: Array1(1..100); type Array2 is array(Integer range <>) of aliased S2 with Pack; Obj2: Array2(1..100); begin X1 := Obj2(17)'Unchecked_Access; X2 := Obj1(17)'Unchecked_Access; end Q; 14.f Loads and stores through X1 would read and write 16 bits, but X1 points to a 32-bit location. Depending on the endianness of the machine, loads might load the wrong 16 bits. Stores would fail to zero the other half in any case. 14.g Loads and stores through X2 would read and write 32 bits, but X2 points to a 16-bit location. Thus, adjacent memory locations would be trashed. 14.h Hence, the above is illegal. Furthermore, the compiler is forbidden from choosing different Sizes by default, for the same reason. 14.i The same issues apply to Alignment. 15/3 {8652/0040} {AI95-00108-01} {AI05-0009-1} {AI05-0295-1} A derived type inherits each type-related representation aspect of its parent type that was directly specified before the declaration of the derived type, or (in the case where the parent is derived) that was inherited by the parent type from the grandparent type. A derived subtype inherits each subtype-specific representation aspect of its parent subtype that was directly specified before the declaration of the derived type, or (in the case where the parent is derived) that was inherited by the parent subtype from the grandparent subtype, but only if the parent subtype statically matches the first subtype of the parent type. An inherited representation aspect is overridden by a subsequent aspect_specification or representation item that specifies a different value for the same aspect of the type or subtype. 15.a To be honest: A record_representation_clause for a record extension does not override the layout of the parent part; if the layout was specified for the parent type, it is inherited by the record extension. 15.b Ramification: If a representation item for the parent appears after the derived_type_definition, then inheritance does not happen for that representation item. 15.b.1/3 {AI05-0009-1} {AI05-0295-1} If an inherited aspect is confirmed by an aspect_specification or a later representation item for a derived type, the confirming specification does not override the inherited one. Thus the derived type has both a specified confirming value and an inherited nonconfirming representation value - this means that rules that apply only to nonconfirming representation values still apply to this type. 15.b.2/4 {AI12-0109-1} If an aspect was specified by an aspect_specification and the parent type has not yet been frozen, then the inherited aspect might not yet have been resolved and evaluated. The implementation will need to have a mechanism to handle such an aspect. 15.1/3 {8652/0040} {AI95-00108-01} {AI95-00444-01} {AI05-0183-1} {AI05-0295-1} In contrast, whether operational aspects are inherited by a derived type depends on each specific aspect; unless specified, an operational aspect is not inherited. When operational aspects are inherited by a derived type, aspects that were directly specified by aspect_specifications or operational items that are visible at the point of the derived type declaration, or (in the case where the parent is derived) that were inherited by the parent type from the grandparent type are inherited. An inherited operational aspect is overridden by a subsequent aspect_specification or operational item that specifies the same aspect of the type. 15.b.3/1 Ramification: As with representation items, if an operational item for the parent appears after the derived_type_definition, then inheritance does not happen for that operational item. 15.2/2 {AI95-00444-01} When an aspect that is a subprogram is inherited, the derived type inherits the aspect in the same way that a derived type inherits a user-defined primitive subprogram from its parent (see 3.4). 15.c/2 Reason: This defines the parameter names and types, and the needed implicit conversions. 16 Each aspect of representation of an entity is as follows: 17 * If the aspect is specified for the entity, meaning that it is either directly specified or inherited, then that aspect of the entity is as specified, except in the case of Storage_Size, which specifies a minimum. 17.a Ramification: This rule implies that queries of the aspect return the specified value. For example, if the user writes "for X'Size use 32;", then a query of X'Size will return 32. 18 * If an aspect of representation of an entity is not specified, it is chosen by default in an unspecified manner. 18.a/3 Ramification: {8652/0009} {AI95-00137-01} {AI05-0295-1} Note that specifying a representation aspect can affect the semantics of the entity. 18.b The rules forbid things like "for S'Base'Alignment use ..." and "for S'Base use record ...". 18.c Discussion: The intent is that implementations will represent the components of a composite value in the same way for all subtypes of a given composite type. Hence, Component_Size and record layout are type-related aspects. 18.d/3 Ramification: {AI05-0083-1} As noted previously, in the case of an object, the entity mentioned in this text is a specific view of an object. That means that only references to the same view of an object that has a specified value for a representation aspect R necessarily have that value for the aspect R. The value of the aspect R for a different view of that object is unspecified. In particular, this means that the representation values for by-reference parameters is unspecified; they do not have to be the same as those of the underlying object. 18.1/1 {8652/0040} {AI95-00108-01} If an operational aspect is specified for an entity (meaning that it is either directly specified or inherited), then that aspect of the entity is as specified. Otherwise, the aspect of the entity has the default value for that aspect. 18.2/3 {AI95-00291-02} {AI05-0295-1} An aspect_specification or representation item that specifies a representation aspect that would have been chosen in the absence of the aspect_specification or representation item is said to be confirming. The aspect value specified in this case is said to be a confirming representation aspect value. Other values of the aspect are said to be nonconfirming, as are the aspect_specifications and representation items that specified them. Dynamic Semantics 19/1 {8652/0009} {AI95-00137-01} For the elaboration of an aspect_clause, any evaluable constructs within it are evaluated. 19.a/3 Ramification: {AI05-0299-1} Elaboration of representation pragmas is covered by the general rules for pragmas in 2.8. Implementation Permissions 20/3 {AI05-0295-1} An implementation may interpret representation aspects in an implementation-defined manner. An implementation may place implementation-defined restrictions on the specification of representation aspects. A recommended level of support is defined for the specification of representation aspects and related features in each subclause. These recommendations are changed to requirements for implementations that support the Systems Programming Annex (see C.2, "Required Representation Support"). 20.a/3 Implementation defined: The interpretation of each representation aspect. 20.b/3 Implementation defined: Any restrictions placed upon the specification of representation aspects. 20.c Ramification: Implementation-defined restrictions may be enforced either at compile time or at run time. There is no requirement that an implementation justify any such restrictions. They can be based on avoiding implementation complexity, or on avoiding excessive inefficiency, for example. 20.c.1/1 {8652/0009} {AI95-00137-01} There is no such permission for operational aspects. Implementation Advice 21/3 {AI05-0295-1} The recommended level of support for the specification of all representation aspects is qualified as follows: 21.1/3 * {AI95-00291-02} {AI05-0295-1} A confirming specification for a representation aspect should be supported. 21.a/3 To be honest: {AI05-0295-1} A confirming representation aspect value might not be possible for some entities. For instance, consider an unconstrained array. The size of such a type is implementation-defined, and might not actually be a representable value, or might not be static. 22/3 * {AI05-0295-1} An implementation need not support the specification for a representation aspect that contains nonstatic expressions, unless each nonstatic expression is a name that statically denotes a constant declared before the entity. 22.a Reason: This is to avoid the following sort of thing: 22.b X : Integer := F(...); Y : Address := G(...); for X'Address use Y; 22.c In the above, we have to evaluate the initialization expression for X before we know where to put the result. This seems like an unreasonable implementation burden. 22.d The above code should instead be written like this: 22.e Y : constant Address := G(...); X : Integer := F(...); for X'Address use Y; 22.f This allows the expression "Y" to be safely evaluated before X is created. 22.g The constant could be a formal parameter of mode in. 22.h An implementation can support other nonstatic expressions if it wants to. Expressions of type Address are hardly ever static, but their value might be known at compile time anyway in many cases. 23 * An implementation need not support a specification for the Size for a given composite subtype, nor the size or storage place for an object (including a component) of a given composite subtype, unless the constraints on the subtype and its composite subcomponents (if any) are all static constraints. 24/3 * {AI95-00291-02} {AI05-0295-1} An implementation need not support specifying a nonconfirming representation aspect value if it could cause an aliased object or an object of a by-reference type to be allocated at a nonaddressable location or, when the alignment attribute of the subtype of such an object is nonzero, at an address that is not an integral multiple of that alignment. 24.a/1 Reason: The intent is that access types, type System.Address, and the pointer used for a by-reference parameter should be implementable as a single machine address - bit-field pointers should not be required. (There is no requirement that this implementation be used - we just want to make sure it's feasible.) 24.b/2 Implementation Note: {AI95-00291-02} We want subprograms to be able to assume the properties of the types of their parameters inside of subprograms. While many objects can be copied to allow this (and thus do not need limitations), aliased or by-reference objects cannot be copied (their memory location is part of their identity). Thus, the above rule does not apply to types that merely allow by-reference parameter passing; for such types, a copy typically needs to be made at the call site when a bit-aligned component is passed as a parameter. 25/3 * {AI95-00291-02} {AI05-0295-1} An implementation need not support specifying a nonconfirming representation aspect value if it could cause an aliased object of an elementary type to have a size other than that which would have been chosen by default. 25.a/2 Reason: Since all bits of elementary objects participate in operations, aliased objects must not have a different size than that assumed by users of the access type. 26/3 * {AI95-00291-02} {AI05-0295-1} An implementation need not support specifying a nonconfirming representation aspect value if it could cause an aliased object of a composite type, or an object whose type is by-reference, to have a size smaller than that which would have been chosen by default. 26.a/2 Reason: Unlike elementary objects, there is no requirement that all bits of a composite object participate in operations. Thus, as long as the object is the same or larger in size than that expected by the access type, all is well. 26.b/2 Ramification: This rule presumes that the implementation allocates an object of a size specified to be larger than the default size in such a way that access of the default size suffices to correctly read and write the value of the object. 27/3 * {AI95-00291-02} {AI05-0295-1} An implementation need not support specifying a nonconfirming subtype-specific representation aspect value for an indefinite or abstract subtype. 27.a/3 Reason: {AI05-0295-1} Representation aspects are often not well-defined for such types. 27.b/3 Ramification: {AI95-00291-02} {AI05-0229-1} A type with the Pack aspect specified will typically not be packed so tightly as to disobey the above rules. A Component_Size clause or record_representation_clause will typically be illegal if it disobeys the above rules. Atomic components have similar restrictions (see C.6, "Shared Variable Control"). 28/3 {AI95-00291-02} {AI05-0295-1} For purposes of these rules, the determination of whether specifying a representation aspect value for a type could cause an object to have some property is based solely on the properties of the type itself, not on any available information about how the type is used. In particular, it presumes that minimally aligned objects of this type might be declared at some point. 28.a/2 Implementation Advice: The recommended level of support for all representation items should be followed. NOTES 29/3 1 {AI05-0229-1} Aspects that can be specified are defined throughout this International Standard, and are summarized in K.1. Incompatibilities With Ada 83 29.a It is now illegal for a representation item to cause a derived by-reference type to have a different record layout from its parent. This is necessary for by-reference parameter passing to be feasible. This only affects programs that specify the representation of types derived from types containing tasks; most by-reference types are new to Ada 95. For example, if A1 is an array of tasks, and A2 is derived from A1, it is illegal to apply a pragma Pack to A2. Extensions to Ada 83 29.b/1 {8652/0009} {AI95-00137-01} Ada 95 allows additional aspect_clauses for objects. Wording Changes from Ada 83 29.c/1 {8652/0009} {AI95-00137-01} The syntax rule for type_representation_clause is removed; the right-hand side of that rule is moved up to where it was used, in aspect_clause. There are two references to "type representation clause" in RM83, both in Section 13; these have been reworded. Also, the representation_clause has been renamed the aspect_clause to reflect that it can be used to control more than just representation aspects. 29.d/2 {8652/0009} {AI95-00137-01} {AI95-00114-01} We have defined a new term "representation item," which includes all representation clauses and representation pragmas, as well as component_clauses. This is convenient because the rules are almost identical for all of them. We have also defined the new terms "operational item" and "operational aspects" in order to conveniently handle new types of specifiable entities. 29.e All of the forcing occurrence stuff has been moved into its own subclause (see 13.14), and rewritten to use the term "freezing". 29.f RM83-13.1(10) requires implementation-defined restrictions on representation items to be enforced at compile time. However, that is impossible in some cases. If the user specifies a junk (nonstatic) address in an address clause, and the implementation chooses to detect the error (for example, using hardware memory management with protected pages), then it's clearly going to be a run-time error. It seems silly to call that "semantics" rather than "a restriction." 29.g RM83-13.1(10) tries to pretend that representation_clauses don't affect the semantics of the program. One counter-example is the Small clause. Ada 95 has more counter-examples. We have noted the opposite above. 29.h Some of the more stringent requirements are moved to C.2, " Required Representation Support". Extensions to Ada 95 29.i/2 {AI95-00291-02} Amendment Correction: Confirming representation items are defined, and the recommended level of support is now that they always be supported. Wording Changes from Ada 95 29.j/2 {8652/0009} {AI95-00137-01} Corrigendum: Added operational items in order to eliminate unnecessary restrictions and permissions on stream attributes. As part of this, representation_clause was renamed to aspect_clause. 29.k/2 {8652/0009} {AI95-00137-01} {AI95-00326-01} Corrigendum: Added wording to say that the partial and full views have the same operational and representation aspects. Ada 2005 extends this to cover all views, including the incomplete view. 29.l/2 {8652/0040} {AI95-00108-01} Corrigendum: Changed operational items to have inheritance specified for each such aspect. 29.m/2 {AI95-00251-01} Added wording to allow the rejection of types with progenitors that have conflicting representation items. 29.n/2 {AI95-00291-02} The description of the representation of an object was clarified (with great difficulty reaching agreement). Added wording to say that representation items on aliased and by-reference objects never need be supported if they would not be implementable without distributed overhead even if other recommended level of support says otherwise. This wording matches the rules with reality. 29.o/3 {AI95-00444-01} {AI05-0005-1} Added wording so that inheritance depends on whether operational items are visible rather than whether they occur before the declaration (we don't want to look into private parts). Also limited operational inheritance to untagged types to avoid anomalies with private extensions (this is not incompatible, no existing operational attribute used this capability). Also added wording to clearly define that subprogram inheritance works like derivation of subprograms. Incompatibilities With Ada 2005 29.p/3 {AI05-0106-1} Correction: Specifying a language-defined aspect for a generic formal parameter is no longer allowed. Most aspects could not be specified on these anyway; moreover, this was not allowed in Ada 83, so it is unlikely that compilers are supporting this as a capability (and it is not likely that they have a consistent definition of what it means if it is allowed). Thus, we expect this to occur rarely in existing programs. Wording Changes from Ada 2005 29.q/3 {AI05-0009-1} Correction: Defined that overriding of an representation aspect only happens for a nonconfirming representation item. This prevents a derived type from being considered to have only a confirming representation item when the value would be nonconfirming if given on a type that does not inherit any aspects of representation. This change just eliminates a wording confusion and ought not change any behavior. 29.r/3 {AI05-0112-1} Correction: Defined a default naming for representation aspects that are representation pragmas. 29.s/3 {AI05-0183-1} Added text ensuring that the rules for representational and operational items also apply appropriately to aspect_specifications; generalized operational aspects so that they can be defined for entities other than types. Any extensions are documented elsewhere. 29.t/3 {AI05-0295-1} Rewrote many rules to be in terms of "specifying a representation aspect" rather than use of a "representation item". This better separates how an aspect is specified from what rules apply to the value of the aspect. Incompatibilities With Ada 2012 29.u/4 {AI12-0109-1} Corrigendum: Added a rule that makes it illegal to specify a representation value after a type is derived from an untagged by-reference type. This restriction is incompatible, but since the implementation would have had to copy an object that does not allow copying in order to change the representation for any implicit or explicit conversion between the original and the derived type, it is unlikely that any program could exist without running into internal compiler errors or bogus results. Wording Changes from Ada 2012 29.v/4 {AI12-0116-1} Corrigendum: Clarified that an aspect (any aspect) can be specified only once for an entity, no matter what means of specifying it are used. We did not document this as an incompatibility as only aspects that are neither operational nor representation could change behavior and there is no known implementation of these new aspects that allows multiple definitions. 13.1.1 Aspect Specifications 1/3 {AI05-0183-1} [Certain representation or operational aspects of an entity may be specified as part of its declaration using an aspect_specification, rather than using a separate representation or operational item.] The declaration with the aspect_specification is termed the associated declaration. Syntax 2/3 {AI05-0183-1} aspect_specification ::= with aspect_mark [=> aspect_definition] {, aspect_mark [=> aspect_definition] } 3/3 {AI05-0183-1} aspect_mark ::= aspect_identifier['Class] 4/3 {AI05-0183-1} aspect_definition ::= name | expression | identifier Language Design Principles 4.a/3 {AI05-0183-1} {AI05-0267-1} The aspect_specification is an optional element in most kinds of declarations. Here is a list of all kinds of declarations and an indication of whether or not they allow aspect clauses, and in some cases a short discussion of why (* = allowed, NO = not allowed). Kinds of declarations with no indication are followed by their subdivisions (which have indications). 4.b/4 {AI12-0005-1} basic_declaration type_declaration full_type_declaration type declaration syntax* task_type_declaration* protected_type_declaration* incomplete_type_declaration -- NO -- Incomplete type aspects cannot be read by an attribute or specified by attribute_definition_clause s -- (the attribute name is illegal), so it would not make sense to allow this in another way. private_type_declaration* private_extension_declaration* subtype_declaration* object_declaration object declaration syntax* single_task_declaration* single_protected_declaration* number_declaration -- NO subprogram_declaration* abstract_subprogram_declaration* null_procedure_declaration* expression_function_declaration* package_declaration* -- via package_specification renaming_declaration* -- There are no language-defined aspects that may be specified -- on renames, but implementations might support some. exception_declaration* generic_declaration generic_subprogram_declaration* generic_package_declaration * -- via package_specification generic_instantiation* enumeration_literal_specification -- NO discriminant_specification -- NO component_declaration* loop_parameter_specification -- NO iterator_specification -- NO parameter_specification -- NO subprogram_body * -- - but language-defined aspects only if there is no explicit specification entry_declaration* entry_index_specification -- NO subprogram_body_stub * -- - but language-defined aspects only if there is no explicit specification choice_parameter_specification -- NO generic_formal_parameter_declaration -- There are no language-defined aspects that may be specified -- on generic formals, but implementations might support some. formal_object_declaration* formal_type_declaration* formal_subprogram_declaration formal_concrete_subprogram_declaration* formal_abstract_subprogram_declaration* formal_package_declaration* extended_return_statement -- NO 4.c/3 -- We also allow aspect_specification s on all kinds of bodies, but are no language-defined aspects -- that may be specified on a body. These are allowed for implementation-defined aspects. -- See above for subprogram bodies and stubs (as these can be declarations). package_body* task_body* protected_body* package_body_stub* task_body_stub* protected_body_stub* 4.d/3 {AI05-0267-1} Syntactically, aspect_specifications generally are located at the end of declarations. When a declaration is all in one piece such as a null_procedure_declaration, object_declaration, or generic_instantiation the aspect_specification goes at the end of the declaration; it is then more visible and less likely to interfere with the layout of the rest of the structure. However, we make an exception for program units (other than subprogram specifications) and bodies, in which the aspect_specification goes before the is. In these cases, the entity could be large and could contain other declarations that also have aspect_specifications, so it is better to put the aspect_specification toward the top of the declaration. (Some aspects - such as Pure - also affect the legality of the contents of a unit, so it would be annoying to only see those after reading the entire unit.) Name Resolution Rules 5/3 {AI05-0183-1} An aspect_mark identifies an aspect of the entity defined by the associated declaration (the associated entity); the aspect denotes an object, a value, an expression, a subprogram, or some other kind of entity. If the aspect_mark identifies: 6/3 * an aspect that denotes an object, the aspect_definition shall be a name. The expected type for the name is the type of the identified aspect of the associated entity; 7/3 * an aspect that is a value or an expression, the aspect_definition shall be an expression. The expected type for the expression is the type of the identified aspect of the associated entity; 8/3 * an aspect that denotes a subprogram, the aspect_definition shall be a name; the expected profile for the name is the profile required for the aspect of the associated entity; 9/3 * an aspect that denotes some other kind of entity, the aspect_definition shall be a name, and the name shall resolve to denote an entity of the appropriate kind; 10/3 * an aspect that is given by an identifier specific to the aspect, the aspect_definition shall be an identifier, and the identifier shall be one of the identifiers specific to the identified aspect. 11/3 {AI05-0183-1} The usage names in an aspect_definition [ are not resolved at the point of the associated declaration, but rather] are resolved at the end of the immediately enclosing declaration list. 12/3 {AI05-0183-1} If the associated declaration is for a subprogram or entry, the names of the formal parameters are directly visible within the aspect_definition, as are certain attributes, as specified elsewhere in this International Standard for the identified aspect. If the associated declaration is a type_declaration, within the aspect_definition the names of any components are directly visible, and the name of the first subtype denotes the current instance of the type (see 8.6). If the associated declaration is a subtype_declaration, within the aspect_definition the name of the new subtype denotes the current instance of the subtype. Legality Rules 13/3 {AI05-0183-1} If the first freezing point of the associated entity comes before the end of the immediately enclosing declaration list, then each usage name in the aspect_definition shall resolve to the same entity at the first freezing point as it does at the end of the immediately enclosing declaration list. 14/3 {AI05-0183-1} At most one occurrence of each aspect_mark is allowed within a single aspect_specification. The aspect identified by the aspect_mark shall be an aspect that can be specified for the associated entity (or view of the entity defined by the associated declaration). 14.a/4 Ramification: {AI12-0116-1} This rule prevents multiple specifications in the same aspect_specification. Rules in 13.1 prevent multiple specifications in different aspect_specifications (on different views of the same type, for instance) or between operational or representation items and an aspect_specification, even for aspects that are neither operational nor representation aspects. 15/3 {AI05-0183-1} The aspect_definition associated with a given aspect_mark may be omitted only when the aspect_mark identifies an aspect of a boolean type, in which case it is equivalent to the aspect_definition being specified as True. 16/3 {AI05-0183-1} If the aspect_mark includes 'Class, then the associated entity shall be a tagged type or a primitive subprogram of a tagged type. 17/3 {AI05-0183-1} {AI05-0267-1} There are no language-defined aspects that may be specified on a renaming_declaration, a generic_formal_parameter_declaration, a subunit, a package_body, a task_body, a protected_body, or a body_stub other than a subprogram_body_stub. 17.a/3 Discussion: Implementation-defined aspects can be allowed on these, of course; the implementation will need to define the semantics. In particular, the implementation will need to define actual type matching rules for any aspects allowed on formal types; there are no default matching rules defined by the language. 18/4 {AI05-0183-1} {AI05-0267-1} {AI12-0105-1} A language-defined aspect shall not be specified in an aspect_specification given on a completion of a subprogram or generic subprogram. 18.a/3 Reason: Most language-defined aspects (for example, preconditions) are intended to be available to callers, and specifying them on a body that has a separate declaration hides them from callers. Specific language-defined aspects may allow this, but they have to do so explicitly (by defining an alternative Legality Rule), and provide any needed rules about visibility. Note that this rule does not apply to implementation-defined aspects, so implementers need to carefully define whether such aspects can be applied to bodies and stubs, and what happens if they are specified on both the declaration and body of a unit. 18.1/4 {AI05-0183-1} {AI12-0138-1} If an aspect of a derived type is inherited from an ancestor type and has the boolean value True, the inherited value shall not be overridden to have the value False for the derived type, unless otherwise specified in this International Standard. 18.2/4 {AI12-0138-1} Certain type-related aspects are defined to be nonoverridable; all such aspects are specified using an aspect_definition that is a name. 18.3/4 {AI12-0138-1} If a nonoverridable aspect is directly specified for a type T, then any explicit specification of that aspect for any other descendant of T shall be confirming; that is, the specified name shall match the inherited aspect, meaning that the specified name shall denote the same declarations as would the inherited name. 18.4/4 {AI12-0138-1} If a full type has a partial view, and a given nonoverridable aspect is allowed for both the full view and the partial view, then the given aspect for the partial view and the full view shall be the same: the aspect shall be directly specified only on the partial view; if the full type inherits the aspect, then a matching definition shall be specified (directly or by inheritance) for the partial view. 18.b Ramification: In order to enforce these rules without breaking privacy, we cannot allow a private type that could have a particular overridable aspect to have a hidden definition of that aspect. There is no problem if the private type does not allow the aspect (as the aspect could not be specified on descendants in that case). 18.5/4 {AI12-0138-1} In addition to the places where Legality Rules normally apply (see 12.3), these rules about nonoverridable aspects also apply in the private part of an instance of a generic unit. 18.6/4 {AI12-0138-1} [The Default_Iterator, Iterator_Element, Implicit_Dereference, Constant_Indexing, and Variable_Indexing aspects are nonoverridable.] 18.c/4 Discussion: We don't need an assume-the-worst rule for most nonoverridable aspects as they only work on tagged types and deriving from formal tagged types is not allowed in generic bodies. In the case of Implicit_Dereference, a derivation in a generic body does not cause problems (the ancestor necessarily cannot have the aspect, else specifying the aspect would be illegal), as there could be no place with visibility on both aspects. Static Semantics 19/3 {AI05-0183-1} Depending on which aspect is identified by the aspect_mark, an aspect_definition specifies: 20/3 * a name that denotes a subprogram, object, or other kind of entity; 21/3 * an expression, which is either evaluated to produce a single value, or which (as in a precondition) is to be evaluated at particular points during later execution; or 22/3 * an identifier specific to the aspect. 23/3 {AI05-0183-1} The identified aspect of the associated entity, or in some cases, the view of the entity defined by the declaration, is as specified by the aspect_definition (or by the default of True when boolean). Whether an aspect_specification applies to an entity or only to the particular view of the entity defined by the declaration is determined by the aspect_mark and the kind of entity. The following aspects are view specific: 24/3 * An aspect specified on an object_declaration; 25/3 * An aspect specified on a subprogram_declaration; 26/3 * An aspect specified on a renaming_declaration. 27/3 {AI05-0183-1} All other aspect_specifications are associated with the entity, and apply to all views of the entity, unless otherwise specified in this International Standard. 28/4 {AI05-0183-1} {AI12-0106-1} If the aspect_mark includes 'Class (a class-wide aspect), then, unless specified otherwise for a particular class-wide aspect: 29/3 * if the associated entity is a tagged type, the specification applies to all descendants of the type; 30/3 * if the associated entity is a primitive subprogram of a tagged type T, the specification applies to the corresponding primitive subprogram of all descendants of T. 31/3 {AI05-0183-1} {AI05-0229-1} All specifiable operational and representation attributes may be specified with an aspect_specification instead of an attribute_definition_clause (see 13.3). 31.a/3 Ramification: The name of the aspect is the same as that of the attribute (see 13.3), so the aspect_mark is the attribute_designator of the attribute. 32/4 {AI05-0229-1} {AI12-0154-1} Any aspect specified by a representation pragma or library unit pragma that has a local_name as its single argument may be specified by an aspect_specification, with the entity being the local_name. The aspect_definition is expected to be of type Boolean. The expression shall be static. Notwithstanding what this International Standard says elsewhere, the expression of an aspect that can be specified by a library unit pragma is resolved and evaluated at the point where it occurs in the aspect_specification[, rather than the first freezing point of the associated package]. 32.a/3 Ramification: The name of the aspect is the same as that of the pragma (see 13.1), so the aspect_mark is the name of the pragma. 33/3 {AI05-0229-1} In addition, other operational and representation aspects not associated with specifiable attributes or representation pragmas may be specified, as specified elsewhere in this International Standard. 34/4 This paragraph was deleted.{AI05-0183-1} {AI12-0138-1} 35/3 {AI05-0183-1} If a Legality Rule or Static Semantics rule only applies when a particular aspect has been specified, the aspect is considered to have been specified only when the aspect_specification or attribute_definition_clause is visible (see 8.3) at the point of the application of the rule. 35.a/3 Reason: Some rules only apply when an aspect has been specified (for instance, an indexable type is one that has aspect Variable_Indexing specified). In order to prevent privacy breaking, this can only be true when the specification of the aspect is visible. In particular, if the Variable_Indexing aspect is specified on the full view of a private type, the private type is not considered an indexable type. 36/3 {AI05-0183-1} Alternative legality and semantics rules may apply for particular aspects, as specified elsewhere in this International Standard. Dynamic Semantics 37/3 {AI05-0183-1} At the freezing point of the associated entity, the aspect_specification is elaborated. The elaboration of the aspect_specification includes the evaluation of the name or expression, if any, unless the aspect itself is an expression. If the corresponding aspect represents an expression (as in a precondition), the elaboration has no effect; the expression is evaluated later at points within the execution as specified elsewhere in this International Standard for the particular aspect. Implementation Permissions 38/3 {AI05-0183-1} Implementations may support implementation-defined aspects. The aspect_specification for an implementation-defined aspect may use an implementation-defined syntax for the aspect_definition, and may follow implementation-defined legality and semantics rules. 38.a/3 Discussion: The intent is to allow implementations to support aspects that are defined, for example, by a subtype_indication rather than an expression or a name. We chose not to try to enumerate all possible aspect_definition syntaxes, but to give implementations maximum freedom. Unrecognized aspects are illegal whether or not they use custom syntax, so this freedom does not reduce portability. 38.a.1/3 Implementation defined: Implementation-defined aspects, inluding the syntax for specifying such aspects and the legality rules for such aspects. Extensions to Ada 2005 38.b/3 {AI05-0183-1} {AI05-0229-1} {AI05-0267-1} Aspect specifications are new. Incompatibilities With Ada 2012 38.c/4 {AI12-0154-1} Corrigendum: Added a clarification that aspects that correspond to library unit pragmas are resolved and evaluated immediately. This is incompatible, as a reference to an entity defined after the aspect will now be illegal. However, this would have require retroactive enforcement of such aspects, which is a new capability not available from the associated pragma, and moreover no known Ada 2012 implementation has ever allowed late evaluation of such aspects. As such, there should be no practical incompatibility. Wording Changes from Ada 2012 38.d/4 {AI125-0105-1} Corrigendum: Clarified the wording so that the restriction against language-defined aspects on subprogram completions includes completions that are expressions functions and null procedures. 38.e/4 {AI125-0106-1} Corrigendum: Defined class-wide aspect for use in rules in 13.13.2. 38.f/4 {AI125-0138-1} Corrigendum: Added a definition of nonoverridable aspects. This is necessary to prevent generic contract problems with formal derived types. 13.2 Packed Types 1/3 {AI05-0229-1} [The Pack aspect having the value True specifies that storage minimization should be the main criterion when selecting the representation of a composite type.] Paragraphs 2 through 4 were moved to Annex J, "Obsolescent Features". Language Design Principles 1.a/4 {AI12-0001-1} If the default representation already uses minimal storage for a particular type, aspect Pack might not cause any representation change. It follows that aspect Pack should always be allowed, even when it has no effect on representation. 1.b/4 As a consequence, the chosen representation for a packed type may change during program maintenance even if the type is unchanged (in particular, if other representation aspects change on a part of the type). This is different than the behavior of most other representation aspects, whose properties remain guaranteed no matter what changes are made to other aspects. 1.c/4 Therefore, aspect Pack should not be used to achieve a representation required by external criteria. For instance, setting Component_Size to 1 should be preferred over using aspect Pack to ensure an array of bits. If future maintenance would make the array components aliased, independent, or atomic, the program would become illegal if Component_Size is used (immediately identifying a problem) while the aspect Pack version would simply change representations (probably causing a hard-to-find bug). Static Semantics 5/3 {AI05-0229-1} For a full type declaration of a composite type, the following language-defined representation aspect may be specified: 5.1/3 Pack The type of aspect Pack is Boolean. When aspect Pack is True for a type, the type (or the extension part) is said to be packed. For a type extension, the parent part is packed as for the parent type, and specifying Pack causes packing only of the extension part. 5.a/3 Aspect Description for Pack: Minimize storage when laying out records and arrays. 5.2/3 If directly specified, the aspect_definition shall be a static expression. If not specified (including by inheritance), the aspect is False. 5.b/3 Ramification: {AI05-0229-1} The only high level semantic effect of specifying the Pack aspect is potential loss of independent addressability (see 9.10, "Shared Variables").] Implementation Advice 6 If a type is packed, then the implementation should try to minimize storage allocated to objects of the type, possibly at the expense of speed of accessing components, subject to reasonable complexity in addressing calculations. 6.a.1/2 Implementation Advice: Storage allocated to objects of a packed type should be minimized. 6.a/3 Ramification: {AI05-0229-1} Specifying the Pack aspect is for gaining space efficiency, possibly at the expense of time. If more explicit control over representation is desired, then a record_representation_clause, a Component_Size clause, or a Size clause should be used instead of, or in addition to, the Pack aspect. 6.1/4 This paragraph was deleted.{AI95-00291-02} {AI12-0001-1} 7/3 {AI05-0229-1} The recommended level of support for the Pack aspect is: 7.1/4 * {AI12-0001-1} Any component of a packed type that is of a by-reference type, that is specified as independently addressable, or that contains an aliased part, shall be aligned according to the alignment of its subtype. 7.a/4 Ramification: This also applies to atomic components. "Atomic" implies "specified as independently addressable", so we don't need to mention atomic here. 7.b/4 Other components do not have to respect the alignment of the subtype when packed; in many cases, the Recommended Level of Support will require the alignment to be ignored. 8/4 * {AI12-0001-1} For a packed record type, the components should be packed as tightly as possible subject to the above alignment requirements, the Sizes of the component subtypes, and any record_representation_clause that applies to the type; the implementation may, but need not, reorder components or cross aligned word boundaries to improve the packing. A component whose Size is greater than the word size may be allocated an integral number of words. 8.a Ramification: The implementation can always allocate an integral number of words for a component that will not fit in a word. The rule also allows small component sizes to be rounded up if such rounding does not waste space. For example, if Storage_Unit = 8, then a component of size 8 is probably more efficient than a component of size 7 plus a 1-bit gap (assuming the gap is needed anyway). 9/4 * {AI05-0009-1} {AI12-0001-1} For a packed array type, if the Size of the component subtype is less than or equal to the word size, Component_Size should be less than or equal to the Size of the component subtype, rounded up to the nearest factor of the word size, unless this would violate the above alignment requirements. 9.a/4 This paragraph was deleted.{AI12-0001-1} 9.b/3 Implementation Advice: The recommended level of support for the Pack aspect should be followed. Wording Changes from Ada 95 9.c/3 {AI95-00291-02} {AI05-0229-1} Added clarification that the Pack aspect can ignore alignment requirements on types that don't have by-reference or aliased parts. This was always intended, but there was no wording to that effect. Extensions to Ada 2005 9.d/3 {AI05-0229-1} Aspect Pack is new; pragma Pack is now obsolescent. Wording Changes from Ada 2005 9.e/3 {AI05-0009-1} Correction: Fixed so that the presence or absence of a confirming Component_Size representation clause does not change the meaning of the Pack aspect. Wording Changes from Ada 2012 9.f/4 {AI12-0001-1} Corrigendum: Fixed so that the Recommended Level of Support does not require packing of components for which such packing would violate other representation items or aspects. This is not incompatible, as either such Pack aspects were treated as illegal or the Recommended Level of Support was ignored as impractical, neither of which would change the behavior of any working programs. (Other behavior cannot be justifed from the Standard.) 13.3 Operational and Representation Attributes 1/1 {8652/0009} {AI95-00137-01} [ The values of certain implementation-dependent characteristics can be obtained by interrogating appropriate operational or representation attributes. Some of these attributes are specifiable via an attribute_definition_clause.] Language Design Principles 1.a In general, the meaning of a given attribute should not depend on whether the attribute was specified via an attribute_definition_clause, or chosen by default by the implementation. Syntax 2 attribute_definition_clause ::= for local_name'attribute_designator use expression; | for local_name'attribute_designator use name; Name Resolution Rules 3 For an attribute_definition_clause that specifies an attribute that denotes a value, the form with an expression shall be used. Otherwise, the form with a name shall be used. 4 For an attribute_definition_clause that specifies an attribute that denotes a value or an object, the expected type for the expression or name is that of the attribute. For an attribute_definition_clause that specifies an attribute that denotes a subprogram, the expected profile for the name is the profile required for the attribute. For an attribute_definition_clause that specifies an attribute that denotes some other kind of entity, the name shall resolve to denote an entity of the appropriate kind. 4.a Ramification: For example, the Size attribute is of type universal_integer. Therefore, the expected type for Y in "for X'Size use Y;" is universal_integer, which means that Y can be of any integer type. 4.b Discussion: For attributes that denote subprograms, the required profile is indicated separately for the individual attributes. 4.c Ramification: For an attribute_definition_clause with a name, the name need not statically denote the entity it denotes. For example, the following kinds of things are allowed: 4.d for Some_Access_Type'Storage_Pool use Storage_Pool_Array(I); for Some_Type'Read use Subprogram_Pointer.all; Legality Rules 5/3 {8652/0009} {AI95-00137-01} {AI05-0183-1} An attribute_designator is allowed in an attribute_definition_clause only if this International Standard explicitly allows it, or for an implementation-defined attribute if the implementation allows it. Each specifiable attribute constitutes an operational aspect or aspect of representation; the name of the aspect is that of the attribute. 5.a Discussion: For each specifiable attribute, we generally say something like, "The ... attribute may be specified for ... via an attribute_definition_clause." 5.b The above wording allows for T'Class'Alignment, T'Class'Size, T'Class'Input, and T'Class'Output to be specifiable. 5.c A specifiable attribute is not necessarily specifiable for all entities for which it is defined. For example, one is allowed to ask T'Component_Size for an array subtype T, but "for T'Component_Size use ..." is only allowed if T is a first subtype, because Component_Size is a type-related aspect. 6 For an attribute_definition_clause that specifies an attribute that denotes a subprogram, the profile shall be mode conformant with the one required for the attribute, and the convention shall be Ada. Additional requirements are defined for particular attributes. 6.a Ramification: This implies, for example, that if one writes: 6.b for T'Read use R; 6.c R has to be a procedure with two parameters with the appropriate subtypes and modes as shown in 13.13.2. Static Semantics 7/2 {AI95-00270-01} A Size clause is an attribute_definition_clause whose attribute_designator is Size. Similar definitions apply to the other specifiable attributes. 7.a To be honest: An attribute_definition_clause is type-related or subtype-specific if the attribute_designator denotes a type-related or subtype-specific attribute, respectively. 8 A storage element is an addressable element of storage in the machine. A word is the largest amount of storage that can be conveniently and efficiently manipulated by the hardware, given the implementation's run-time model. A word consists of an integral number of storage elements. 8.a Discussion: A storage element is not intended to be a single bit, unless the machine can efficiently address individual bits. 8.b Ramification: For example, on a machine with 8-bit storage elements, if there exist 32-bit integer registers, with a full set of arithmetic and logical instructions to manipulate those registers, a word ought to be 4 storage elements - that is, 32 bits. 8.c Discussion: The "given the implementation's run-time model" part is intended to imply that, for example, on an 80386 running MS-DOS, the word might be 16 bits, even though the hardware can support 32 bits. 8.d A word is what ACID refers to as a "natural hardware boundary". 8.e Storage elements may, but need not be, independently addressable (see 9.10, "Shared Variables"). Words are expected to be independently addressable. 8.1/3 {AI95-00133-01} {AI05-0092-1} A machine scalar is an amount of storage that can be conveniently and efficiently loaded, stored, or operated upon by the hardware. Machine scalars consist of an integral number of storage elements. The set of machine scalars is implementation defined, but includes at least the storage element and the word. Machine scalars are used to interpret component_clauses when the nondefault bit ordering applies. 8.e.1/2 Implementation defined: The set of machine scalars. 8.f/3 Ramification: {AI05-0092-1} A single storage element is a machine scalar in all Ada implementations. Similarly, a word is a machine scalar in all implementations (although it might be the same as a storage element). An implementation may define other machine scalars that make sense on the target (a half-word, for instance). 9/3 {8652/0009} {AI95-00137-01} {AI05-0191-1} The following representation attributes are defined: Address, Alignment, Size, Storage_Size, Component_Size, Has_Same_Storage, and Overlaps_Storage. 10/1 For a prefix X that denotes an object, program unit, or label: 11 X'Address Denotes the address of the first of the storage elements allocated to X. For a program unit or label, this value refers to the machine code associated with the corresponding body or statement. The value of this attribute is of type System.Address. 11.a Ramification: Here, the "first of the storage elements" is intended to mean the one with the lowest address; the endianness of the machine doesn't matter. 11.1/3 {AI05-0095-1} The prefix of X'Address shall not statically denote a subprogram that has convention Intrinsic. X'Address raises Program_Error if X denotes a subprogram that has convention Intrinsic. 12 Address may be specified for stand-alone objects and for program units via an attribute_definition_clause. 12.a Ramification: Address is not allowed for enumeration literals, predefined operators, derived task types, or derived protected types, since they are not program units. 12.b/3 Address is not allowed for intrinsic subprograms, either. That can be checked statically unless the prefix is a generic formal subprogram and the attribute reference is in the body of a generic unit. We define that case to raise Program_Error, in order that the compiler does not have to build a wrapper for intrinsic subprograms. 12.c The validity of a given address depends on the run-time model; thus, in order to use Address clauses correctly, one needs intimate knowledge of the run-time model. 12.d/3 {AI05-0229-1} If the Address of an object is specified, any explicit or implicit initialization takes place as usual, unless the Import aspect is also specified for the object (in which case any necessary initialization is presumably done in the foreign language). 12.e Any compilation unit containing an attribute_reference of a given type depends semantically on the declaration of the package in which the type is declared, even if not mentioned in an applicable with_clause - see 10.1.1. In this case, it means that if a compilation unit contains X'Address, then it depends on the declaration of System. Otherwise, the fact that the value of Address is of a type in System wouldn't make sense; it would violate the "legality determinable via semantic dependences" Language Design Principle. 12.f AI83-00305 - If X is a task type, then within the body of X, X denotes the current task object; thus, X'Address denotes the object's address. 12.g Interrupt entries and their addresses are described in J.7.1, " Interrupt Entries". 12.h If X is not allocated on a storage element boundary, X'Address points at the first of the storage elements that contains any part of X. This is important for the definition of the Position attribute to be sensible. 12.i/3 Aspect Description for Address: Machine address of an entity. Erroneous Execution 13/3 {AI05-0009-1} If an Address is specified, it is the programmer's responsibility to ensure that the address is valid and appropriate for the entity and its use; otherwise, program execution is erroneous. 13.a Discussion: "Appropriate for the entity and its use" covers cases such as misaligned addresses, read-only code addresses for variable data objects (and nonexecutable data addresses for code units), and addresses which would force objects that are supposed to be independently addressable to not be. Such addresses may be "valid" as they designate locations that are accessible to the program, but the program execution is still erroneous (meaning that implementations do not have to worry about these cases). Implementation Advice 14 For an array X, X'Address should point at the first component of the array, and not at the array bounds. 14.a.1/2 Implementation Advice: For an array X, X'Address should point at the first component of the array rather than the array bounds. 14.a Ramification: On the other hand, we have no advice to offer about discriminants and tag fields; whether or not the address points at them is not specified by the language. If discriminants are stored separately, then the Position of a discriminant might be negative, or might raise an exception. 15 The recommended level of support for the Address attribute is: 16 * X'Address should produce a useful result if X is an object that is aliased or of a by-reference type, or is an entity whose Address has been specified. 16.a Reason: Aliased objects are the ones for which the Unchecked_Access attribute is allowed; hence, these have to be allocated on an addressable boundary anyway. Similar considerations apply to objects of a by-reference type. 16.b An implementation need not go to any trouble to make Address work in other cases. For example, if an object X is not aliased and not of a by-reference type, and the implementation chooses to store it in a register, X'Address might return System.Null_Address (assuming registers are not addressable). For a subprogram whose calling convention is Intrinsic, or for a package, the implementation need not generate an out-of-line piece of code for it. 17 * An implementation should support Address clauses for imported subprograms. 18/2 * This paragraph was deleted.{AI95-00291-02} 18.a/2 This paragraph was deleted. 19 * If the Address of an object is specified, or it is imported or exported, then the implementation should not perform optimizations based on assumptions of no aliases. 19.a/2 Implementation Advice: The recommended level of support for the Address attribute should be followed. NOTES 20 2 The specification of a link name with the Link_Name aspect (see B.1) for a subprogram or object is an alternative to explicit specification of its link-time address, allowing a link-time directive to place the subprogram or object within memory. 21 3 The rules for the Size attribute imply, for an aliased object X, that if X'Size = Storage_Unit, then X'Address points at a storage element containing all of the bits of X, and only the bits of X. Wording Changes from Ada 83 21.a The intended meaning of the various attributes, and their attribute_definition_clauses, is more explicit. 21.b The address_clause has been renamed to at_clause and moved to Annex J, "Obsolescent Features". One can use an Address clause ("for T'Address use ...;") instead. 21.c The attributes defined in RM83-13.7.3 are moved to Annex G, A.5.3, and A.5.4. Wording Changes from Ada 2005 21.c.1/3 {AI05-0183-1} Defined that the names of aspects are the same as the name of the attribute; that gives a name to use in aspect_specifications (see 13.1.1). Language Design Principles 21.d By default, the Alignment of a subtype should reflect the " natural" alignment for objects of the subtype on the machine. The Alignment, whether specified or default, should be known at compile time, even though Addresses are generally not known at compile time. (The generated code should never need to check at run time the number of zero bits at the end of an address to determine an alignment). 21.e There are two symmetric purposes of Alignment clauses, depending on whether or not the implementation has control over object allocation. If the implementation allocates an object, the implementation should ensure that the Address and Alignment are consistent with each other. If something outside the implementation allocates an object, the implementation should be allowed to assume that the Address and Alignment are consistent, but should not assume stricter alignments than that. Static Semantics 22/2 {AI95-00291-02} For a prefix X that denotes an object: 23/2 X'Alignment {AI95-00291-02} The value of this attribute is of type universal_integer, and nonnegative; zero means that the object is not necessarily aligned on a storage element boundary. If X'Alignment is not zero, then X is aligned on a storage unit boundary and X'Address is an integral multiple of X'Alignment (that is, the Address modulo the Alignment is zero). 24/2 This paragraph was deleted.{AI95-00291-02} 24.a Ramification: The Alignment is passed by an allocator to the Allocate operation; the implementation has to choose a value such that if the address returned by Allocate is aligned as requested, the generated code can correctly access the object. 24.b The above mention of "modulo" is referring to the "mod" operator declared in System.Storage_Elements; if X mod N = 0, then X is by definition aligned on an N-storage-element boundary. 25/2 {AI95-00291-02} Alignment may be specified for [stand-alone] objects via an attribute_definition_clause; the expression of such a clause shall be static, and its value nonnegative. 25.a/3 Aspect Description for Alignment (object): Alignment of an object. 26/2 This paragraph was deleted.{AI95-00247-01} 26.1/2 {AI95-00291-02} For every subtype S: 26.2/2 S'Alignment {AI95-00291-02} The value of this attribute is of type universal_integer, and nonnegative. 26.3/2 {AI95-00051-02} {AI95-00291-02} For an object X of subtype S, if S'Alignment is not zero, then X'Alignment is a nonzero integral multiple of S'Alignment unless specified otherwise by a representation item. 26.4/2 {AI95-00291-02} Alignment may be specified for first subtypes via an attribute_definition_clause; the expression of such a clause shall be static, and its value nonnegative. 26.a/3 Aspect Description for Alignment (subtype): Alignment of a subtype. Erroneous Execution 27 Program execution is erroneous if an Address clause is given that conflicts with the Alignment. 27.a Ramification: The user has to either give an Alignment clause also, or else know what Alignment the implementation will choose by default. 28/2 {AI95-00051-02} {AI95-00291-02} For an object that is not allocated under control of the implementation, execution is erroneous if the object is not aligned according to its Alignment. Implementation Advice 28.1/3 {AI05-0116-1} For any tagged specific subtype S, S'Class'Alignment should equal S'Alignment. 28.a/3 Reason: A tagged object should never be less aligned than the alignment of the type of its view, so for a class-wide type T'Class, the alignment should be no greater than that of any type covered by T'Class. If the implementation only supports alignments that are required by the recommended level of support (and this is most likely), then the alignment of any covered type has to be the same or greater than that of T - which leaves the only reasonable value of T'Class'Alignment being T'Alignment. Thus we recommend this, but don't require it so that in the unlikely case that the implementation does support smaller alignments for covered types, it can select a smaller value for T'Class'Alignment. 28.a.1/3 Implementation Advice: For any tagged specific subtype S, S'Class'Alignment should equal S'Alignment. 29 The recommended level of support for the Alignment attribute for subtypes is: 30/2 * {AI95-00051-02} An implementation should support an Alignment clause for a discrete type, fixed point type, record type, or array type, specifying an Alignment value that is zero or a power of two, subject to the following: 31/2 * {AI95-00051-02} An implementation need not support an Alignment clause for a signed integer type specifying an Alignment greater than the largest Alignment value that is ever chosen by default by the implementation for any signed integer type. A corresponding limitation may be imposed for modular integer types, fixed point types, enumeration types, record types, and array types. 32/2 * {AI95-00051-02} An implementation need not support a nonconfirming Alignment clause which could enable the creation of an object of an elementary type which cannot be easily loaded and stored by available machine instructions. 32.1/2 * {AI95-00291-02} An implementation need not support an Alignment specified for a derived tagged type which is not a multiple of the Alignment of the parent type. An implementation need not support a nonconfirming Alignment specified for a derived untagged by-reference type. 32.a/2 Ramification: {AI95-00291-02} There is no recommendation to support any nonconfirming Alignment clauses for types not mentioned above. Remember that 13.1 requires support for confirming Alignment clauses for all types. 32.b/3 Implementation Note: {AI05-0116-1} An implementation that tries to support other alignments for derived tagged types will need to allow inherited subprograms to be passed objects that are less aligned than expected by the parent subprogram and type. This is unlikely to work if alignment has any effect on code selection. Similar issues arise for untagged derived types whose parameters are passed by reference. 33 The recommended level of support for the Alignment attribute for objects is: 34/2 * This paragraph was deleted.{AI95-00291-02} 35 * For stand-alone library-level objects of statically constrained subtypes, the implementation should support all Alignments supported by the target linker. For example, page alignment is likely to be supported for such objects, but not for subtypes. 35.1/2 * {AI95-00291-02} For other objects, an implementation should at least support the alignments supported for their subtype, subject to the following: 35.2/2 * {AI95-00291-02} An implementation need not support Alignments specified for objects of a by-reference type or for objects of types containing aliased subcomponents if the specified Alignment is not a multiple of the Alignment of the subtype of the object. 35.a/2 Implementation Advice: The recommended level of support for the Alignment attribute should be followed. NOTES 36 4 Alignment is a subtype-specific attribute. 37/2 This paragraph was deleted.{AI95-00247-01} 37.a/2 This paragraph was deleted. 38/3 5 {AI05-0229-1} {AI05-0269-1} A component_clause, Component_Size clause, or specifying the Pack aspect as True can override a specified Alignment. 38.a Discussion: Most objects are allocated by the implementation; for these, the implementation obeys the Alignment. The implementation is of course allowed to make an object more aligned than its Alignment requires - an object whose Alignment is 4 might just happen to land at an address that's a multiple of 4096. For formal parameters, the implementation might want to force an Alignment stricter than the parameter's subtype. For example, on some systems, it is customary to always align parameters to 4 storage elements. 38.b Hence, one might initially assume that the implementation could evilly make all Alignments 1 by default, even though integers, say, are normally aligned on a 4-storage-element boundary. However, the implementation cannot get away with that - if the Alignment is 1, the generated code cannot assume an Alignment of 4, at least not for objects allocated outside the control of the implementation. 38.c Of course implementations can assume anything they can prove, but typically an implementation will be unable to prove much about the alignment of, say, an imported object. Furthermore, the information about where an address "came from" can be lost to the compiler due to separate compilation. 38.d/3 {AI95-00114-01} {AI05-0229-1} The Alignment of an object that is a component of a packed composite object will usually be 0, to indicate that the component is not necessarily aligned on a storage element boundary. For a subtype, an Alignment of 0 means that objects of the subtype are not normally aligned on a storage element boundary at all. For example, an implementation might choose to make Component_Size be 1 for an array of Booleans, even when the Pack aspect has not been specified for the array. In this case, Boolean'Alignment would be 0. (In the presence of tasking, this would in general be feasible only on a machine that had atomic test-bit and set-bit instructions.) 38.e If the machine has no particular natural alignments, then all subtype Alignments will probably be 1 by default. 38.f Specifying an Alignment of 0 in an attribute_definition_clause does not require the implementation to do anything (except return 0 when the Alignment is queried). However, it might be taken as advice on some implementations. 38.g It is an error for an Address clause to disobey the object's Alignment. The error cannot be detected at compile time, in general, because the Address is not necessarily known at compile time (and is almost certainly not static). We do not require a run-time check, since efficiency seems paramount here, and Address clauses are treading on thin ice anyway. Hence, this misuse of Address clauses is just like any other misuse of Address clauses - it's erroneous. 38.h A type extension can have a stricter Alignment than its parent. This can happen, for example, if the Alignment of the parent is 4, but the extension contains a component with Alignment 8. The Alignment of a class-wide type or object will have to be the maximum possible Alignment of any extension. 38.i The recommended level of support for the Alignment attribute is intended to reflect a minimum useful set of capabilities. An implementation can assume that all Alignments are multiples of each other - 1, 2, 4, and 8 might be the only supported Alignments for subtypes. An Alignment of 3 or 6 is unlikely to be useful. For objects that can be allocated statically, we recommend that the implementation support larger alignments, such as 4096. We do not recommend such large alignments for subtypes, because the maximum subtype alignment will also have to be used as the alignment of stack frames, heap objects, and class-wide objects. Similarly, we do not recommend such large alignments for stack-allocated objects. 38.j If the maximum default Alignment is 8 (say, Long_Float'Alignment = 8), then the implementation can refuse to accept stricter alignments for subtypes. This simplifies the generated code, since the compiler can align the stack and class-wide types to this maximum without a substantial waste of space (or time). 38.k Note that the recommended level of support takes into account interactions between Size and Alignment. For example, on a 32-bit machine with 8-bit storage elements, where load and store instructions have to be aligned according to the size of the thing being loaded or stored, the implementation might accept an Alignment of 1 if the Size is 8, but might reject an Alignment of 1 if the Size is 32. On a machine where unaligned loads and stores are merely inefficient (as opposed to causing hardware traps), we would expect an Alignment of 1 to be supported for any Size. Wording Changes from Ada 83 38.l The nonnegative part is missing from RM83 (for mod_clauses, nee alignment_clauses, which are an obsolete version of Alignment clauses). Static Semantics 39/1 For a prefix X that denotes an object: 40 X'Size Denotes the size in bits of the representation of the object. The value of this attribute is of the type universal_integer. 40.a Ramification: Note that Size is in bits even if Machine_Radix is 10. Each decimal digit (and the sign) is presumably represented as some number of bits. 41 Size may be specified for [stand-alone] objects via an attribute_definition_clause; the expression of such a clause shall be static and its value nonnegative. 41.a/3 Aspect Description for Size (object): Size in bits of an object. Implementation Advice 41.1/2 {AI95-00051-02} The size of an array object should not include its bounds. 41.a.1/2 Implementation Advice: The Size of an array object should not include its bounds. 42/2 {AI95-00051-02} {AI95-00291-02} The recommended level of support for the Size attribute of objects is the same as for subtypes (see below), except that only a confirming Size clause need be supported for an aliased elementary object. 43/2 * This paragraph was deleted.{AI95-00051-02} Static Semantics 44 For every subtype S: 45 S'Size If S is definite, denotes the size [(in bits)] that the implementation would choose for the following objects of subtype S: 46 * A record component of subtype S when the record type is packed. 47 * The formal parameter of an instance of Unchecked_Conversion that converts from subtype S to some other subtype. 48 If S is indefinite, the meaning is implementation defined. The value of this attribute is of the type universal_integer. The Size of an object is at least as large as that of its subtype, unless the object's Size is determined by a Size clause, a component_clause, or a Component_Size clause. Size may be specified for first subtypes via an attribute_definition_- clause; the expression of such a clause shall be static and its value nonnegative. 48.a Implementation defined: The meaning of Size for indefinite subtypes. 48.b Reason: The effects of specifying the Size of a subtype are: 48.c * Unchecked_Conversion works in a predictable manner. 48.d * A composite type cannot be packed so tightly as to override the specified Size of a component's subtype. 48.e * Assuming the Implementation Advice is obeyed, if the specified Size allows independent addressability, then the Size of certain objects of the subtype should be equal to the subtype's Size. This applies to stand-alone objects and to components (unless a component_clause or a Component_Size clause applies). 48.f/3 {AI05-0229-1} A component_clause or a Component_Size clause can cause an object to be smaller than its subtype's specified size. The aspect Pack cannot; if a component subtype's size is specified, this limits how tightly the composite object can be packed. 48.g The Size of a class-wide (tagged) subtype is unspecified, because it's not clear what it should mean; it should certainly not depend on all of the descendants that happen to exist in a given program. Note that this cannot be detected at compile time, because in a generic unit, it is not necessarily known whether a given subtype is class-wide. It might raise an exception on some implementations. 48.h Ramification: A Size clause for a numeric subtype need not affect the underlying numeric type. For example, if I say: 48.i type S is range 1..2; for S'Size use 64; 48.j I am not guaranteed that S'Base'Last >= 2**63-1, nor that intermediate results will be represented in 64 bits. 48.k Reason: There is no need to complicate implementations for this sort of thing, because the right way to affect the base range of a type is to use the normal way of declaring the base range: 48.l type Big is range -2**63 .. 2**63 - 1; subtype Small is Big range 1..1000; 48.m Ramification: The Size of a large unconstrained subtype (e.g. String'Size) is likely to raise Constraint_Error, since it is a nonstatic expression of type universal_integer that might overflow the largest signed integer type. There is no requirement that the largest integer type be able to represent the size in bits of the largest possible object. 48.n/3 Aspect Description for Size (subtype): Size in bits of a subtype. Implementation Requirements 49 In an implementation, Boolean'Size shall be 1. Implementation Advice 50/2 {AI95-00051-02} If the Size of a subtype allows for efficient independent addressability (see 9.10) on the target architecture, then the Size of the following objects of the subtype should equal the Size of the subtype: 51 * Aliased objects (including components). 52 * Unaliased components, unless the Size of the component is determined by a component_clause or Component_Size clause. 52.a.1/2 Implementation Advice: If the Size of a subtype allows for efficient independent addressability, then the Size of most objects of the subtype should equal the Size of the subtype. 52.a Ramification: Thus, on a typical 32-bit machine, "for S'Size use 32;" will guarantee that aliased objects of subtype S, and components whose subtype is S, will have Size = 32 (assuming the implementation chooses to obey this Implementation Advice). On the other hand, if one writes, "for S2'Size use 5;" then stand-alone objects of subtype S2 will typically have their Size rounded up to ensure independent addressability. 52.b Note that "for S'Size use 32;" does not cause things like formal parameters to have Size = 32 - the implementation is allowed to make all parameters be at least 64 bits, for example. 52.c Note that "for S2'Size use 5;" requires record components whose subtype is S2 to be exactly 5 bits if the record type is packed. The same is not true of array components; their Size may be rounded up to the nearest factor of the word size. 52.d/2 Implementation Note: {AI95-00291-02} On most machines, arrays don't contain gaps between elementary components; if the Component_Size is greater than the Size of the component subtype, the extra bits are generally considered part of each component, rather than gaps between components. On the other hand, a record might contain gaps between elementary components, depending on what sorts of loads, stores, and masking operations are generally done by the generated code. 52.e/2 {AI95-00291-02} For an array, any extra bits stored for each elementary component will generally be part of the component - the whole point of storing extra bits is to make loads and stores more efficient by avoiding the need to mask out extra bits. The PDP-10 is one counter-example; since the hardware supports byte strings with a gap at the end of each word, one would want to pack in that manner. 53 A Size clause on a composite subtype should not affect the internal layout of components. 53.a.1/2 Implementation Advice: A Size clause on a composite subtype should not affect the internal layout of components. 53.a/3 Reason: {AI05-0229-1} That's what Pack aspects, record_representation_clauses, and Component_Size clauses are for. 54 The recommended level of support for the Size attribute of subtypes is: 55 * The Size (if not specified) of a static discrete or fixed point subtype should be the number of bits needed to represent each value belonging to the subtype using an unbiased representation, leaving space for a sign bit only if the subtype contains negative values. If such a subtype is a first subtype, then an implementation should support a specified Size for it that reflects this representation. 55.a Implementation Note: This applies to static enumeration subtypes, using the internal codes used to represent the values. 55.b For a two's-complement machine, this implies that for a static signed integer subtype S, if all values of S are in the range 0 .. 2(n)-1, or all values of S are in the range -2(n-1) .. 2(n-1)-1, for some n less than or equal to the word size, then S'Size should be <= the smallest such n. For a one's-complement machine, it is the same except that in the second range, the lower bound " -2(n-1)" is replaced by "-2(n-1)+1". 55.c If an integer subtype (whether signed or unsigned) contains no negative values, the Size should not include space for a sign bit. 55.d Typically, the implementation will choose to make the Size of a subtype be exactly the smallest such n. However, it might, for example, choose a biased representation, in which case it could choose a smaller value. 55.e/3 {AI05-0229-1} On most machines, it is in general not a good idea to pack (parts of) multiple stand-alone objects into the same storage element, because (1) it usually doesn't save much space, and (2) it requires locking to prevent tasks from interfering with each other, since separate stand-alone objects are independently addressable. Therefore, if S'Size = 2 on a machine with 8-bit storage elements, the size of a stand-alone object of subtype S will probably not be 2. It might, for example, be 8, 16 or 32, depending on the availability and efficiency of various machine instructions. The same applies to components of composite types, unless Pack, Component_Size, or record layout is specified. 55.f For an unconstrained discriminated object, if the implementation allocates the maximum possible size, then the Size attribute should return that maximum possible size. 55.g Ramification: The Size of an object X is not usually the same as that of its subtype S. If X is a stand-alone object or a parameter, for example, most implementations will round X'Size up to a storage element boundary, or more, so X'Size might be greater than S'Size. On the other hand, X'Size cannot be less than S'Size, even if the implementation can prove, for example, that the range of values actually taken on by X during execution is smaller than the range of S. 55.h For example, if S is a first integer subtype whose range is 0..3, S'Size will be probably be 2 bits, and components of packed composite types of this subtype will be 2 bits (assuming Storage_Unit is a multiple of 2), but stand-alone objects and parameters will probably not have a size of 2 bits; they might be rounded up to 32 bits, for example. On the other hand, Unchecked_Conversion will use the 2-bit size, even when converting a stand-alone object, as one would expect. 55.i Another reason for making the Size of an object bigger than its subtype's Size is to support the run-time detection of uninitialized variables. The implementation might add an extra value to a discrete subtype that represents the uninitialized state, and check for this value on use. In some cases, the extra value will require an extra bit in the representation of the object. Such detection is not required by the language. If it is provided, the implementation has to be able to turn it off. For example, if the programmer gives a record_representation_clause or Component_Size clause that makes a component too small to allow the extra bit, then the implementation will not be able to perform the checking (not using this method, anyway). 55.j The fact that the size of an object is not necessarily the same as its subtype can be confusing: 55.k type Device_Register is range 0..2**8 - 1; for Device_Register'Size use 8; -- Confusing! My_Device : Device_Register; for My_Device'Address use To_Address(16#FF00#); 55.l The programmer might think that My_Device'Size is 8, and that My_Device'Address points at an 8-bit location. However, this is not true. In Ada 83 (and in Ada 95), My_Device'Size might well be 32, and My_Device'Address might well point at the high-order 8 bits of the 32-bit object, which are always all zero bits. If My_Device'Address is passed to an assembly language subprogram, based on the programmer's assumption, the program will not work properly. 55.m Reason: It is not reasonable to require that an implementation allocate exactly 8 bits to all objects of subtype Device_Register. For example, in many run-time models, stand-alone objects and parameters are always aligned to a word boundary. Such run-time models are generally based on hardware considerations that are beyond the control of the implementer. (It is reasonable to require that an implementation allocate exactly 8 bits to all components of subtype Device_Register, if packed.) 55.n Ramification: The correct way to write the above code is like this: 55.o type Device_Register is range 0..2**8 - 1; My_Device : Device_Register; for My_Device'Size use 8; for My_Device'Address use To_Address(16#FF00#); 55.p If the implementation cannot accept 8-bit stand-alone objects, then this will be illegal. However, on a machine where an 8-bit device register exists, the implementation will probably be able to accept 8-bit stand-alone objects. Therefore, My_Device'Size will be 8, and My_Device'Address will point at those 8 bits, as desired. 55.q If an object of subtype Device_Register is passed to a foreign language subprogram, it will be passed according to that subprogram's conventions. Most foreign language implementations have similar run-time model restrictions. For example, when passing to a C function, where the argument is of the C type char* (that is, pointer to char), the C compiler will generally expect a full word value, either on the stack, or in a register. It will not expect a single byte. Thus, Size clauses for subtypes really have nothing to do with passing parameters to foreign language subprograms. 56 * For a subtype implemented with levels of indirection, the Size should include the size of the pointers, but not the size of what they point at. 56.a Ramification: For example, if a task object is represented as a pointer to some information (including a task stack), then the size of the object should be the size of the pointer. The Storage_Size, on the other hand, should include the size of the stack. 56.1/2 * {AI95-00051-02} An implementation should support a Size clause for a discrete type, fixed point type, record type, or array type, subject to the following: 56.2/2 * {AI95-00051-02} An implementation need not support a Size clause for a signed integer type specifying a Size greater than that of the largest signed integer type supported by the implementation in the absence of a size clause (that is, when the size is chosen by default). A corresponding limitation may be imposed for modular integer types, fixed point types, enumeration types, record types, and array types. 56.b/2 Discussion: {AI95-00051-02} Note that the "corresponding limitation" for a record or array type implies that an implementation may impose some reasonable maximum size for records and arrays (e.g. 2**32 bits), which is an upper bound ("capacity" limit) on the size, whether chosen by default or by being specified by the user. The largest size supported for records need not be the same as the largest size supported for arrays. 56.b.1/3 Ramification: {AI05-0155-1} Only Size clauses with a size greater than or equal to the Size that would be chosen by default may be safely presumed to be supported on nonstatic elementary subtypes. Implementations may choose to support smaller sizes, but only if the Size allows any value of the subtype to be represented, for any possible value of the bounds. 56.3/2 * {AI95-00291-02} A nonconfirming size clause for the first subtype of a derived untagged by-reference type need not be supported. 56.c/2 Implementation Advice: The recommended level of support for the Size attribute should be followed. 56.d/2 Ramification: {AI95-00291-02} There is no recommendation to support any nonconfirming Size clauses for types not mentioned above. Remember that 13.1 requires support for confirming Size clauses for all types. NOTES 57 6 Size is a subtype-specific attribute. 58/3 7 {AI05-0229-1} A component_clause or Component_Size clause can override a specified Size. Aspect Pack cannot. Inconsistencies With Ada 83 58.a.1/2 {AI95-00114-01} We specify the meaning of Size in much more detail than Ada 83. This is not technically an inconsistency, but it is in practice, as most Ada 83 compilers use a different definition for Size than is required here. This should have been documented more explicitly during the Ada 9X process. Wording Changes from Ada 83 58.a The requirement for a nonnegative value in a Size clause was not in RM83, but it's hard to see how it would make sense. For uniformity, we forbid negative sizes, rather than letting implementations define their meaning. Static Semantics 59/1 For a prefix T that denotes a task object [(after any implicit dereference)]: 60/3 T'Storage_Size {AI05-0229-1} Denotes the number of storage elements reserved for the task. The value of this attribute is of the type universal_integer. The Storage_Size includes the size of the task's stack, if any. The language does not specify whether or not it includes other storage associated with the task (such as the "task control block" used by some implementations.) If the aspect Storage_Size is specified for the type of the object, the value of the Storage_Size attribute is at least the value determined by the aspect. 60.a Ramification: The value of this attribute is never negative, since it is impossible to "reserve" a negative number of storage elements. 60.b If the implementation chooses to allocate an initial amount of storage, and then increase this as needed, the Storage_Size cannot include the additional amounts (assuming the allocation of the additional amounts can raise Storage_Error); this is inherent in the meaning of "reserved." 60.c The implementation is allowed to allocate different amounts of storage for different tasks of the same subtype. 60.d Storage_Size is also defined for access subtypes - see 13.11. 61/3 {AI95-0229-1} [Aspect Storage_Size specifies the amount of storage to be reserved for the execution of a task.] Paragraphs 62 through 65 were moved to Annex J, "Obsolescent Features". Static Semantics 65.1/3 {AI05-0229-1} {AI05-0269-1} For a task type (including the anonymous type of a single_task_declaration), the following language-defined representation aspect may be specified: 65.2/3 Storage_Size The Storage_Size aspect is an expression, which shall be of any integer type. 65.a/3 To be honest: This definition somewhat conflicts with the "automatic" one for the obsolescent attribute Storage_Size (which can be specified). The only difference is where the given expression is evaluated. We intend for the above definition to supersede that "automatic" definition for this attribute. 65.b/3 Ramification: Note that the value of the Storage_Size aspect is an expression; it is not the value of an expression. The expression is evaluated for each object of the type (see below). 65.c/3 Aspect Description for Storage_Size (task): Size in storage elements reserved for a task type or single task object. Legality Rules 65.3/3 {AI05-0229-1} The Storage_Size aspect shall not be specified for a task interface type. Dynamic Semantics 66/3 {AI05-0229-1} When a task object is created, the expression (if any) associated with the Storage_Size aspect of its type is evaluated; the Storage_Size attribute of the newly created task object is at least the value of the expression. 66.a Ramification: The implementation is allowed to round up a specified Storage_Size amount. For example, if the implementation always allocates in chunks of 4096 bytes, the number 200 might be rounded up to 4096. Also, if the user specifies a negative number, the implementation has to normalize this to 0, or perhaps to a positive number. 66.b/3 {AI05-0229-1} If the Storage_Size aspect is not specified for the type of the task object, the value of the Storage_Size attribute is unspecified. 67 At the point of task object creation, or upon task activation, Storage_Error is raised if there is insufficient free storage to accommodate the requested Storage_Size. Static Semantics 68/1 For a prefix X that denotes an array subtype or array object [(after any implicit dereference)]: 69 X'Component_Size Denotes the size in bits of components of the type of X. The value of this attribute is of type universal_integer. 70 Component_Size may be specified for array types via an attribute_definition_clause; the expression of such a clause shall be static, and its value nonnegative. 70.a Implementation Note: The intent is that the value of X'Component_Size is always nonnegative. If the array is stored " backwards" in memory (which might be caused by an implementation-defined pragma), X'Component_Size is still positive. 70.b Ramification: For an array object A, A'Component_Size = A(I)'Size for any index I. 70.c/3 Aspect Description for Component_Size: Size in bits of a component of an array type. Implementation Advice 71 The recommended level of support for the Component_Size attribute is: 72 * An implementation need not support specified Component_Sizes that are less than the Size of the component subtype. 73/3 * {AI05-0229-1} An implementation should support specified Component_Sizes that are factors and multiples of the word size. For such Component_Sizes, the array should contain no gaps between components. For other Component_Sizes (if supported), the array should contain no gaps between components when Pack is also specified; the implementation should forbid this combination in cases where it cannot support a no-gaps representation. 73.a/3 Ramification: {AI05-0229-1} For example, if Storage_Unit = 8, and Word_Size = 32, then the user is allowed to specify a Component_Size of 1, 2, 4, 8, 16, and 32, with no gaps. In addition, n*32 is allowed for positive integers n, again with no gaps. If the implementation accepts Component_Size = 3, then it might allocate 10 components per word, with a 2-bit gap at the end of each word (unless Pack is also specified), or it might not have any internal gaps at all. (There can be gaps at either end of the array.) 73.b/2 Implementation Advice: The recommended level of support for the Component_Size attribute should be followed. Static Semantics 73.1/3 {AI05-0191-1} For a prefix X that denotes an object: 73.2/4 X'Has_Same_Storage {AI05-0191-1} X'Has_Same_Storage denotes a function with the following specification: 73.3/3 function X'Has_Same_Storage (Arg : any_type) return Boolean 73.4/4 {AI05-0191-1} {AI05-0264-1} {AI12-0077-1} The actual parameter shall be a name that denotes an object. The object denoted by the actual parameter can be of any type. This function evaluates the names of the objects involved. It returns True if the representation of the object denoted by the actual parameter occupies exactly the same bits as the representation of the object denoted by X and the objects occupy at least one bit; otherwise, it returns False. 73.c/3 Discussion: Has_Same_Storage means that, if the representation is contiguous, the objects sit at the same address and occupy the same length of memory. 73.5/3 {AI05-0191-1} For a prefix X that denotes an object: 73.6/3 X'Overlaps_Storage {AI05-0191-1} X'Overlaps_Storage denotes a function with the following specification: 73.7/3 function X'Overlaps_Storage (Arg : any_type) return Boolean 73.8/3 {AI05-0191-1} {AI05-0264-1} The actual parameter shall be a name that denotes an object. The object denoted by the actual parameter can be of any type. This function evaluates the names of the objects involved and returns True if the representation of the object denoted by the actual parameter shares at least one bit with the representation of the object denoted by X; otherwise, it returns False. NOTES 73.9/3 8 {AI05-0191-1} X'Has_Same_Storage(Y) implies X'Overlaps_Storage(Y). 73.10/3 9 {AI05-0191-1} X'Has_Same_Storage(Y) and X'Overlaps_Storage(Y) are not considered to be reads of X and Y. Static Semantics 73.11/3 {8652/0009} {AI95-00137-01} {AI05-0183-1} The following type-related operational attribute is defined: External_Tag. 74/1 {8652/0009} {AI95-00137-01} For every subtype S of a tagged type T (specific or class-wide): 75/3 S'External_Tag {8652/0040} {AI95-00108-01} {AI05-0092-1} S'External_Tag denotes an external string representation for S'Tag; it is of the predefined type String. External_Tag may be specified for a specific tagged type via an attribute_definition_clause; the expression of such a clause shall be static. The default external tag representation is implementation defined. See 13.13.2. The value of External_Tag is never inherited[; the default value is always used unless a new value is directly specified for a type]. 75.a Implementation defined: The default external representation for a type tag. 75.b/3 Aspect Description for External_Tag: Unique identifier for a tagged type in streams. Dynamic Semantics 75.1/3 {AI05-0113-1} If a user-specified external tag S'External_Tag is the same as T'External_Tag for some other tagged type declared by a different declaration in the partition, Program_Error is raised by the elaboration of the attribute_definition_clause. 75.c/3 Ramification: This rule does not depend on the visibility of the other tagged type, but it does depend on the existence of the other tagged type. The other tagged type could have the default external tag or a user-specified external tag. 75.d/3 This rule allows the same declaration to be elaborated multiple times. In that case, different types could have the same external tag. If that happens, Internal_Tag would return some unspecified tag, and Descendant_Tag probably would return the intended tag (using the given ancestor to determine which type is intended). However, in some cases (such as multiple instantiations of a derived tagged type declared in a generic body), Tag_Error might be raised by Descendant_Tag if multiple types are identified. 75.e/3 Note that while there is a race condition inherent in this definition (which attribute_definition_clause raises Program_Error depends on the order of elaboration), it doesn't matter as a program with two such clauses is simply wrong. Two types that both come from the same declaration are allowed, as noted previously. Implementation Requirements 76 In an implementation, the default external tag for each specific tagged type declared in a partition shall be distinct, so long as the type is declared outside an instance of a generic body. If the compilation unit in which a given tagged type is declared, and all compilation units on which it semantically depends, are the same in two different partitions, then the external tag for the type shall be the same in the two partitions. What it means for a compilation unit to be the same in two different partitions is implementation defined. At a minimum, if the compilation unit is not recompiled between building the two different partitions that include it, the compilation unit is considered the same in the two partitions. 76.a Implementation defined: What determines whether a compilation unit is the same in two different partitions. 76.b Reason: These requirements are important because external tags are used for input/output of class-wide types. These requirements ensure that what is written by one program can be read back by some other program so long as they share the same declaration for the type (and everything it depends on). 76.c The user may specify the external tag if (s)he wishes its value to be stable even across changes to the compilation unit in which the type is declared (or changes in some unit on which it depends). 76.d/2 {AI95-00114-01} We use a String rather than a Stream_Element_Array to represent an external tag for portability. 76.e Ramification: Note that the characters of an external tag need not all be graphic characters. In other words, the external tag can be a sequence of arbitrary 8-bit bytes. Implementation Permissions 76.1/3 {AI05-0113-1} If a user-specified external tag S'External_Tag is the same as T'External_Tag for some other tagged type declared by a different declaration in the partition, the partition may be rejected. 76.f/3 Ramification: This is, in general, a post-compilation check. This permission is intended for implementations that do link-time construction of the external tag lookup table; implementations that dynamically construct the table will likely prefer to raise Program_Error upon elaboration of the problem construct. We don't want this check to require any implementation complexity, as it will be very rare that there would be a problem. NOTES 77/2 10 {AI95-00270-01} The following language-defined attributes are specifiable, at least for some of the kinds of entities to which they apply: Address, Alignment, Bit_Order, Component_Size, External_Tag, Input, Machine_Radix, Output, Read, Size, Small, Storage_Pool, Storage_Size, Stream_Size, and Write. 78 11 It follows from the general rules in 13.1 that if one writes "for X'Size use Y;" then the X'Size attribute_reference will return Y (assuming the implementation allows the Size clause). The same is true for all of the specifiable attributes except Storage_Size. 78.a Ramification: An implementation may specify that an implementation-defined attribute is specifiable for certain entities. This follows from the fact that the semantics of implementation-defined attributes is implementation defined. An implementation is not allowed to make a language-defined attribute specifiable if it isn't. Examples 79 Examples of attribute definition clauses: 80 Byte : constant := 8; Page : constant := 2**12; 81 type Medium is range 0 .. 65_000; for Medium'Size use 2*Byte; for Medium'Alignment use 2; Device_Register : Medium; for Device_Register'Size use Medium'Size; for Device_Register'Address use System.Storage_Elements.To_Address(16#FFFF_0020#); 82 type Short is delta 0.01 range -100.0 .. 100.0; for Short'Size use 15; 83 for Car_Name'Storage_Size use -- specify access type's storage pool size 2000*((Car'Size/System.Storage_Unit) +1); -- approximately 2000 cars 84/2 {AI95-00441-01} function My_Input(Stream : not null access Ada.Streams.Root_Stream_Type'Class) return T; for T'Input use My_Input; -- see 13.13.2 NOTES 85 12 Notes on the examples: In the Size clause for Short, fifteen bits is the minimum necessary, since the type definition requires Short'Small <= 2**(-7). Extensions to Ada 83 85.a The syntax rule for length_clause is replaced with the new syntax rule for attribute_definition_clause, and it is modified to allow a name (as well as an expression). Wording Changes from Ada 83 85.b The syntax rule for attribute_definition_clause now requires that the prefix of the attribute be a local_name; in Ada 83 this rule was stated in the text. 85.c/2 {AI95-00114-01} In Ada 83, the relationship between a aspect_clause specifying a certain aspect and an attribute that queried that aspect was unclear. In Ada 95, they are the same, except for certain explicit exceptions. Wording Changes from Ada 95 85.d/2 {8652/0009} {AI95-00137-01} Corrigendum: Added wording to specify for each attribute whether it is an operational or representation attribute. 85.e/2 {8652/0040} {AI95-00108-01} Corrigendum: Added wording to specify that External_Tag is never inherited. 85.f/2 {AI95-00051-01} {AI95-00291-01} Adjusted the Recommended Level of Support for Alignment to eliminate nonsense requirements and to ensure that useful capabilities are required. 85.g/2 {AI95-00051-01} {AI95-00291-01} Adjusted the Recommended Level of Support for Size to eliminate nonsense requirements and to ensure that useful capabilities are required. Also eliminated any dependence on whether an aspect was specified (a confirming representation item should not affect the semantics). 85.h/2 {AI95-00133-01} Added the definition of machine scalar. 85.i/2 {AI95-00247-01} Removed the requirement that specified alignments for a composite type cannot override those for their components, because it was never intended to apply to components whose location was specified with a representation item. Moreover, it causes a difference in legality when a confirming alignment is specified for one of the composite types. 85.j/2 {AI95-00291-02} Removed recommended level of support rules about types with by-reference and aliased parts, because there are now blanket rules covering all recommended level of support rules. 85.k/2 {AI95-00291-02} Split the definition of Alignment for subtypes and for objects. This simplified the wording and eliminated confusion about which rules applied to objects, which applied to subtypes, and which applied to both. Inconsistencies With Ada 2005 85.l/3 {AI95-0095-1} Correction: An address attribute with a prefix of a generic formal subprogram whose actual parameter has convention Intrinsic now raises Program_Error. Since it is unlikely that such an attribute would have done anything useful (a subprogram with convention Intrinsic is not expected to have a normal subprogram body), it is highly unlikely that any existing programs would notice the difference, and any that do probably are buggy. 85.m/3 {AI95-0113-1} Correction: User-specified external tags that conflict with other external tags raise Program_Error (or are optionally illegal). This was legal and did not raise an exception in the past, although the effects were not defined. So while a program might depend on such behavior, the results were not portable (even to different versions of the same implementation). Such programs should be rare. Incompatibilities With Ada 2005 85.n/3 {AI05-0095-1} Correction: An address attribute with a prefix of a subprogram with convention Intrinsic is now illegal. Such attributes are very unlikely to have provided a useful answer (the intended meaning of convention Intrinsic is that there is no actual subprogram body for the operation), so this is highly unlikely to affect any existing programs unless they have a hidden bug. Extensions to Ada 2005 85.o/3 {AI05-0191-1} Attributes Has_Same_Storage and Overlaps_Storage are new. 85.p/3 {AI05-0229-1} Aspect Storage_Size is new; pragma Storage_Size is now obsolescent, joining attribute Storage_Size for task types. Wording Changes from Ada 2005 85.q/3 {AI05-0009-1} Correction: Improved the description of erroneous execution for address clauses to make it clear that specifying an address inappropriate for the entity will lead to erroneous execution. 85.r/3 {AI05-0116-1} Correction: Added Implementation Advice for the alignment of class-wide types. Wording Changes from Ada 2012 85.s/4 {AI12-0070-1} Corrigendum: Clarified the behavior of Has_Same_Storage when 'Size = 0. 13.4 Enumeration Representation Clauses 1 [An enumeration_representation_clause specifies the internal codes for enumeration literals.] Syntax 2 enumeration_representation_clause ::= for first_subtype_local_name use enumeration_aggregate; 3 enumeration_aggregate ::= array_aggregate Name Resolution Rules 4 The enumeration_aggregate shall be written as a one-dimensional array_aggregate, for which the index subtype is the unconstrained subtype of the enumeration type, and each component expression is expected to be of any integer type. 4.a Ramification: The "full coverage rules" for aggregates applies. An others is not allowed - there is no applicable index constraint in this context. Legality Rules 5 The first_subtype_local_name of an enumeration_representation_clause shall denote an enumeration subtype. 5.a Ramification: As for all type-related representation items, the local_name is required to denote a first subtype. 6/2 {AI95-00287-01} Each component of the array_aggregate shall be given by an expression rather than a <>. The expressions given in the array_aggregate shall be static, and shall specify distinct integer codes for each value of the enumeration type; the associated integer codes shall satisfy the predefined ordering relation of the type. 6.a Reason: Each value of the enumeration type has to be given an internal code, even if the first subtype of the enumeration type is constrained to only a subrange (this is only possible if the enumeration type is a derived type). This "full coverage" requirement is important because one may refer to Enum'Base'First and Enum'Base'Last, which need to have defined representations. Static Semantics 7 An enumeration_representation_clause specifies the coding aspect of representation. The coding consists of the internal code for each enumeration literal, that is, the integral value used internally to represent each literal. 7.a/3 Aspect Description for Coding: Internal representation of enumeration literals. Specified by an enumeration_representation_clause, not by an aspect_specification. Implementation Requirements 8 For nonboolean enumeration types, if the coding is not specified for the type, then for each value of the type, the internal code shall be equal to its position number. 8.a Reason: This default representation is already used by all known Ada compilers for nonboolean enumeration types. Therefore, we make it a requirement so users can depend on it, rather than feeling obliged to supply for every enumeration type an enumeration representation clause that is equivalent to this default rule. 8.b Discussion: For boolean types, it is relatively common to use all ones for True, and all zeros for False, since some hardware supports that directly. Of course, for a one-bit Boolean object (like in a packed array), False is presumably zero and True is presumably one (choosing the reverse would be extremely unfriendly!). Implementation Advice 9 The recommended level of support for enumeration_representation_clauses is: 10 * An implementation should support at least the internal codes in the range System.Min_Int..System.Max_Int. An implementation need not support enumeration_representation_clauses for boolean types. 10.a Ramification: The implementation may support numbers outside the above range, such as numbers greater than System.Max_Int. See AI83-00564. 10.b Reason: The benefits of specifying the internal coding of a boolean type do not outweigh the implementation costs. Consider, for example, the implementation of the logical operators on a packed array of booleans with strange internal codes. It's implementable, but not worth it. 10.c/2 Implementation Advice: The recommended level of support for enumeration_representation_clauses should be followed. NOTES 11/3 13 {8652/0009} {AI95-00137-01} {AI05-0299-1} Unchecked_Conversion may be used to query the internal codes used for an enumeration type. The attributes of the type, such as Succ, Pred, and Pos, are unaffected by the enumeration_representation_clause. For example, Pos always returns the position number, not the internal integer code that might have been specified in an enumeration_representation_clause. 11.a Discussion: Suppose the enumeration type in question is derived: 11.b type T1 is (Red, Green, Blue); subtype S1 is T1 range Red .. Green; type S2 is new S1; for S2 use (Red => 10, Green => 20, Blue => 30); 11.c/1 {8652/0009} {AI95-00137-01} The enumeration_representation_clause has to specify values for all enumerals, even ones that are not in S2 (such as Blue). The Base attribute can be used to get at these values. For example: 11.d for I in S2'Base loop ... -- When I equals Blue, the internal code is 30. end loop; 11.e We considered allowing or requiring "for S2'Base use ..." in cases like this, but it didn't seem worth the trouble. Examples 12 Example of an enumeration representation clause: 13 type Mix_Code is (ADD, SUB, MUL, LDA, STA, STZ); 14 for Mix_Code use (ADD => 1, SUB => 2, MUL => 3, LDA => 8, STA => 24, STZ =>33); Extensions to Ada 83 14.a As in other similar contexts, Ada 95 allows expressions of any integer type, not just expressions of type universal_integer, for the component expressions in the enumeration_aggregate. The preference rules for the predefined operators of root_integer eliminate any ambiguity. 14.b For portability, we now require that the default coding for an enumeration type be the "obvious" coding using position numbers. This is satisfied by all known implementations. Wording Changes from Ada 95 14.c/2 {8652/0009} {AI95-00137-01} Corrigendum: Updated to reflect that we no longer have something called representation_clause. 14.d/2 {AI95-00287-01} Added wording to prevent the use of <> in a enumeration_representation_clause. (<> is newly added to array_aggregates.) 13.5 Record Layout 1 The (record) layout aspect of representation consists of the storage places for some or all components, that is, storage place attributes of the components. The layout can be specified with a record_representation_clause. 13.5.1 Record Representation Clauses 1 [A record_representation_clause specifies the storage representation of records and record extensions, that is, the order, position, and size of components (including discriminants, if any). ] Language Design Principles 1.a/2 {AI95-00114-01} It should be feasible for an implementation to use negative offsets in the representation of composite types. However, no implementation should be forced to support negative offsets. Therefore, in the interest of uniformity, negative offsets should be disallowed in record_representation_clauses. Syntax 2 record_representation_clause ::= for first_subtype_local_name use record [mod_clause] {component_clause} end record; 3 component_clause ::= component_local_name at position range first_bit .. last_bit; 4 position ::= static_expression 5 first_bit ::= static_simple_expression 6 last_bit ::= static_simple_expression 6.a Reason: First_bit and last_bit need to be simple_expression instead of expression for the same reason as in range (see 3.5 , "Scalar Types"). Name Resolution Rules 7 Each position, first_bit, and last_bit is expected to be of any integer type. 7.a Ramification: These need not have the same integer type. Legality Rules 8/2 {AI95-00436-01} The first_subtype_local_name of a record_representation_clause shall denote a specific record or record extension subtype. 8.a Ramification: As for all type-related representation items, the local_name is required to denote a first subtype. 9 If the component_local_name is a direct_name, the local_name shall denote a component of the type. For a record extension, the component shall not be inherited, and shall not be a discriminant that corresponds to a discriminant of the parent type. If the component_local_name has an attribute_designator, the direct_name of the local_name shall denote either the declaration of the type or a component of the type, and the attribute_designator shall denote an implementation-defined implicit component of the type. 10 The position, first_bit, and last_bit shall be static expressions. The value of position and first_bit shall be nonnegative. The value of last_bit shall be no less than first_bit - 1. 10.a Ramification: A component_clause such as "X at 4 range 0..-1;" is allowed if X can fit in zero bits. 10.1/2 {AI95-00133-01} If the nondefault bit ordering applies to the type, then either: 10.2/2 * the value of last_bit shall be less than the size of the largest machine scalar; or 10.3/2 * the value of first_bit shall be zero and the value of last_bit + 1 shall be a multiple of System.Storage_Unit. 11 At most one component_clause is allowed for each component of the type, including for each discriminant (component_clauses may be given for some, all, or none of the components). Storage places within a component_list shall not overlap, unless they are for components in distinct variants of the same variant_part. 12 A name that denotes a component of a type is not allowed within a record_representation_clause for the type, except as the component_local_name of a component_clause. 12.a Reason: It might seem strange to make the record_representation_clause part of the declarative region, and then disallow mentions of the components within almost all of the record_representation_clause. The alternative would be to treat the component_local_name like a formal parameter name in a subprogram call (in terms of visibility). However, this rule would imply slightly different semantics, because (given the actual rule) the components can hide other declarations. This was the rule in Ada 83, and we see no reason to change it. The following, for example, was and is illegal: 12.b type T is record X : Integer; end record; X : constant := 31; -- Same defining name as the component. for T use record X at 0 range 0..X; -- Illegal! end record; 12.c The component X hides the named number X throughout the record_representation_clause. Static Semantics 13/2 {AI95-00133-01} A record_representation_clause (without the mod_clause) specifies the layout. 13.a/3 Aspect Description for Layout (record): Layout of record components. Specified by a record_representation_clause, not by an aspect_specification. 13.b/3 Aspect Description for Record layout: See Layout. 13.1/2 {AI95-00133-01} If the default bit ordering applies to the type, the position, first_bit, and last_bit of each component_clause directly specify the position and size of the corresponding component. 13.2/3 {AI95-00133-01} {AI05-0264-1} If the nondefault bit ordering applies to the type, then the layout is determined as follows: 13.3/2 * the component_clauses for which the value of last_bit is greater than or equal to the size of the largest machine scalar directly specify the position and size of the corresponding component; 13.4/2 * for other component_clauses, all of the components having the same value of position are considered to be part of a single machine scalar, located at that position; this machine scalar has a size which is the smallest machine scalar size larger than the largest last_bit for all component_clauses at that position; the first_bit and last_bit of each component_clause are then interpreted as bit offsets in this machine scalar. 13.c/2 This paragraph was deleted.{AI95-00133-01} 13.d Ramification: A component_clause also determines the value of the Size attribute of the component, since this attribute is related to First_Bit and Last_Bit. 14 [A record_representation_clause for a record extension does not override the layout of the parent part;] if the layout was specified for the parent type, it is inherited by the record extension. Implementation Permissions 15 An implementation may generate implementation-defined components (for example, one containing the offset of another component). An implementation may generate names that denote such implementation-defined components; such names shall be implementation-defined attribute_references. An implemen- tation may allow such implementation-defined names to be used in record_- representation_clauses. An implementation can restrict such component_clauses in any manner it sees fit. 15.a Implementation defined: Implementation-defined components. 15.b Ramification: Of course, since the semantics of implementation-defined attributes is implementation defined, the implementation need not support these names in all situations. They might be purely for the purpose of component_clauses, for example. The visibility rules for such names are up to the implementation. 15.c We do not allow such component names to be normal identifiers - that would constitute blanket permission to do all kinds of evil things. 15.d Discussion: Such implementation-defined components are known in the vernacular as "dope." Their main purpose is for storing offsets of components that depend on discriminants. 16 If a record_representation_clause is given for an untagged derived type, the storage place attributes for all of the components of the derived type may differ from those of the corresponding components of the parent type, even for components whose storage place is not specified explicitly in the record_- representation_clause. 16.a Reason: This is clearly necessary, since the whole record may need to be laid out differently. Implementation Advice 17 The recommended level of support for record_representation_clauses is: 17.1/2 * {AI95-00133-01} An implementation should support machine scalars that correspond to all of the integer, floating point, and address formats supported by the machine. 18 * An implementation should support storage places that can be extracted with a load, mask, shift sequence of machine code, and set with a load, shift, mask, store sequence, given the available machine instructions and run-time model. 19 * A storage place should be supported if its size is equal to the Size of the component subtype, and it starts and ends on a boundary that obeys the Alignment of the component subtype. 20/2 * {AI95-00133-01} For a component with a subtype whose Size is less than the word size, any storage place that does not cross an aligned word boundary should be supported. 20.a Reason: The above recommendations are sufficient to define interfaces to most interesting hardware. This causes less implementation burden than the definition in ACID, which requires arbitrary bit alignments of arbitrarily large components. Since the ACID definition is neither enforced by the ACVC, nor supported by all implementations, it seems OK for us to weaken it. 21 * An implementation may reserve a storage place for the tag field of a tagged type, and disallow other components from overlapping that place. 21.a Ramification: Similar permission for other dope is not granted. 22 * An implementation need not support a component_clause for a component of an extension part if the storage place is not after the storage places of all components of the parent type, whether or not those storage places had been specified. 22.a Reason: These restrictions are probably necessary if block equality operations are to be feasible for class-wide types. For block comparison to work, the implementation typically has to fill in any gaps with zero (or one) bits. If a "gap" in the parent type is filled in with a component in a type extension, then this won't work when a class-wide object is passed by reference, as is required. 22.b/2 Implementation Advice: The recommended level of support for record_representation_clauses should be followed. NOTES 23 14 If no component_clause is given for a component, then the choice of the storage place for the component is left to the implementation. If component_clauses are given for all components, the record_representation_clause completely specifies the representation of the type and will be obeyed exactly by the implementation. 23.a Ramification: The visibility rules prevent the name of a component of the type from appearing in a record_representation_clause at any place except for the component_local_name of a component_clause. However, since the record_representation_clause is part of the declarative region of the type declaration, the component names hide outer homographs throughout. 23.b/1 {8652/0009} {AI95-00137-01} A record_representation_clause cannot be given for a protected type, even though protected types, like record types, have components. The primary reason for this rule is that there is likely to be too much dope in a protected type - entry queues, bit maps for barrier values, etc. In order to control the representation of the user-defined components, simply declare a record type, give it a record_representation_clause, and give the protected type one component whose type is the record type. Alternatively, if the protected object is protecting something like a device register, it makes more sense to keep the thing being protected outside the protected object (possibly with a pointer to it in the protected object), in order to keep implementation-defined components out of the way. Examples 24 Example of specifying the layout of a record type: 25 Word : constant := 4; -- storage element is byte, 4 bytes per word 26 type State is (A,M,W,P); type Mode is (Fix, Dec, Exp, Signif); 27 type Byte_Mask is array (0..7) of Boolean; type State_Mask is array (State) of Boolean; type Mode_Mask is array (Mode) of Boolean; 28 type Program_Status_Word is record System_Mask : Byte_Mask; Protection_Key : Integer range 0 .. 3; Machine_State : State_Mask; Interrupt_Cause : Interruption_Code; Ilc : Integer range 0 .. 3; Cc : Integer range 0 .. 3; Program_Mask : Mode_Mask; Inst_Address : Address; end record; 29 for Program_Status_Word use record System_Mask at 0*Word range 0 .. 7; Protection_Key at 0*Word range 10 .. 11; -- bits 8,9 unused Machine_State at 0*Word range 12 .. 15; Interrupt_Cause at 0*Word range 16 .. 31; Ilc at 1*Word range 0 .. 1; -- second word Cc at 1*Word range 2 .. 3; Program_Mask at 1*Word range 4 .. 7; Inst_Address at 1*Word range 8 .. 31; end record; 30 for Program_Status_Word'Size use 8*System.Storage_Unit; for Program_Status_Word'Alignment use 8; NOTES 31 15 Note on the example: The record_representation_clause defines the record layout. The Size clause guarantees that (at least) eight storage elements are used for objects of the type. The Alignment clause guarantees that aliased, imported, or exported objects of the type will have addresses divisible by eight. Wording Changes from Ada 83 31.a The alignment_clause has been renamed to mod_clause and moved to Annex J, "Obsolescent Features". 31.b We have clarified that implementation-defined component names have to be in the form of an attribute_reference of a component or of the first subtype itself; surely Ada 83 did not intend to allow arbitrary identifiers. 31.c The RM83-13.4(7) wording incorrectly allows components in nonvariant records to overlap. We have corrected that oversight. Incompatibilities With Ada 95 31.d/2 {AI95-00133-01} Amendment Correction: The meaning of a record_representation_clause for the nondefault bit order is now clearly defined. Thus, such clauses can be portably written. In order to do that though, the equivalence of bit 1 in word 1 to bit 9 in word 0 (for a machine with Storage_Unit = 8) had to be dropped for the nondefault bit order. Any record_representation_clauses which depends on that equivalence will break (although such code would imply a noncontiguous representation for a component, and it seems unlikely that compilers were supporting that anyway). Extensions to Ada 95 31.e/2 {AI95-00436-01} Amendment Correction: The undocumented (and likely unintentional) incompatibility with Ada 83 caused by not allowing record_representation_clauses on limited record types is removed. 13.5.2 Storage Place Attributes Static Semantics 1 For a component C of a composite, non-array object R, the storage place attributes are defined: 1.a Ramification: The storage place attributes are not (individually) specifiable, but the user may control their values by giving a record_representation_clause. 2/2 R.C'Position {AI95-00133-01} If the nondefault bit ordering applies to the composite type, and if a component_clause specifies the placement of C, denotes the value given for the position of the component_clause; otherwise, denotes the same value as R.C'Address - R'Address. The value of this attribute is of the type universal_integer. 2.a/2 Ramification: {AI95-00133-01} Thus, for the default bit order, R.C'Position is the offset of C in storage elements from the beginning of the object, where the first storage element of an object is numbered zero. R'Address + R.C'Position = R.C'Address. For record extensions, the offset is not measured from the beginning of the extension part, but from the beginning of the whole object, as usual. 2.b In "R.C'Address - R'Address", the "-" operator is the one in System.Storage_Elements that takes two Addresses and returns a Storage_Offset. 3/2 R.C'First_Bit {AI95-00133-01} If the nondefault bit ordering applies to the composite type, and if a component_clause specifies the placement of C, denotes the value given for the first_bit of the component_clause; otherwise, denotes the offset, from the start of the first of the storage elements occupied by C, of the first bit occupied by C. This offset is measured in bits. The first bit of a storage element is numbered zero. The value of this attribute is of the type universal_integer. 4/2 R.C'Last_Bit {AI95-00133-01} If the nondefault bit ordering applies to the composite type, and if a component_clause specifies the placement of C, denotes the value given for the last_bit of the component_clause; otherwise, denotes the offset, from the start of the first of the storage elements occupied by C, of the last bit occupied by C. This offset is measured in bits. The value of this attribute is of the type universal_integer. 4.a/2 Ramification: {AI95-00114-01} The ordering of bits in a storage element is defined in 13.5.3, "Bit Ordering". 4.b R.C'Size = R.C'Last_Bit - R.C'First_Bit + 1. (Unless the implementation chooses an indirection representation.) 4.c If a component_clause applies to a component, then that component will be at the same relative storage place in all objects of the type. Otherwise, there is no such requirement. Implementation Advice 5 If a component is represented using some form of pointer (such as an offset) to the actual data of the component, and this data is contiguous with the rest of the object, then the storage place attributes should reflect the place of the actual data, not the pointer. If a component is allocated discontiguously from the rest of the object, then a warning should be generated upon reference to one of its storage place attributes. 5.a Reason: For discontiguous components, these attributes make no sense. For example, an implementation might allocate dynamic-sized components on the heap. For another example, an implementation might allocate the discriminants separately from the other components, so that multiple objects of the same subtype can share discriminants. Such representations cannot happen if there is a component_clause for that component. 5.b/2 Implementation Advice: If a component is represented using a pointer to the actual data of the component which is contiguous with the rest of the object, then the storage place attributes should reflect the place of the actual data. If a component is allocated discontiguously from the rest of the object, then a warning should be generated upon reference to one of its storage place attributes. Incompatibilities With Ada 95 5.c/2 {AI95-00133-01} Amendment Correction: The meaning of the storage place attributes for the nondefault bit order is now clearly defined, and can be different than that given by strictly following the Ada 95 wording. Any code which depends on the original Ada 95 values for a type using the nondefault bit order where they are different will break. 13.5.3 Bit Ordering 1 [The Bit_Order attribute specifies the interpretation of the storage place attributes.] 1.a Reason: The intention is to provide uniformity in the interpretation of storage places across implementations on a particular machine by allowing the user to specify the Bit_Order. It is not intended to fully support data interoperability across different machines, although it can be used for that purpose in some situations. 1.b/2 {AI95-00114-01} We can't require all implementations on a given machine to use the same bit ordering by default; if the user cares, a Bit_Order attribute_definition_clause can be used to force all implementations to use the same bit ordering. Static Semantics 2 A bit ordering is a method of interpreting the meaning of the storage place attributes. High_Order_First [(known in the vernacular as "big endian" )] means that the first bit of a storage element (bit 0) is the most significant bit (interpreting the sequence of bits that represent a component as an unsigned integer value). Low_Order_First [(known in the vernacular as "little endian")] means the opposite: the first bit is the least significant. 3 For every specific record subtype S, the following attribute is defined: 4 S'Bit_Order Denotes the bit ordering for the type of S. The value of this attribute is of type System.Bit_Order. Bit_Order may be specified for specific record types via an attribute_definition_clause; the expression of such a clause shall be static. 4.a/3 Aspect Description for Bit_Order: Order of bit numbering in a record_representation_clause. 5 If Word_Size = Storage_Unit, the default bit ordering is implementation defined. If Word_Size > Storage_Unit, the default bit ordering is the same as the ordering of storage elements in a word, when interpreted as an integer. 5.a Implementation defined: If Word_Size = Storage_Unit, the default bit ordering. 5.b Ramification: Consider machines whose Word_Size = 32, and whose Storage_Unit = 8. Assume the default bit ordering applies. On a machine with big-endian addresses, the most significant storage element of an integer is at the address of the integer. Therefore, bit zero of a storage element is the most significant bit. On a machine with little-endian addresses, the least significant storage element of an integer is at the address of the integer. Therefore, bit zero of a storage element is the least significant bit. 6 The storage place attributes of a component of a type are interpreted according to the bit ordering of the type. 6.a Ramification: This implies that the interpretation of the position, first_bit, and last_bit of a component_clause of a record_representation_clause obey the bit ordering given in a representation item. Implementation Advice 7 The recommended level of support for the nondefault bit ordering is: 8/2 * {AI95-00133-01} The implementation should support the nondefault bit ordering in addition to the default bit ordering. 8.a/2 Ramification: {AI95-00133-01} The implementation should support both bit orderings. Implementations are required to support storage positions that cross storage element boundaries when Word_Size > Storage_Unit but the definition of the storage place attributes for the nondefault bit order ensures that such storage positions will not be split into two or three pieces. Thus, there is no significant implementation burden to supporting the nondefault bit order, given that the set of machine scalars is implementation-defined. 8.b/2 Implementation Advice: The recommended level of support for the nondefault bit ordering should be followed. NOTES 9/2 16 {AI95-00133-01} Bit_Order clauses make it possible to write record_representation_clauses that can be ported between machines having different bit ordering. They do not guarantee transparent exchange of data between such machines. Extensions to Ada 83 9.a The Bit_Order attribute is new to Ada 95. Wording Changes from Ada 95 9.b/2 {AI95-00133-01} We now suggest that all implementations support the nondefault bit order. 13.6 Change of Representation 1/3 {AI05-0229-1} [ A type_conversion (see 4.6) can be used to convert between two different representations of the same array or record. To convert an array from one representation to another, two array types need to be declared with matching component subtypes, and convertible index types. If one type has Pack specified and the other does not, then explicit conversion can be used to pack or unpack an array. 2 To convert a record from one representation to another, two record types with a common ancestor type need to be declared, with no inherited subprograms. Distinct representations can then be specified for the record types, and explicit conversion between the types can be used to effect a change in representation.] 2.a Ramification: This technique does not work if the first type is an untagged type with user-defined primitive subprograms. It does not work at all for tagged types. Examples 3 Example of change of representation: 4 -- Packed_Descriptor and Descriptor are two different types -- with identical characteristics, apart from their -- representation 5 type Descriptor is record -- components of a descriptor end record; 6 type Packed_Descriptor is new Descriptor; 7 for Packed_Descriptor use record -- component clauses for some or for all components end record; 8 -- Change of representation can now be accomplished by explicit type conversions: 9 D : Descriptor; P : Packed_Descriptor; 10 P := Packed_Descriptor(D); -- pack D D := Descriptor(P); -- unpack P 13.7 The Package System 1 [For each implementation there is a library package called System which includes the definitions of certain configuration-dependent characteristics.] Static Semantics 2 The following language-defined library package exists: 2.a/2 Implementation defined: The contents of the visible part of package System. 3/2 {AI95-00362-01} package System is pragma Pure(System); 4 type Name is implementation-defined-enumeration-type; System_Name : constant Name := implementation-defined; 5 -- System-Dependent Named Numbers: 6 Min_Int : constant := root_integer'First; Max_Int : constant := root_integer'Last; 7 Max_Binary_Modulus : constant := implementation-defined; Max_Nonbinary_Modulus : constant := implementation-defined; 8 Max_Base_Digits : constant := root_real'Digits; Max_Digits : constant := implementation-defined; 9 Max_Mantissa : constant := implementation-defined; Fine_Delta : constant := implementation-defined; 10 Tick : constant := implementation-defined; 11 -- Storage-related Declarations: 12 type Address is implementation-defined; Null_Address : constant Address; 13 Storage_Unit : constant := implementation-defined; Word_Size : constant := implementation-defined * Storage_Unit; Memory_Size : constant := implementation-defined; 14/3 {AI05-0229-1} -- Address Comparison: function "<" (Left, Right : Address) return Boolean with Convention => Intrinsic; function "<="(Left, Right : Address) return Boolean with Convention => Intrinsic; function ">" (Left, Right : Address) return Boolean with Convention => Intrinsic; function ">="(Left, Right : Address) return Boolean with Convention => Intrinsic; function "=" (Left, Right : Address) return Boolean with Convention => Intrinsic; -- function "/=" (Left, Right : Address) return Boolean; -- "/=" is implicitly defined 15/2 {AI95-00221-01} -- Other System-Dependent Declarations: type Bit_Order is (High_Order_First, Low_Order_First); Default_Bit_Order : constant Bit_Order := implementation-defined; 16 -- Priority-related declarations (see D.1): subtype Any_Priority is Integer range implementation-defined; subtype Priority is Any_Priority range Any_Priority'First .. implementation-defined; subtype Interrupt_Priority is Any_Priority range Priority'Last+1 .. Any_Priority'Last; 17 Default_Priority : constant Priority := (Priority'First + Priority'Last)/2; 18 private ... -- not specified by the language end System; 19 Name is an enumeration subtype. Values of type Name are the names of alternative machine configurations handled by the implementation. System_Name represents the current machine configuration. 20 The named numbers Fine_Delta and Tick are of the type universal_real; the others are of the type universal_integer. 21 The meanings of the named numbers are: 22 [ Min_Int The smallest (most negative) value allowed for the expressions of a signed_integer_type_definition. 23 Max_Int The largest (most positive) value allowed for the expressions of a signed_integer_type_definition. 24 Max_Binary_Modulus A power of two such that it, and all lesser positive powers of two, are allowed as the modulus of a modular_type_definition. 25 Max_Nonbinary_Modulus A value such that it, and all lesser positive integers, are allowed as the modulus of a modular_type_definition. 25.a Ramification: There is no requirement that Max_Nonbinary_Modulus be less than or equal to Max_Binary_Modulus, although that's what makes most sense. On a typical 32-bit machine, for example, Max_Binary_Modulus will be 2**32 and Max_Nonbinary_Modulus will be 2**31, because supporting nonbinary moduli in above 2**31 causes implementation difficulties. 26 Max_Base_Digits The largest value allowed for the requested decimal precision in a floating_point_definition. 27 Max_Digits The largest value allowed for the requested decimal precision in a floating_point_definition that has no real_range_specification. Max_Digits is less than or equal to Max_Base_Digits. 28 Max_Mantissa The largest possible number of binary digits in the mantissa of machine numbers of a user-defined ordinary fixed point type. (The mantissa is defined in Annex G.) 29 Fine_Delta The smallest delta allowed in an ordinary_fixed_point_definition that has the real_range_- specification range -1.0 .. 1.0. ] 30 Tick A period in seconds approximating the real time interval during which the value of Calendar.Clock remains constant. 30.a Ramification: There is no required relationship between System.Tick and Duration'Small, other than the one described here. 30.b The inaccuracy of the delay_statement has no relation to Tick. In particular, it is possible that the clock used for the delay_statement is less accurate than Calendar.Clock. 30.c We considered making Tick a run-time-determined quantity, to allow for easier configurability. However, this would not be upward compatible, and the desired configurability can be achieved using functionality defined in Annex D, "Real-Time Systems". 31 Storage_Unit The number of bits per storage element. 32 Word_Size The number of bits per word. 33 Memory_Size An implementation-defined value [that is intended to reflect the memory size of the configuration in storage elements.] 33.a Discussion: It is unspecified whether this refers to the size of the address space, the amount of physical memory on the machine, or perhaps some other interpretation of "memory size." In any case, the value has to be given by a static expression, even though the amount of memory on many modern machines is a dynamic quantity in several ways. Thus, Memory_Size is not very useful. 34/2 {AI95-00161-01} Address is a definite, nonlimited type with preelaborable initialization (see 10.2.1). Address represents machine addresses capable of addressing individual storage elements. Null_Address is an address that is distinct from the address of any object or program unit. 34.a Ramification: The implementation has to ensure that there is at least one address that nothing will be allocated to; Null_Address will be one such address. 34.b Ramification: Address is the type of the result of the attribute Address. 34.c Reason: Address is required to be nonlimited and definite because it is important to be able to assign addresses, and to declare uninitialized address variables. 34.d/2 Ramification: {AI95-00161-01} If System.Address is defined as a private type (as suggested below), it might be necessary to add a pragma Preelaborable_Initialization to the specification of System in order that Address have preelaborable initialization as required. 35/2 {AI95-00221-01} Default_Bit_Order shall be a static constant. See 13.5.3 for an explanation of Bit_Order and Default_Bit_Order. Implementation Permissions 36/2 {AI95-00362-01} An implementation may add additional implementation-defined declarations to package System and its children. [However, it is usually better for the implementation to provide additional functionality via implementation-defined children of System.] 36.a Ramification: The declarations in package System and its children can be implicit. For example, since Address is not limited, the predefined "=" and "/=" operations are probably sufficient. However, the implementation is not required to use the predefined "=". Implementation Advice 37 Address should be a private type. 37.a Reason: This promotes uniformity by avoiding having implementation-defined predefined operations for the type. We don't require it, because implementations may want to stick with what they have. 37.a.1/2 Implementation Advice: Type System.Address should be a private type. 37.b Implementation Note: It is not necessary for Address to be able to point at individual bits within a storage element. Nor is it necessary for it to be able to point at machine registers. It is intended as a memory address that matches the hardware's notion of an address. 37.c The representation of the null value of a general access type should be the same as that of Null_Address; instantiations of Unchecked_Conversion should work accordingly. If the implementation supports interfaces to other languages, the representation of the null value of a general access type should be the same as in those other languages, if appropriate. 37.d Note that the children of the Interfaces package will generally provide foreign-language-specific null values where appropriate. See UI-0065 regarding Null_Address. NOTES 38 17 There are also some language-defined child packages of System defined elsewhere. Extensions to Ada 83 38.a.1/1 The declarations Max_Binary_Modulus, Max_Nonbinary_Modulus, Max_Base_Digits, Null_Address, Word_Size, Bit_Order, Default_Bit_Order, Any_Priority, Interrupt_Priority, and Default_Priority are added to System in Ada 95. The presence of ordering operators for type Address is also guaranteed (the existence of these depends on the definition of Address in an Ada 83 implementation). We do not list these as incompatibilities, as the contents of System can vary between implementations anyway; thus a program that depends on the contents of System (by using use System; for example) is already at risk of being incompatible when moved between Ada implementations. Wording Changes from Ada 83 38.a Much of the content of System is standardized, to provide more uniformity across implementations. Implementations can still add their own declarations to System, but are encouraged to do so via children of System. 38.b Some of the named numbers are defined more explicitly in terms of the standard numeric types. 38.c The pragmas System_Name, Storage_Unit, and Memory_Size are no longer defined by the language. However, the corresponding declarations in package System still exist. Existing implementations may continue to support the three pragmas as implementation-defined pragmas, if they so desire. 38.d Priority semantics, including subtype Priority, have been moved to the Real Time Annex. Extensions to Ada 95 38.e/2 {AI95-00161-01} Amendment Correction: Type Address is defined to have preelaborable initialization, so that it can be used without restriction in preelaborated units. (If Address is defined to be a private type, as suggested by the Implementation Advice, in Ada 95 it cannot be used in some contexts in a preelaborated units. This is an unnecessary portability issue.) 38.f/2 {AI95-00221-01} Amendment Correction: Default_Bit_Order is now a static constant. 38.g/2 {AI95-00362-01} Package System is now Pure, so it can be portably used in more places. (Ada 95 allowed it to be Pure, but did not require that.) 13.7.1 The Package System.Storage_Elements Static Semantics 1 The following language-defined library package exists: 2/2 {AI95-00362-01} package System.Storage_Elements is pragma Pure(Storage_Elements); 3 type Storage_Offset is range implementation-defined; 4 subtype Storage_Count is Storage_Offset range 0..Storage_Offset'Last; 5 type Storage_Element is mod implementation-defined; for Storage_Element'Size use Storage_Unit; type Storage_Array is array (Storage_Offset range <>) of aliased Storage_Element; for Storage_Array'Component_Size use Storage_Unit; 6 -- Address Arithmetic: 7/3 {AI05-0229-1} function "+"(Left : Address; Right : Storage_Offset) return Address with Convention => Intrinsic; function "+"(Left : Storage_Offset; Right : Address) return Address with Convention => Intrinsic; function "-"(Left : Address; Right : Storage_Offset) return Address with Convention => Intrinsic; function "-"(Left, Right : Address) return Storage_Offset with Convention => Intrinsic; 8/3 {AI05-0229-1} function "mod"(Left : Address; Right : Storage_Offset) return Storage_Offset with Convention => Intrinsic; 9 -- Conversion to/from integers: 10/3 {AI05-0229-1} type Integer_Address is implementation-defined; function To_Address(Value : Integer_Address) return Address with Convention => Intrinsic; function To_Integer(Value : Address) return Integer_Address with Convention => Intrinsic; 11/3 {AI05-0229-1} end System.Storage_Elements; 11.a/3 Reason: {AI05-0229-1} The Convention aspects imply that the attribute Access is not allowed for those operations. 11.b The mod function is needed so that the definition of Alignment makes sense. 11.c/2 Implementation defined: The range of Storage_Elements.Storage_Offset, the modulus of Storage_Elements.Storage_Element, and the declaration of Storage_Elements.Integer_Address.. 12 Storage_Element represents a storage element. Storage_Offset represents an offset in storage elements. Storage_Count represents a number of storage elements. Storage_Array represents a contiguous sequence of storage elements. 12.a Reason: The index subtype of Storage_Array is Storage_Offset because we wish to allow maximum flexibility. Most Storage_Arrays will probably have a lower bound of 0 or 1, but other lower bounds, including negative ones, make sense in some situations. 12.b/2 This paragraph was deleted.{AI95-00114-01} 13 Integer_Address is a [(signed or modular)] integer subtype. To_Address and To_Integer convert back and forth between this type and Address. Implementation Requirements 14 Storage_Offset'Last shall be greater than or equal to Integer'Last or the largest possible storage offset, whichever is smaller. Storage_Offset'First shall be <= (-Storage_Offset'Last). Paragraph 15 was deleted. Implementation Advice 16 Operations in System and its children should reflect the target environment semantics as closely as is reasonable. For example, on most machines, it makes sense for address arithmetic to "wrap around." Operations that do not make sense should raise Program_Error. 16.a.1/2 Implementation Advice: Operations in System and its children should reflect the target environment; operations that do not make sense should raise Program_Error. 16.a Discussion: For example, on a segmented architecture, X < Y might raise Program_Error if X and Y do not point at the same segment (assuming segments are unordered). Similarly, on a segmented architecture, the conversions between Integer_Address and Address might not make sense for some values, and so might raise Program_Error. 16.b Reason: We considered making Storage_Element a private type. However, it is better to declare it as a modular type in the visible part, since code that uses it is already low level, and might as well have access to the underlying representation. We also considered allowing Storage_Element to be any integer type, signed integer or modular, but it is better to have uniformity across implementations in this regard, and viewing storage elements as unsigned seemed to make the most sense. 16.c Implementation Note: To_Address is intended for use in Address clauses. Implementations should overload To_Address if appropriate. For example, on a segmented architecture, it might make sense to have a record type representing a segment/offset pair, and have a To_Address conversion that converts from that record type to type Address. Extensions to Ada 95 16.d/2 {AI95-00362-01} Package System.Storage_Elements is now Pure, so it can be portably used in more places. (Ada 95 allowed it to be Pure, but did not require that.) 13.7.2 The Package System.Address_To_Access_Conversions Static Semantics 1 The following language-defined generic library package exists: 2 generic type Object(<>) is limited private; package System.Address_To_Access_Conversions is pragma Preelaborate(Address_To_Access_Conversions); 3/3 {AI05-0229-1} type Object_Pointer is access all Object; function To_Pointer(Value : Address) return Object_Pointer with Convention => Intrinsic; function To_Address(Value : Object_Pointer) return Address with Convention => Intrinsic; 4/3 {AI05-0229-1} end System.Address_To_Access_Conversions; 5/2 {AI95-00230-01} The To_Pointer and To_Address subprograms convert back and forth between values of types Object_Pointer and Address. To_Pointer(X'Address) is equal to X'Unchecked_Access for any X that allows Unchecked_Access. To_Pointer(Null_Address) returns null. For other addresses, the behavior is unspecified. To_Address(null) returns Null_Address. To_Address(Y), where Y /= null, returns Y.all'Address. 5.a/3 Discussion: {AI95-00114-01} {AI05-0005-1} The programmer should ensure that the address passed to To_Pointer is either Null_Address, or the address of an object of type Object. (If Object is not a by-reference type, the object ought to be aliased; recall that the Address attribute is not required to provide a useful result for other objects.) Otherwise, the behavior of the program is unspecified; it might raise an exception or crash, for example. 5.b Reason: Unspecified is almost the same thing as erroneous; they both allow arbitrarily bad behavior. We don't say erroneous here, because the implementation might allow the address passed to To_Pointer to point at some memory that just happens to "look like" an object of type Object. That's not necessarily an error; it's just not portable. However, if the actual type passed to Object is (for example) an array type, the programmer would need to be aware of any dope that the implementation expects to exist, when passing an address that did not come from the Address attribute of an object of type Object. 5.c One might wonder why To_Pointer and To_Address are any better than unchecked conversions. The answer is that Address does not necessarily have the same representation as an access type. For example, an access value might point at the bounds of an array when an address would point at the first element. Or an access value might be an offset in words from someplace, whereas an address might be an offset in bytes from the beginning of memory. Implementation Permissions 6 An implementation may place restrictions on instantiations of Address_To_Access_Conversions. 6.a Ramification: For example, if the hardware requires aligned loads and stores, then dereferencing an access value that is not properly aligned might raise an exception. 6.b For another example, if the implementation has chosen to use negative component offsets (from an access value), it might not be possible to preserve the semantics, since negative offsets from the Address are not allowed. (The Address attribute always points at "the first of the storage elements....") Note that while the implementation knows how to convert an access value into an address, it might not be able to do the reverse. To avoid generic contract model violations, the restriction might have to be detected at run time in some cases. 13.8 Machine Code Insertions 1 [ A machine code insertion can be achieved by a call to a subprogram whose sequence_of_statements contains code_statements.] Syntax 2 code_statement ::= qualified_expression; 3 A code_statement is only allowed in the handled_sequence_of_statements of a subprogram_body. If a subprogram_body contains any code_statements, then within this subprogram_body the only allowed form of statement is a code_statement (labeled or not), the only allowed declarative_items are use_clauses, and no exception_handler is allowed (comments and pragmas are allowed as usual). Name Resolution Rules 4 The qualified_expression is expected to be of any type. Legality Rules 5 The qualified_expression shall be of a type declared in package System.Machine_Code. 5.a Ramification: This includes types declared in children of System.Machine_Code. 6 A code_statement shall appear only within the scope of a with_clause that mentions package System.Machine_Code. 6.a Ramification: Note that this is not a note; without this rule, it would be possible to write machine code in compilation units which depend on System.Machine_Code only indirectly. Static Semantics 7 The contents of the library package System.Machine_Code (if provided) are implementation defined. The meaning of code_statements is implementation defined. [Typically, each qualified_expression represents a machine instruction or assembly directive.] 7.a Discussion: For example, an instruction might be a record with an Op_Code component and other components for the operands. 7.b Implementation defined: The contents of the visible part of package System.Machine_Code, and the meaning of code_statements. Implementation Permissions 8 An implementation may place restrictions on code_statements. An implementation is not required to provide package System.Machine_Code. NOTES 9 18 An implementation may provide implementation-defined pragmas specifying register conventions and calling conventions. 10/2 19 {AI95-00318-02} Machine code functions are exempt from the rule that a return statement is required. In fact, return statements are forbidden, since only code_statements are allowed. 10.a Discussion: The idea is that the author of a machine code subprogram knows the calling conventions, and refers to parameters and results accordingly. The implementation should document where to put the result of a machine code function, for example, " Scalar results are returned in register 0." 11 20 Intrinsic subprograms (see 6.3.1, "Conformance Rules") can also be used to achieve machine code insertions. Interface to assembly language can be achieved using the features in Annex B, " Interface to Other Languages". Examples 12 Example of a code statement: 13/3 {AI05-0229-1} M : Mask; procedure Set_Mask with Inline; 14 procedure Set_Mask is use System.Machine_Code; -- assume "with System.Machine_Code;" appears somewhere above begin SI_Format'(Code => SSM, B => M'Base_Reg, D => M'Disp); -- Base_Reg and Disp are implementation-defined attributes end Set_Mask; Extensions to Ada 83 14.a Machine code functions are allowed in Ada 95; in Ada 83, only procedures were allowed. Wording Changes from Ada 83 14.b The syntax for code_statement is changed to say " qualified_expression" instead of "subtype_mark'record_aggregate ". Requiring the type of each instruction to be a record type is overspecification. 13.9 Unchecked Type Conversions 1 [ An unchecked type conversion can be achieved by a call to an instance of the generic function Unchecked_Conversion.] Static Semantics 2 The following language-defined generic library function exists: 3/3 {AI05-0229-1} generic type Source(<>) is limited private; type Target(<>) is limited private; function Ada.Unchecked_Conversion(S : Source) return Target with Convention => Intrinsic; pragma Pure(Ada.Unchecked_Conversion); 3.a/3 Reason: {AI05-0229-1} The aspect Convention implies that the attribute Access is not allowed for instances of Unchecked_Conversion. Dynamic Semantics 4 The size of the formal parameter S in an instance of Unchecked_Conversion is that of its subtype. [This is the actual subtype passed to Source, except when the actual is an unconstrained composite subtype, in which case the subtype is constrained by the bounds or discriminants of the value of the actual expression passed to S.] 5 If all of the following are true, the effect of an unchecked conversion is to return the value of an object of the target subtype whose representation is the same as that of the source object S: 6 * S'Size = Target'Size. 6.a Ramification: Note that there is no requirement that the Sizes be known at compile time. 7/3 * {AI05-0078-1} S'Alignment is a multiple of Target'Alignment or Target'Alignment is zero. 8 * The target subtype is not an unconstrained composite subtype. 9 * S and the target subtype both have a contiguous representation. 10 * The representation of S is a representation of an object of the target subtype. 11/2 {AI95-00426-01} Otherwise, if the result type is scalar, the result of the function is implementation defined, and can have an invalid representation (see 13.9.1). If the result type is nonscalar, the effect is implementation defined; in particular, the result can be abnormal (see 13.9.1). 11.a.1/2 Implementation defined: The result of unchecked conversion for instances with scalar result types whose result is not defined by the language. 11.a/2 Implementation defined: The effect of unchecked conversion for instances with nonscalar result types whose effect is not defined by the language. 11.a.1/2 Reason: {AI95-00426-01} Note the difference between these sentences; the first only says that the bits returned are implementation defined, while the latter allows any effect. The difference is because scalar objects should never be abnormal unless their assignment was disrupted or if they are a subcomponent of an abnormal composite object. Neither exception applies to instances of Unchecked_Conversion. 11.a.2/2 Ramification: {AI95-00426-01} Whenever unchecked conversions are used, it is the programmer's responsibility to ensure that these conversions maintain the properties that are guaranteed by the language for objects of the target type. For nonscalar types, this requires the user to understand the underlying run-time model of the implementation. The execution of a program that violates these properties by means of unchecked conversions returning a nonscalar type is erroneous. Properties of scalar types can be checked by using the Valid attribute (see 13.9.2); programs can avoid violating properties of the type (and erroneous execution) by careful use of this attribute. 11.b An instance of Unchecked_Conversion can be applied to an object of a private type, assuming the implementation allows it. Implementation Permissions 12 An implementation may return the result of an unchecked conversion by reference, if the Source type is not a by-copy type. [In this case, the result of the unchecked conversion represents simply a different (read-only) view of the operand of the conversion.] 12.a Ramification: In other words, the result object of a call on an instance of Unchecked_Conversion can occupy the same storage as the formal parameter S. 13 An implementation may place restrictions on Unchecked_Conversion. 13.a Ramification: For example, an instantiation of Unchecked_Conversion for types for which unchecked conversion doesn't make sense may be disallowed. Implementation Advice 14/2 {AI95-00051-02} Since the Size of an array object generally does not include its bounds, the bounds should not be part of the converted data. 14.a.1/2 Implementation Advice: Since the Size of an array object generally does not include its bounds, the bounds should not be part of the converted data in an instance of Unchecked_Conversion. 14.a Ramification: On the other hand, we have no advice to offer about discriminants and tag fields. 15 The implementation should not generate unnecessary run-time checks to ensure that the representation of S is a representation of the target type. It should take advantage of the permission to return by reference when possible. Restrictions on unchecked conversions should be avoided unless required by the target environment. 15.a.1/2 Implementation Advice: There should not be unnecessary run-time checks on the result of an Unchecked_Conversion; the result should be returned by reference when possible. Restrictions on Unchecked_Conversions should be avoided. 15.a Implementation Note: As an example of an unnecessary run-time check, consider a record type with gaps between components. The compiler might assume that such gaps are always zero bits. If a value is produced that does not obey that assumption, then the program might misbehave. The implementation should not generate extra code to check for zero bits (except, perhaps, in a special error-checking mode). 16 The recommended level of support for unchecked conversions is: 17/3 * {AI05-0299-1} Unchecked conversions should be supported and should be reversible in the cases where this subclause defines the result. To enable meaningful use of unchecked conversion, a contiguous representation should be used for elementary subtypes, for statically constrained array subtypes whose component subtype is one of the subtypes described in this paragraph, and for record subtypes without discriminants whose component subtypes are described in this paragraph. 17.a/2 Implementation Advice: The recommended level of support for Unchecked_Conversion should be followed. Wording Changes from Ada 95 17.b/2 {AI95-00051-02} The implementation advice about the size of array objects was moved to 13.3 so that all of the advice about Size is in one place. 17.c/2 {AI95-00426-01} Clarified that the result of Unchecked_Conversion for scalar types can be invalid, but not abnormal. Wording Changes from Ada 2005 17.d/3 {AI05-0078-1} Correction: Relaxed the alignment requirement slightly, giving a defined result in more cases. 13.9.1 Data Validity 1 Certain actions that can potentially lead to erroneous execution are not directly erroneous, but instead can cause objects to become abnormal. Subsequent uses of abnormal objects can be erroneous. 2 A scalar object can have an invalid representation, which means that the object's representation does not represent any value of the object's subtype. The primary cause of invalid representations is uninitialized variables. 3 Abnormal objects and invalid representations are explained in this subclause. Dynamic Semantics 4 When an object is first created, and any explicit or default initializations have been performed, the object and all of its parts are in the normal state. Subsequent operations generally leave them normal. However, an object or part of an object can become abnormal in the following ways: 5 * An assignment to the object is disrupted due to an abort (see 9.8) or due to the failure of a language-defined check (see 11.6). 6/2 * {AI95-00426-01} The object is not scalar, and is passed to an in out or out parameter of an imported procedure, the Read procedure of an instance of Sequential_IO, Direct_IO, or Storage_IO, or the stream attribute T'Read, if after return from the procedure the representation of the parameter does not represent a value of the parameter's subtype. 6.1/2 * {AI95-00426-01} The object is the return object of a function call of a nonscalar type, and the function is an imported function, an instance of Unchecked_Conversion, or the stream attribute T'Input, if after return from the function the representation of the return object does not represent a value of the function's subtype. 6.a/2 Discussion: We explicitly list the routines involved in order to avoid future arguments. All possibilities are listed. 6.b/2 We did not include Stream_IO.Read in the list above. A Stream_Element should include all possible bit patterns, and thus it cannot be invalid. Therefore, the parameter will always represent a value of its subtype. By omitting this routine, we make it possible to write arbitrary I/O operations without any possibility of abnormal objects. 6.2/2 {AI95-00426-01} [For an imported object, it is the programmer's responsibility to ensure that the object remains in a normal state.] 6.c/2 Proof: This follows (and echos) the standard rule of interfacing; the programmer must ensure that Ada semantics are followed (see B.1). 7 Whether or not an object actually becomes abnormal in these cases is not specified. An abnormal object becomes normal again upon successful completion of an assignment to the object as a whole. Erroneous Execution 8 It is erroneous to evaluate a primary that is a name denoting an abnormal object, or to evaluate a prefix that denotes an abnormal object. 8.a/2 This paragraph was deleted.{AI95-00114-01} 8.b Ramification: The in out or out parameter case does not apply to scalars; bad scalars are merely invalid representations, rather than abnormal, in this case. 8.c/2 Reason: {AI95-00114-01} The reason we allow access objects, and objects containing subcomponents of an access type, to become abnormal is because the correctness of an access value cannot necessarily be determined merely by looking at the bits of the object. The reason we allow scalar objects to become abnormal is that we wish to allow the compiler to optimize assuming that the value of a scalar object belongs to the object's subtype, if the compiler can prove that the object is initialized with a value that belongs to the subtype. The reason we allow composite objects to become abnormal is that such object might be represented with implicit levels of indirection; if those are corrupted, then even assigning into a component of the object, or simply asking for its Address, might have an unpredictable effect. The same is true if the discriminants have been destroyed. Bounded (Run-Time) Errors 9 If the representation of a scalar object does not represent a value of the object's subtype (perhaps because the object was not initialized), the object is said to have an invalid representation. It is a bounded error to evaluate the value of such an object. If the error is detected, either Constraint_Error or Program_Error is raised. Otherwise, execution continues using the invalid representation. The rules of the language outside this subclause assume that all objects have valid representations. The semantics of operations on invalid representations are as follows: 9.a Discussion: The AARM is more explicit about what happens when the value of the case expression is an invalid representation. 9.b/2 Ramification: {AI95-00426-01} This includes the result object of functions, including the result of Unchecked_Conversion, T'Input, and imported functions. 10 * If the representation of the object represents a value of the object's type, the value of the type is used. 11 * If the representation of the object does not represent a value of the object's type, the semantics of operations on such representations is implementation-defined, but does not by itself lead to erroneous or unpredictable execution, or to other objects becoming abnormal. 11.a/2 Implementation Note: {AI95-00426-01} This means that the implementation must take care not to use an invalid representation in a way that might cause erroneous execution. For instance, the exception mandated for case_statements must be raised. Array indexing must not cause memory outside of the array to be written (and usually, not read either). These cases and similar cases may require explicit checks by the implementation. Erroneous Execution 12/3 {AI95-00167-01} {AI05-0279-1} A call to an imported function or an instance of Unchecked_Conversion is erroneous if the result is scalar, the result object has an invalid representation, and the result is used other than as the expression of an assignment_statement or an object_declaration, as the object_name of an object_renaming_declaration, or as the prefix of a Valid attribute. If such a result object is used as the source of an assignment, and the assigned value is an invalid representation for the target of the assignment, then any use of the target object prior to a further assignment to the target object, other than as the prefix of a Valid attribute reference, is erroneous. 12.a/2 Ramification: {AI95-00167-01} In a typical implementation, every bit pattern that fits in an object of a signed integer subtype will represent a value of the type, if not of the subtype. However, for an enumeration or floating point type, as well as some modular types, there are typically bit patterns that do not represent any value of the type. In such cases, the implementation ought to define the semantics of operations on the invalid representations in the obvious manner (assuming the bounded error is not detected): a given representation should be equal to itself, a representation that is in between the internal codes of two enumeration literals should behave accordingly when passed to comparison operators and membership tests, etc. We considered requiring such sensible behavior, but it resulted in too much arcane verbiage, and since implementations have little incentive to behave irrationally, such verbiage is not important to have. 12.b/2 {AI95-00167-01} If a stand-alone scalar object is initialized to a an in-range value, then the implementation can take advantage of the fact that the use of any out-of-range value has to be erroneous. Such an out-of-range value can be produced only by things like unchecked conversion, imported functions, and abnormal values caused by disruption of an assignment due to abort or to failure of a language-defined check. This depends on out-of-range values being checked before assignment (that is, checks are not optimized away unless they are proven redundant). 12.c Consider the following example: 12.d/2 {AI95-00167-01} type My_Int is range 0..99; function Safe_Convert is new Unchecked_Conversion(My_Int, Integer); function Unsafe_Convert is new Unchecked_Conversion(My_Int, Positive); X : Positive := Safe_Convert(0); -- Raises Constraint_Error. Y : Positive := Unsafe_Convert(0); -- Bounded Error, may be invalid. B : Boolean := Y'Valid; -- OK, B = False. Z : Positive := Y+1; -- Erroneous to use Y. 12.e/2 {AI95-00167-01} {AI95-00426-01} The call to Unsafe_Convert is a bounded error, which might raise Constraint_Error, Program_Error, or return an invalid value. Moreover, if an exception is not raised, most uses of that invalid value (including the use of Y) cause erroneous execution. The call to Safe_Convert is not erroneous. The result object is an object of subtype Integer containing the value 0. The assignment to X is required to do a constraint check; the fact that the conversion is unchecked does not obviate the need for subsequent checks required by the language rules. 12.e.1/2 {AI95-00167-01} {AI95-00426-01} The reason for delaying erroneous execution until the object is used is so that the invalid representation can be tested for validity using the Valid attribute (see 13.9.2) without causing execution to become erroneous. Note that this delay does not imply an exception will not be raised; an implementation could treat both conversions in the example in the same way and raise Constraint_Error. 12.e.2/3 {AI05-0279-1} The rules are defined in terms of the result object, and thus the name used to reference that object is irrelevant. That is why we don't need any special rules to describe what happens when the function result is renamed. 12.f Implementation Note: If an implementation wants to have a " friendly" mode, it might always assign an uninitialized scalar a default initial value that is outside the object's subtype (if there is one), and check for this value on some or all reads of the object, so as to help detect references to uninitialized scalars. Alternatively, an implementation might want to provide an "unsafe" mode where it presumed even uninitialized scalars were always within their subtype. 12.g Ramification: The above rules imply that it is a bounded error to apply a predefined operator to an object with a scalar subcomponent having an invalid representation, since this implies reading the value of each subcomponent. Either Program_Error or Constraint_Error is raised, or some result is produced, which if composite, might have a corresponding scalar subcomponent still with an invalid representation. 12.h Note that it is not an error to assign, convert, or pass as a parameter a composite object with an uninitialized scalar subcomponent. In the other hand, it is a (bounded) error to apply a predefined operator such as =, <, and xor to a composite operand with an invalid scalar subcomponent. 13/3 {AI05-0054-2} The dereference of an access value is erroneous if it does not designate an object of an appropriate type or a subprogram with an appropriate profile, if it designates a nonexistent object, or if it is an access-to-variable value that designates a constant object and it did not originate from an attribute_reference applied to an aliased variable view of a controlled or immutably limited object. [An access value whose dereference is erroneous can exist, for example, because of Unchecked_Deallocation, Unchecked_Access, or Unchecked_Conversion.] 13.a Ramification: The above mentioned Unchecked_... features are not the only causes of such access values. For example, interfacing to other languages can also cause the problem. 13.b/3 {AI05-0054-2} We permit the use of access-to-variable values that designate constant objects so long as they originate from an aliased variable view of a controlled or immutably limited constant, such as during the initialization of a constant (both via the "current instance" and during a call to Initialize) or during an assignment (during a call to Adjust). NOTES 14 21 Objects can become abnormal due to other kinds of actions that directly update the object's representation; such actions are generally considered directly erroneous, however. Wording Changes from Ada 83 14.a In order to reduce the amount of erroneousness, we separate the concept of an undefined value into objects with invalid representation (scalars only) and abnormal objects. 14.b Reading an object with an invalid representation is a bounded error rather than erroneous; reading an abnormal object is still erroneous. In fact, the only safe thing to do to an abnormal object is to assign to the object as a whole. Wording Changes from Ada 95 14.c/2 {AI95-00167-01} The description of erroneous execution for Unchecked_Conversion and imported objects was tightened up so that using the Valid attribute to test such a value is not erroneous. 14.d/2 {AI95-00426-01} Clarified the definition of objects that can become abnormal; made sure that all of the possibilities are included. Wording Changes from Ada 2005 14.e/3 {AI05-0054-2} Correction: Common programming techniques such as squirreling away an access to a controlled object during initialization and using a self-referencing discriminant (the so-called "Rosen trick") no longer are immediately erroneous if the object is declared constant, so these techniques can be used portably and safely. Practically, these techniques already worked as compilers did not take much advantage of this rule, so the impact of this change will be slight. 14.f/3 {AI05-0279-1} Correction: The description of erroneous execution for Unchecked_Conversion and imported objects was adjusted to clarify that renaming such an object is not, by itself, erroneous. 13.9.2 The Valid Attribute 1 The Valid attribute can be used to check the validity of data produced by unchecked conversion, input, interface to foreign languages, and the like. Static Semantics 2 For a prefix X that denotes a scalar object [(after any implicit dereference)], the following attribute is defined: 3/4 X'Valid {AI05-0153-3} {AI12-0071-1} Yields True if and only if the object denoted by X is normal, has a valid representation, and then, if the preceding conditions hold, the value of X also satisfies the predicates of the nominal subtype of X. The value of this attribute is of the predefined type Boolean. 3.a Ramification: Having checked that X'Valid is True, it is safe to read the value of X without fear of erroneous execution caused by abnormality, or a bounded error caused by an invalid representation. Such a read will produce a value in the subtype of X. NOTES 4 22 Invalid data can be created in the following cases (not counting erroneous or unpredictable execution): 5 * an uninitialized scalar object, 6 * the result of an unchecked conversion, 7 * input, 8 * interface to another language (including machine code), 9 * aborting an assignment, 10 * disrupting an assignment due to the failure of a language-defined check (see 11.6), and 11 * use of an object whose Address has been specified. 12/4 23 {AI12-0071-1} Determining whether X is normal and has a valid representation as part of the evaluation of X'Valid is not considered to include an evaluation of X; hence, it is not an error to check the validity of an object that is invalid or abnormal. Determining whether X satisfies the predicates of its nominal subtype may include an evaluation of X, but only after it has been determined that X has a valid representation. 13/4 {AI12-0071-1} If X is volatile, the evaluation of X'Valid is considered a read of X. 13.a/4 Reason: Since an implementation is not allowed to add, remove, or reorder accesses to volatile objects, we have to define X'Valid as a read so that it is implementable for most subtypes as the value of the object is required. 14/2 24 {AI95-00426-01} The Valid attribute may be used to check the result of calling an instance of Unchecked_Conversion (or any other operation that can return invalid values). However, an exception handler should also be provided because implementations are permitted to raise Constraint_Error or Program_Error if they detect the use of an invalid representation (see 13.9.1). 14.a Ramification: If X is of an enumeration type with a representation clause, then X'Valid checks that the value of X when viewed as an integer is one of the specified internal codes. 14.b Reason: Valid is defined only for scalar objects because the implementation and description burden would be too high for other types. For example, given a typical run-time model, it is impossible to check the validity of an access value. The same applies to composite types implemented with internal pointers. One can check the validity of a composite object by checking the validity of each of its scalar subcomponents. The user should ensure that any composite types that need to be checked for validity are represented in a way that does not involve implementation-defined components, or gaps between components. Furthermore, such types should not contain access subcomponents. 14.c/2 This paragraph was deleted.{AI95-00114-01} Extensions to Ada 83 14.d X'Valid is new in Ada 95. Wording Changes from Ada 95 14.e/2 {AI95-00426-01} Added a note explaining that handlers for Constraint_Error and Program_Error are needed in the general case of testing for validity. (An implementation could document cases where these are not necessary, but there is no language requirement.) Wording Changes from Ada 2005 14.f/3 {AI05-0153-3} The validity check now also includes a check of the predicate aspects (see 3.2.4), if any, of the subtype of the object. Wording Changes from Ada 2012 14.g/4 {AI12-0071-1} Corrigendum: Updated wording of the attributes X'Valid to use the new term "satisfies the predicates" (see 3.2.4). Also updated the notes to make sense when evaluating predicates and testing validity of volatile objects. 13.10 Unchecked Access Value Creation 1 [The attribute Unchecked_Access is used to create access values in an unsafe manner - the programmer is responsible for preventing "dangling references."] Static Semantics 2 The following attribute is defined for a prefix X that denotes an aliased view of an object: 3 X'Unchecked_Access All rules and semantics that apply to X'Access (see 3.10.2) apply also to X'Unchecked_Access, except that, for the purposes of accessibility rules and checks, it is as if X were declared immediately within a library package. 3.a/3 Ramification: {AI05-0005-1} We say "rules and semantics" here so that library-level accessibility applies to the value created by X'Unchecked_Access as well as to the checks needed for the attribute itself. This means that any anonymous access values that inherit the accessibility of this attribute (such as access parameters) also act as if they have library-level accessibility. We don't want the "real" accessibility of the created value re-emerging at a later point - that would create hard-to-understand bugs. NOTES 4 25 This attribute is provided to support the situation where a local object is to be inserted into a global linked data structure, when the programmer knows that it will always be removed from the data structure prior to exiting the object's scope. The Access attribute would be illegal in this case (see 3.10.2, " Operations of Access Types"). 4.a Ramification: The expected type for X'Unchecked_Access is as for X'Access. 4.b If an attribute_reference with Unchecked_Access is used as the actual parameter for an access parameter, an Accessibility_Check can never fail on that access parameter. 5 26 There is no Unchecked_Access attribute for subprograms. 5.a/2 Reason: {AI95-00254-01} Such an attribute would allow unsafe " downward closures", where an access value designating a more nested subprogram is passed to a less nested subprogram. (Anonymous access-to-subprogram parameters provide safe "downward closures".) This requires some means of reconstructing the global environment for the more nested subprogram, so that it can do up-level references to objects. The two methods of implementing up-level references are displays and static links. If unsafe downward closures were supported, each access-to-subprogram value would have to carry the static link or display with it. We don't want to require the space and time overhead of requiring the extra information for all access-to-subprogram types, especially as including it would make interfacing to other languages (like C) harder. 5.b If desired, an instance of Unchecked_Conversion can be used to create an access value of a global access-to-subprogram type that designates a local subprogram. The semantics of using such a value are not specified by the language. In particular, it is not specified what happens if such subprograms make up-level references; even if the frame being referenced still exists, the up-level reference might go awry if the representation of a value of a global access-to-subprogram type doesn't include a static link. 13.11 Storage Management 1 [ Each access-to-object type has an associated storage pool. The storage allocated by an allocator comes from the pool; instances of Unchecked_Deallocation return storage to the pool. Several access types can share the same pool.] 2/2 {AI95-00435-01} [A storage pool is a variable of a type in the class rooted at Root_Storage_Pool, which is an abstract limited controlled type. By default, the implementation chooses a standard storage pool for each access-to-object type. The user may define new pool types, and may override the choice of pool for an access-to-object type by specifying Storage_Pool for the type.] 2.a Ramification: By default, the implementation might choose to have a single global storage pool, which is used (by default) by all access types, which might mean that storage is reclaimed automatically only upon partition completion. Alternatively, it might choose to create a new pool at each accessibility level, which might mean that storage is reclaimed for an access type when leaving the appropriate scope. Other schemes are possible. 2.a.1/3 Glossary entry: Each access-to-object type has an associated storage pool object. The storage for an object created by an allocator comes from the storage pool of the type of the allocator. Some storage pools may be partitioned into subpools in order to support finer-grained storage management. Legality Rules 3 If Storage_Pool is specified for a given access type, Storage_Size shall not be specified for it. 3.a Reason: The Storage_Pool determines the Storage_Size; hence it would not make sense to specify both. Note that this rule is simplified by the fact that the aspects in question cannot be specified for derived types, nor for nonfirst subtypes, so we don't have to worry about whether, say, Storage_Pool on a derived type overrides Storage_Size on the parent type. For the same reason, "specified" means the same thing as "directly specified" here. Static Semantics 4 The following language-defined library package exists: 5 with Ada.Finalization; with System.Storage_Elements; package System.Storage_Pools is pragma Preelaborate(System.Storage_Pools); 6/2 {AI95-00161-01} type Root_Storage_Pool is abstract new Ada.Finalization.Limited_Controlled with private; pragma Preelaborable_Initialization(Root_Storage_Pool); 7 procedure Allocate( Pool : in out Root_Storage_Pool; Storage_Address : out Address; Size_In_Storage_Elements : in Storage_Elements.Storage_Count; Alignment : in Storage_Elements.Storage_Count) is abstract; 8 procedure Deallocate( Pool : in out Root_Storage_Pool; Storage_Address : in Address; Size_In_Storage_Elements : in Storage_Elements.Storage_Count; Alignment : in Storage_Elements.Storage_Count) is abstract; 9 function Storage_Size(Pool : Root_Storage_Pool) return Storage_Elements.Storage_Count is abstract; 10 private ... -- not specified by the language end System.Storage_Pools; 10.a Reason: The Alignment parameter is provided to Deallocate because some allocation strategies require it. If it is not needed, it can be ignored. 11 A storage pool type (or pool type) is a descendant of Root_Storage_Pool. The elements of a storage pool are the objects allocated in the pool by allocators. 11.a Discussion: In most cases, an element corresponds to a single memory block allocated by Allocate. However, in some cases the implementation may choose to associate more than one memory block with a given pool element. 12/2 {8652/0009} {AI95-00137-01} {AI95-00435-01} For every access-to-object subtype S, the following representation attributes are defined: 13 S'Storage_Pool Denotes the storage pool of the type of S. The type of this attribute is Root_Storage_Pool'Class. 14 S'Storage_Size Yields the result of calling Storage_Size(S'Storage_Pool)[, which is intended to be a measure of the number of storage elements reserved for the pool.] The type of this attribute is universal_integer. 14.a Ramification: Storage_Size is also defined for task subtypes and objects - see 13.3. 14.b Storage_Size is not a measure of how much un-allocated space is left in the pool. That is, it includes both allocated and unallocated space. Implementations and users may provide a Storage_Available function for their pools, if so desired. 15 Storage_Size or Storage_Pool may be specified for a nonderived access-to-object type via an attribute_definition_clause; the name in a Storage_Pool clause shall denote a variable. 15.a/3 Aspect Description for Storage_Pool: Pool of memory from which new will allocate for a given access type. 15.b/3 Aspect Description for Storage_Size (access): Sets memory size for allocations for an access type. 16/3 {AI05-0107-1} {AI05-0111-3} {AI05-0116-1} An allocator of a type T that does not support subpools allocates storage from T's storage pool. If the storage pool is a user-defined object, then the storage is allocated by calling Allocate as described below. Allocators for types that support subpools are described in 13.11.4. 16.a Ramification: If the implementation chooses to represent the designated subtype in multiple pieces, one allocator evaluation might result in more than one call upon Allocate. In any case, allocators for the access type obtain all the required storage for an object of the designated type by calling the specified Allocate procedure. 16.b/3 This paragraph was deleted.{AI05-0107-1} 16.b.1/1 {8652/0111} {AI95-00103-01} If D (the designated type of T) includes subcomponents of other access types, they will be allocated from the storage pools for those types, even if those allocators are executed as part of the allocator of T (as part of the initialization of the object). For instance, an access-to-task type TT may allocate the data structures used to implement the task value from other storage pools. (In particular, the task stack does not necessarily need to be allocated from the storage pool for TT.) 17 If Storage_Pool is not specified for a type defined by an access_to_object_definition, then the implementation chooses a standard storage pool for it in an implementation-defined manner. In this case, the exception Storage_Error is raised by an allocator if there is not enough storage. It is implementation defined whether or not the implementation provides user-accessible names for the standard pool type(s). 17.a/2 This paragraph was deleted. 17.a.1/2 Discussion: The manner of choosing a storage pool is covered by a Documentation Requirement below, so it is not summarized here. 17.b Implementation defined: Whether or not the implementation provides user-accessible names for the standard pool type(s). 17.c/2 Ramification: {AI95-00230-01} An access-to-object type defined by a derived_type_definition inherits its pool from its parent type, so all access-to-object types in the same derivation class share the same pool. Hence the "defined by an access_to_object_definition" wording above. 17.d There is no requirement that all storage pools be implemented using a contiguous block of memory (although each allocation returns a pointer to a contiguous block of memory). 18/4 {AI12-0043-1} If Storage_Size is specified for an access type T, an implementation-defined pool P is used for the type. The Storage_Size of P is at least that requested, and the storage for P is reclaimed when the master containing the declaration of the access type is left. If the implementation cannot satisfy the request, Storage_Error is raised at the freezing point of type T. The storage pool P is used only for allocators returning type T or other access types specified to use T'Storage_Pool. Storage_Error is raised by an allocator returning such a type if the storage space of P is exhausted (additional memory is not allocated). 18.1/4 {AI12-0043-1} If neither Storage_Pool nor Storage_Size are specified, then the meaning of Storage_Size is implementation defined. 18.a/2 Implementation defined: The meaning of Storage_Size when neither the Storage_Size nor the Storage_Pool is specified for an access type. 18.b Ramification: The Storage_Size function and attribute will return the actual size, rather than the requested size. Comments about rounding up, zero, and negative on task Storage_Size apply here, as well. See also AI83-00557, AI83-00558, and AI83-00608. 18.c The expression in a Storage_Size clause need not be static. 18.d The reclamation happens after the master is finalized. 18.e Implementation Note: For a pool allocated on the stack, normal stack cut-back can accomplish the reclamation. For a library-level pool, normal partition termination actions can accomplish the reclamation. 19 If Storage_Pool is specified for an access type, then the specified pool is used. 20 The effect of calling Allocate and Deallocate for a standard storage pool directly (rather than implicitly via an allocator or an instance of Unchecked_Deallocation) is unspecified. 20.a Ramification: For example, an allocator might put the pool element on a finalization list. If the user directly Deallocates it, instead of calling an instance of Unchecked_Deallocation, then the implementation would probably try to finalize the object upon master completion, which would be bad news. Therefore, the implementation should define such situations as erroneous. Erroneous Execution 21 If Storage_Pool is specified for an access type, then if Allocate can satisfy the request, it should allocate a contiguous block of memory, and return the address of the first storage element in Storage_Address. The block should contain Size_In_Storage_Elements storage elements, and should be aligned according to Alignment. The allocated storage should not be used for any other purpose while the pool element remains in existence. If the request cannot be satisfied, then Allocate should propagate an exception [(such as Storage_Error)]. If Allocate behaves in any other manner, then the program execution is erroneous. Implementation Requirements 21.1/3 {AI05-0107-1} {AI05-0262-1} The Allocate procedure of a user-defined storage pool object P may be called by the implementation only to allocate storage for a type T whose pool is P, only at the following points: 21.2/3 * During the execution of an allocator of type T; 21.a/3 Ramification: This includes during the evaluation of the initializing expression such as an aggregate; this is important if the initializing expression is built in place. We need to allow allocation to be deferred until the size of the object is known. 21.3/3 * During the execution of a return statement for a function whose result is built-in-place in the result of an allocator of type T; 21.b/3 Reason: We need this bullet as well as the preceding one in order that exceptions that propagate from such a call to Allocate can be handled within the return statement. We don't want to require the generation of special handling code in this unusual case, as it would add overhead to most return statements of composite types. 21.4/3 * During the execution of an assignment operation with a target of an allocated object of type T with a part that has an unconstrained discriminated subtype with defaults. 21.c/3 Reason: We allow Allocate to be called during assignment of objects with mutable parts so that mutable objects can be implemented with reallocation on assignment. (Unfortunately, the term "mutable" is only defined in the AARM, so we have to use the long-winded wording shown here.) 21.d/3 Discussion: Of course, explicit calls to Allocate are also allowed and are not bound by any of the rules found here. 21.5/3 {AI05-0107-1} {AI05-0116-1} {AI05-0193-1} {AI05-0262-1} {AI05-0269-1} For each of the calls of Allocate described above, P (equivalent to T'Storage_Pool) is passed as the Pool parameter. The Size_In_Storage_Elements parameter indicates the number of storage elements to be allocated, and is no more than D'Max_Size_In_Storage_Elements, where D is the designated subtype of T. The Alignment parameter is a nonzero integral multiple of D'Alignment if D is a specific type, and otherwise is a nonzero integral multiple of the alignment of the specific type identified by the tag of the object being created; it is unspecified if there is no such value. The Alignment parameter is no more than D'Max_Alignment_For_Allocation. The result returned in the Storage_Address parameter is used as the address of the allocated storage, which is a contiguous block of memory of Size_In_Storage_Elements storage elements. [Any exception propagated by Allocate is propagated by the construct that contained the call.] 21.e/3 Ramification: Note that the implementation does not turn other exceptions into Storage_Error. 21.f/3 "Nonzero integral multiple" of an alignment includes the alignment value itself, of course. The value is unspecified if the alignment of the specific type is zero. 21.6/3 {AI05-0107-1} The number of calls to Allocate needed to implement an allocator for any particular type is unspecified. The number of calls to Deallocate needed to implement an instance of Unchecked_Deallocation (see 13.11.2) for any particular object is the same as the number of Allocate calls for that object. 21.g/3 Reason: This supports objects that are allocated in one or more parts. The second sentence prevents extra or missing calls to Deallocate. 21.h/3 To be honest: {AI05-0005-1} The number of calls to Deallocate from all sources for an object always will be the same as the number of calls to Allocate from all sources for that object. However, in unusual cases, not all of those Deallocate calls may be made by an instance of Unchecked_Deallocation. Specifically, in the unusual case of assigning to an object of a mutable variant record type such that the variant changes, some of the Deallocate calls may be made by the assignment (as may some of the Allocate calls). 21.i/3 Ramification: We do not define the relative order of multiple calls used to deallocate the same object - that is, if the allocator allocated two pieces x and y, then an instance of Unchecked_Deallocation might deallocate x and then y, or it might deallocate y and then x. 21.7/3 {AI05-0107-1} The Deallocate procedure of a user-defined storage pool object P may be called by the implementation to deallocate storage for a type T whose pool is P only at the places when an Allocate call is allowed for P, during the execution of an instance of Unchecked_Deallocation for T, or as part of the finalization of the collection of T. For such a call of Deallocate, P (equivalent to T'Storage_Pool) is passed as the Pool parameter. The value of the Storage_Address parameter for a call to Deallocate is the value returned in the Storage_Address parameter of the corresponding successful call to Allocate. The values of the Size_In_Storage_Elements and Alignment parameters are the same values passed to the corresponding Allocate call. Any exception propagated by Deallocate is propagated by the construct that contained the call. 21.j/3 Reason: We allow Deallocate to be called anywhere that Allocate is, in order to allow the recovery of storage from failed allocations (that is, those that raise exceptions); from extended return statements that exit via a goto, exit, or locally handled exception; and from objects that are reallocated when they are assigned. In each of these cases, we would have a storage leak if the implementation did not recover the storage (there is no way for the programmer to do it). We do not require such recovery, however, as it could be a serious performance drag on these operations. Documentation Requirements 22 An implementation shall document the set of values that a user-defined Allocate procedure needs to accept for the Alignment parameter. An implementation shall document how the standard storage pool is chosen, and how storage is allocated by standard storage pools. 22.a/2 This paragraph was deleted. 22.b/2 Documentation Requirement: The set of values that a user-defined Allocate procedure needs to accept for the Alignment parameter. How the standard storage pool is chosen, and how storage is allocated by standard storage pools. Implementation Advice 23 An implementation should document any cases in which it dynamically allocates heap storage for a purpose other than the evaluation of an allocator. 23.a.1/2 Implementation Advice: Any cases in which heap storage is dynamically allocated other than as part of the evaluation of an allocator should be documented. 23.a Reason: This is "Implementation Advice" because the term "heap storage" is not formally definable; therefore, it is not testable whether the implementation obeys this advice. 24 A default (implementation-provided) storage pool for an access-to-constant type should not have overhead to support deallocation of individual objects. 24.a.1/2 Implementation Advice: A default storage pool for an access-to-constant type should not have overhead to support deallocation of individual objects. 24.a Ramification: Unchecked_Deallocation is not defined for such types. If the access-to-constant type is library-level, then no deallocation (other than at partition completion) will ever be necessary, so if the size needed by an allocator of the type is known at link-time, then the allocation should be performed statically. If, in addition, the initial value of the designated object is known at compile time, the object can be allocated to read-only memory. 24.b Implementation Note: If the Storage_Size for an access type is specified, the storage pool should consist of a contiguous block of memory, possibly allocated on the stack. The pool should contain approximately this number of storage elements. These storage elements should be reserved at the place of the Storage_Size clause, so that allocators cannot raise Storage_Error due to running out of pool space until the appropriate number of storage elements has been used up. This approximate (possibly rounded-up) value should be used as a maximum; the implementation should not increase the size of the pool on the fly. If the Storage_Size for an access type is specified as zero, then the pool should not take up any storage space, and any allocator for the type should raise Storage_Error. 24.c Ramification: Note that most of this is approximate, and so cannot be (portably) tested. That's why we make it an Implementation Note. There is no particular number of allocations that is guaranteed to succeed, and there is no particular number of allocations that is guaranteed to fail. 25/2 {AI95-00230-01} The storage pool used for an allocator of an anonymous access type should be determined as follows: 25.1/2 * {AI95-00230-01} {AI95-00416-01} If the allocator is defining a coextension (see 3.10.2) of an object being created by an outer allocator, then the storage pool used for the outer allocator should also be used for the coextension; 25.2/2 * {AI95-00230-01} For other access discriminants and access parameters, the storage pool should be created at the point of the allocator, and be reclaimed when the allocated object becomes inaccessible; 25.3/3 * {AI05-0051-1} If the allocator defines the result of a function with an access result, the storage pool is determined as though the allocator were in place of the call of the function. If the call is the operand of a type conversion, the storage pool is that of the target access type of the conversion. If the call is itself defining the result of a function with an access result, this rule is applied recursively; 25.4/2 * {AI95-00230-01} Otherwise, a default storage pool should be created at the point where the anonymous access type is elaborated; such a storage pool need not support deallocation of individual objects. 25.a.1/2 Implementation Advice: Usually, a storage pool for an access discriminant or access parameter should be created at the point of an allocator, and be reclaimed when the designated object becomes inaccessible. For other anonymous access types, the pool should be created at the point where the type is elaborated and need not support deallocation of individual objects. 25.a/2 Implementation Note: {AI95-00230-01} For access parameters and access discriminants, the "storage pool" for an anonymous access type would not normally exist as a separate entity. Instead, the designated object of the allocator would be allocated, in the case of an access parameter, as a local aliased variable at the call site, and in the case of an access discriminant, contiguous with the object containing the discriminant. This is similar to the way storage for aggregates is typically managed. 25.b/2 {AI95-00230-01} For other sorts of anonymous access types, this implementation is not possible in general, as the accessibility of the anonymous access type is that of its declaration, while the allocator could be more nested. In this case, a "real" storage pool is required. Note, however, that this storage pool need not support (separate) deallocation, as it is not possible to instantiate Unchecked_Deallocation with an anonymous access type. (If deallocation is needed, the object should be allocated for a named access type and converted.) Thus, deallocation only need happen when the anonymous access type itself goes out of scope; this is similar to the case of an access-to-constant type. NOTES 26 27 A user-defined storage pool type can be obtained by extending the Root_Storage_Pool type, and overriding the primitive subprograms Allocate, Deallocate, and Storage_Size. A user-defined storage pool can then be obtained by declaring an object of the type extension. The user can override Initialize and Finalize if there is any need for nontrivial initialization and finalization for a user-defined pool type. For example, Finalize might reclaim blocks of storage that are allocated separately from the pool object itself. 27 28 The writer of the user-defined allocation and deallocation procedures, and users of allocators for the associated access type, are responsible for dealing with any interactions with tasking. In particular: 28 * If the allocators are used in different tasks, they require mutual exclusion. 29 * If they are used inside protected objects, they cannot block. 30 * If they are used by interrupt handlers (see C.3, " Interrupt Support"), the mutual exclusion mechanism has to work properly in that context. 31 29 The primitives Allocate, Deallocate, and Storage_Size are declared as abstract (see 3.9.3), and therefore they have to be overridden when a new (nonabstract) storage pool type is declared. 31.a Ramification: Note that the Storage_Pool attribute denotes an object, rather than a value, which is somewhat unusual for attributes. 31.b The calls to Allocate, Deallocate, and Storage_Size are dispatching calls - this follows from the fact that the actual parameter for Pool is T'Storage_Pool, which is of type Root_Storage_Pool'Class. In many cases (including all cases in which Storage_Pool is not specified), the compiler can determine the tag statically. However, it is possible to construct cases where it cannot. 31.c All access types in the same derivation class share the same pool, whether implementation defined or user defined. This is necessary because we allow type conversions among them (even if they are pool-specific), and we want pool-specific access values to always designate an element of the right pool. 31.d Implementation Note: If an access type has a standard storage pool, then the implementation doesn't actually have to follow the pool interface described here, since this would be semantically invisible. For example, the allocator could conceivably be implemented with inline code. Examples 32 To associate an access type with a storage pool object, the user first declares a pool object of some type derived from Root_Storage_Pool. Then, the user defines its Storage_Pool attribute, as follows: 33 Pool_Object : Some_Storage_Pool_Type; 34 type T is access Designated; for T'Storage_Pool use Pool_Object; 35 Another access type may be added to an existing storage pool, via: 36 for T2'Storage_Pool use T'Storage_Pool; 37 The semantics of this is implementation defined for a standard storage pool. 37.a Reason: For example, the implementation is allowed to choose a storage pool for T that takes advantage of the fact that T is of a certain size. If T2 is not of that size, then the above will probably not work. 38/3 {AI05-0111-3} As usual, a derivative of Root_Storage_Pool may define additional operations. For example, consider the Mark_Release_Pool_Type defined in 13.11.6, that has two additional operations, Mark and Release, the following is a possible use: 39/3 {8652/0041} {AI95-00066-01} {AI05-0111-3} type Mark_Release_Pool_Type (Pool_Size : Storage_Elements.Storage_Count) is new Subpools.Root_Storage_Pool_With_Subpools with private; -- As defined in package MR_Pool, see 13.11.6 40 ... 41/3 {AI05-0111-3} Our_Pool : Mark_Release_Pool_Type (Pool_Size => 2000); My_Mark : MR_Pool.Subpool_Handle; -- See 13.11.6 42/3 {AI05-0111-3} type Acc is access ...; for Acc'Storage_Pool use Our_Pool; ... 43/3 {AI05-0111-3} My_Mark := Mark(Our_Pool); ... -- Allocate objects using "new (My_Mark) Designated(...)". Release(My_Mark); -- Finalize objects and reclaim storage. Extensions to Ada 83 43.a User-defined storage pools are new to Ada 95. Wording Changes from Ada 83 43.b/3 {AI05-0005-1} {AI05-0190-1} Ada 83 originally introduced the concept called a "collection," which is similar to what we call a storage pool. All access types in the same derivation class share the same collection. Ada 95 introduces the storage pool, which is similar in that all access types in the same derivation class share the same storage pool, but other (unrelated) access types can also share the same storage pool, either by default, or as specified by the user. A collection is an amorphous grouping of objects (mainly used to describe finalization of access types); a storage pool is a more concrete concept - hence the different name. 43.c RM83 states the erroneousness of reading or updating deallocated objects incorrectly by missing various cases. Incompatibilities With Ada 95 43.d/2 {AI95-00435-01} Amendment Correction: Storage pools (and Storage_Size) are not defined for access-to-subprogram types. The original Ada 95 wording defined the attributes, but said nothing about their values. If a program uses attributes Storage_Pool or Storage_Size on an access-to-subprogram type, it will need to be corrected for Ada 2005. That's a good thing, as such a use is a bug - the concepts never were defined for such types. Extensions to Ada 95 43.e/2 {AI95-00161-01} Amendment Correction: Added pragma Preelaborable_Initialization to type Root_Storage_Pool, so that extensions of it can be used to declare default-initialized objects in preelaborated units. Wording Changes from Ada 95 43.f/2 {8652/0009} {AI95-00137-01} Corrigendum: Added wording to specify that these are representation attributes. 43.g/2 {AI95-00230-01} {AI95-00416-01} Added wording to clarify that an allocator for a coextension nested inside an outer allocator shares the pool with the outer allocator. Wording Changes from Ada 2005 43.h/3 {AI05-0051-1} Correction: Added the missing definition of the storage pool of an allocator for an anonymous access result type. 43.i/3 {AI05-0107-1} Correction: Clarified when an implementation is allowed to call Allocate and Deallocate, and the requirements on such calls. 43.j/3 {AI05-0111-3} Added wording to support subpools and refer to the subpool example, see 13.11.4. 43.k/3 {AI05-0116-1} Correction: Added wording to specify that the alignment for an allocator with a class-wide designated type comes from the specific type that is allocated. 43.l/3 {AI05-0193-1} Added wording to allow larger alignments for calls to Allocate made by allocators, up to Max_Alignment_For_Allocation. This eases implementation in some cases. Wording Changes from Ada 2012 43.m/4 {AI12-0043-1} Corrigendum: Tightened up the description of the implementation-defined pool used when Storage_Size is specified. This is not intended to change any implementation. 13.11.1 Storage Allocation Attributes 1/3 {AI05-0193-1} [The Max_Size_In_Storage_Elements and Max_Alignment_For_Allocation attributes may be useful in writing user-defined pool types.] Static Semantics 2/3 {AI05-0193-1} For every subtype S, the following attributes are defined: 3/3 S'Max_Size_In_Storage_Elements {AI95-00256-01} {AI95-00416-01} {AI05-0193-1} Denotes the maximum value for Size_In_Storage_Elements that could be requested by the implementation via Allocate for an access type whose designated subtype is S. The value of this attribute is of type universal_integer. 3.a Ramification: If S is an unconstrained array subtype, or an unconstrained subtype with discriminants, S'Max_Size_In_Storage_Elements might be very large. 4/3 S'Max_Alignment_For_Allocation {AI05-0193-1} Denotes the maximum value for Alignment that could be requested by the implementation via Allocate for an access type whose designated subtype is S. The value of this attribute is of type universal_integer. 5/3 {AI05-0193-1} For a type with access discriminants, if the implementation allocates space for a coextension in the same pool as that of the object having the access discriminant, then these attributes account for any calls on Allocate that could be performed to provide space for such coextensions. 5.a/3 Reason: {AI05-0193-1} The values of these attributes should reflect only the calls that might be made to the pool specified for an access type with designated type S. Thus, if the coextensions would normally be allocated from a different pool than the one used for the main object (that is, the Implementation Advice of 13.11 for determining the pool of an anonymous access discriminant is not followed), then these attributes should not reflect any calls on Allocate used to allocate the coextensions. 5.b/3 Ramification: {AI05-0193-1} Coextensions of coextensions of this type (and so on) are included in the values of these attributes if they are allocated from the same pool. Wording Changes from Ada 95 5.c/2 {AI95-00256-01} Corrected the wording so that a fortune-telling compiler that can see the future execution of the program is not required. Extensions to Ada 2005 5.d/3 {AI05-0193-1} The Max_Alignment_For_Allocation attribute is new. 13.11.2 Unchecked Storage Deallocation 1 [ Unchecked storage deallocation of an object designated by a value of an access type is achieved by a call to an instance of the generic procedure Unchecked_Deallocation.] Static Semantics 2 The following language-defined generic library procedure exists: 3/3 {AI05-0229-1} generic type Object(<>) is limited private; type Name is access Object; procedure Ada.Unchecked_Deallocation(X : in out Name) with Convention => Intrinsic; pragma Preelaborate(Ada.Unchecked_Deallocation); 3.a/3 Reason: {AI05-0229-1} The aspect Convention implies that the attribute Access is not allowed for instances of Unchecked_Deallocation. Legality Rules 3.1/3 {AI05-0157-1} A call on an instance of Unchecked_Deallocation is illegal if the actual access type of the instance is a type for which the Storage_Size has been specified by a static expression with value zero or is defined by the language to be zero. In addition to the places where Legality Rules normally apply (see 12.3), this rule applies also in the private part of an instance of a generic unit. 3.b/3 Discussion: This rule is the same as the rule for allocators. We could have left the last sentence out, as a call to Unchecked_Deallocation cannot occur in a specification as it is a procedure call, but we left it for consistency and to avoid future maintenance hazards. Dynamic Semantics 4 Given an instance of Unchecked_Deallocation declared as follows: 5 procedure Free is new Ada.Unchecked_Deallocation( object_subtype_name, access_to_variable_subtype_name); 6 Procedure Free has the following effect: 7 1. After executing Free(X), the value of X is null. 8 2. Free(X), when X is already equal to null, has no effect. 9/3 3. {AI95-00416-01} {AI05-0107-1} Free(X), when X is not equal to null first performs finalization of the object designated by X (and any coextensions of the object - see 3.10.2), as described in 7.6.1. It then deallocates the storage occupied by the object designated by X (and any coextensions). If the storage pool is a user-defined object, then the storage is deallocated by calling Deallocate as described in 13.11. There is one exception: if the object being freed contains tasks, the object might not be deallocated. 9.a/3 Ramification: {AI05-0107-1} Free calls only the specified Deallocate procedure to do deallocation. 10/4 {AI95-00416-01} {AI12-0148-1} After the finalization step of Free(X), the object designated by X, and any subcomponents (and coextensions) thereof, no longer exist; their storage can be reused for other purposes. Bounded (Run-Time) Errors 11 It is a bounded error to free a discriminated, unterminated task object. The possible consequences are: 11.a Reason: This is an error because the task might refer to its discriminants, and the discriminants might be deallocated by freeing the task object. 12 * No exception is raised. 13 * Program_Error or Tasking_Error is raised at the point of the deallocation. 14 * Program_Error or Tasking_Error is raised in the task the next time it references any of the discriminants. 14.a Implementation Note: This last case presumes an implementation where the task references its discriminants indirectly, and the pointer is nulled out when the task object is deallocated. 15 In the first two cases, the storage for the discriminants (and for any enclosing object if it is designated by an access discriminant of the task) is not reclaimed prior to task termination. 15.a Ramification: The storage might never be reclaimed. 15.1/4 {AI12-0148-1} An access value that designates a nonexistent object is called a dangling reference. 15.b/4 Discussion: These can result from use of Unchecked_Deallocation, Unchecked_Deallocate_Subpool, and attribute Unchecked_Access. Bad results from Unchecked_Conversion and from stream-oriented attributes are abnormal by 13.9.1, which is stronger and thus takes precedence. 15.2/4 {AI12-0148-1} [If a dangling reference is dereferenced (implicitly or explicitly), execution is erroneous (see below).] If there is no explicit or implicit dereference, then it is a bounded error to evaluate an expression whose result is a dangling reference. If the error is detected, either Constraint_Error or Program_Error is raised. Otherwise, execution proceeds normally, but with the possibility that the access value designates some other existing object. 15.c/4 Reason: If a dangling reference is compared with another access value, a result of either True or False is allowed. We need to allow this so that simple implementations of access values (for instance, as a bare address) can work if the memory in question is reused. (The formal definition of access equality is that it returns True if both access values designate the same object; that can never be True if one of the values is a dangling reference, and the other is not, but both values could refer to the same memory.) Membership tests that do not involve an implicit dereference generally do not depend on the access value at all. 15.d/4 We allow Constraint_Error to be raised here so that dangling reference and null pointer checks can be combined into a single check. If different exceptions are required, then the checks have to be made separately - but there's little semantic difference (neither designate a usable object). 15.e/4 Ramification: If a dangling reference is assigned into an object, including being passed to a formal parameter, that object also contains a dangling reference afterwards. 15.f/4 Discussion: For equality and membership operations on composite types, this applies to any parts that are access types, as these operations are created based on the operations of the components (which triggers the bounded error). For other operations on composite types, the bounded error is not triggered. For instance, an assignment of a composite object with a subcomponent that is a dangling reference has to work normally; no exception can be raised, but the target object will have a subcomponent that is a dangling references, and a (direct) use of that subcomponent is again a bounded error. This is similar to the way that assignments of invalid subcomponents are handled (see 13.9.1). Erroneous Execution 16/3 {AI05-0033-1} {AI05-0262-1} Evaluating a name that denotes a nonexistent object, or a protected subprogram or subprogram renaming whose associated object (if any) is nonexistent, is erroneous. The execution of a call to an instance of Unchecked_Deallocation is erroneous if the object was created other than by an allocator for an access type whose pool is Name'Storage_Pool. 16.a/3 Reason: {AI05-0033-1} {AI05-0262-1} The part about a protected subprogram is intended to cover the case of an access-to-protected-subprogram where the associated object has been deallocated. The part about a subprogram renaming is intended to cover the case of a renaming of a prefixed view where the prefix object has been deallocated, or the case of a renaming of an entry or protected subprogram where the associated task or protected object has been deallocated. 16.b/3 Ramification: {AI05-0157-1} This text does not cover the case of a name that contains a null access value, as null does not denote an object (rather than denoting a nonexistent object). Implementation Advice 17 For a standard storage pool, Free should actually reclaim the storage. 17.a.1/2 Implementation Advice: For a standard storage pool, an instance of Unchecked_Deallocation should actually reclaim the storage. 17.a/2 Ramification: {AI95-00114-01} This is not a testable property, since we do not know how much storage is used by a given pool element, nor whether fragmentation can occur. 17.1/3 {AI05-0157-1} A call on an instance of Unchecked_Deallocation with a nonnull access value should raise Program_Error if the actual access type of the instance is a type for which the Storage_Size has been specified to be zero or is defined by the language to be zero. 17.a.1/3 Implementation Advice: A call on an instance of Unchecked_Deallocation with a nonnull access value should raise Program_Error if the actual access type of the instance is a type for which the Storage_Size has been specified to be zero or is defined by the language to be zero. 17.b Discussion: If the call is not illegal (as in a generic body), we recommend that it raise Program_Error. Since the execution of this call is erroneous (any allocator from the pool will have raised Storage_Error, so the nonnull access value must have been allocated from a different pool or be a stack-allocated object), we can't require any behavior - anything at all would be a legitimate implementation. NOTES 18 30 The rules here that refer to Free apply to any instance of Unchecked_Deallocation. 19 31 Unchecked_Deallocation cannot be instantiated for an access-to-constant type. This is implied by the rules of 12.5.4. Wording Changes from Ada 95 19.a/2 {AI95-00416-01} The rules for coextensions are clarified (mainly by adding that term). In theory, this reflects no change from Ada 95 (coextensions existed in Ada 95, they just didn't have a name). Wording Changes from Ada 2005 19.b/3 {AI05-0033-1} Correction: Added a rule that using an access-to-protected-subprogram is erroneous if the associated object no longer exists. It is hard to imagine an alternative meaning here, and this has no effect on correct programs. 19.c/3 {AI05-0107-1} Correction: Moved the requirements on an implementation-generated call to Deallocate to 13.11, in order to put all of the rules associated with implementation-generated calls to Allocate and Deallocate together. 19.d/3 {AI05-0157-1} Correction: Added wording so that calling an instance of Unchecked_Deallocation is treated similarly to allocators for access types where allocators would be banned. Inconsistencies With Ada 2012 19.e/4 {AI12-0148-1} Corrigendum: Defined a "dangling reference", and specified that a dangling reference might designate some other existing object. This allows simple implementations of access values and reuse of object memory after deallocation. In prior versions of Ada, "=" between a dangling reference and an access to an existing object has to return False, even if the existing object and the object designated by the dangling reference are allocated in the same memory. A program that depended upon that could break with this revised rule. However, as a practical matter, almost all Ada implementations use simple implementations of access types that do not meet that requirement. So such a program would not work (consistently) on most Ada implementations; thus the change shouldn't break any existing programs - it just aligns the Standard with actual practice. 19.f/4 {AI12-0148-1} A side effect of this change is to allow an Ada implementation to detect dangling references in more places. This does not require any Ada implementation to change, and if the implementation does change, it just means that errors will be detected earlier. Wording Changes from Ada 2012 19.g/4 {AI12-0148-1} Corrigendum: Clarified that deallocated objects cease to exist after finalization but before Deallocate is called. This is necessary to prevent erroneous execution from being triggered by the rules in 13.11 in the time between the end of finalization and the end of the call to the instance of Unchecked_Deallocation. 13.11.3 Default Storage Pools 1/4 {AI05-0229-1} {AI12-0003-1} [Pragma and aspect Default_Storage_Pool specify the storage pool that will be used in the absence of an explicit specification of a storage pool or storage size for an access type.] Syntax 2/3 {AI05-0190-1} {AI05-0229-1} The form of a pragma Default_Storage_Pool is as follows: 3/3 {AI05-0190-1} {AI05-0229-1} pragma Default_Storage_Pool (storage_pool_indicator); 3.1/4 {AI05-0190-1} {AI12-0003-1} storage_pool_indicator ::= storage_pool_name | null | Standard 3.2/3 {AI05-0190-1} A pragma Default_Storage_Pool is allowed immediately within the visible part of a package_specification, immediately within a declarative_part, or as a configuration pragma. Name Resolution Rules 3.3/3 {AI05-0190-1} The storage_pool_name is expected to be of type Root_Storage_Pool'Class. Legality Rules 4/3 {AI05-0190-1} {AI05-0229-1} The storage_pool_name shall denote a variable. 4.1/4 {AI12-0003-1} The Standard storage_pool_indicator is an identifier specific to a pragma (see 2.8) and does not denote any declaration. If the storage_pool_indicator is Standard, then there shall not be a declaration with defining_identifier Standard that is immediately visible at the point of the pragma, other than package Standard itself. 4.a.1/4 Reason: We considered having the Standard storage_pool_indicator resolve to package Standard rather than being an identifier specific to a pragma. That would eliminate the need for a special check. But it would be bizarre to have something that could resolve to either an object or a (single) package, and resolving to package Standard would imply that the standard pool is an object declared in that package. A storage pool object must be a variable (see 13.11), yet preelaborable packages depend on package Standard, which would require implementers to implement the standard storage pool with Preelaborable_Initialization, which is an unnecessary restriction. 4.a.2/4 No declaration of Standard can ever be use-visible, as the language-defined nonoverloadable definition of Standard will hide any use-visible declarations. Thus we need only concern ourselves with eliminating any possible confusion with regard to immediately visible declarations with the defining_identifier Standard. 4.2/4 {AI05-0190-1} {AI12-0003-1} If the pragma is used as a configuration pragma, the storage_pool_indicator shall be either null or Standard, and it defines the default pool to be the given storage_pool_indicator within all applicable compilation units (see 10.1.5), except within the immediate scope of another pragma Default_Storage_Pool. Otherwise, [the pragma occurs immediately within a sequence of declarations, and] it defines the default pool within the immediate scope of the pragma to be the given storage_pool_indicator, except within the immediate scope of a later pragma Default_Storage_Pool. [Thus, an inner pragma overrides an outer one.] 4.3/4 {AI05-0190-1} {AI05-0262-1} A pragma Default_Storage_Pool shall not be used as a configuration pragma that applies to a compilation unit that is within the immediate scope of another pragma Default_Storage_Pool. 4.a/3 Reason: This is to prevent confusion in cases like this: 4.b/3 package Parent is pragma Default_Storage_Pool(...); ... end Parent; 4.c/3 pragma Default_Storage_Pool(...); -- Illegal! package Parent.Child is ... end Parent.Child; 4.d/3 where the Default_Storage_Pool on Parent.Child would not (if it were legal) override the one in Parent. Static Semantics 5/4 {AI05-0190-1} {AI05-0229-1} {AI12-0003-1} The language-defined aspect Default_Storage_Pool may be specified for a generic instance; it defines the default pool for access types within an instance. . 5.1/4 {AI12-0003-1} The Default_Storage_Pool aspect may be specified as Standard, which is an identifier specific to an aspect (see 13.1.1) and defines the default pool to be Standard. In this case, there shall not be a declaration with defining_identifier Standard that is immediately visible at the point of the aspect specification, other than package Standard itself. 5.2/4 {AI12-0003-1} Otherwise, the expected type for the Default_Storage_Pool aspect is Root_Storage_Pool'Class and the aspect_definition shall be a name that denotes a variable. This aspect overrides any Default_Storage_Pool pragma that might apply to the generic unit; if the aspect is not specified, the default pool of the instance is that defined for the generic unit. 5.a/3 Aspect Description for Default_Storage_Pool: Default storage pool for a generic instance. 5.3/4 {AI12-0136-1} The effect of specifying the aspect Default_Storage_Pool on an instance of a language-defined generic unit is implementation-defined. 5.a.1/4 Implementation defined: The effect of specifying aspect Default_Storage_Pool on an instance of a language-defined generic unit. 6/3 {AI05-0190-1} {AI05-0229-1} For nonderived access types declared in places where the default pool is defined by the pragma or aspect, their Storage_Pool or Storage_Size attribute is determined as follows, unless Storage_Pool or Storage_Size is specified for the type: 6.1/3 * {AI05-0190-1} If the default pool is null, the Storage_Size attribute is defined by the language to be zero. [Therefore, an allocator for such a type is illegal.] 6.2/4 * {AI05-0190-1} {AI12-0003-1} If the default pool is neither null nor Standard, the Storage_Pool attribute is that pool. 6.3/4 {AI05-0190-1} {AI12-0003-1} Otherwise (including when the default pool is specified as Standard), the standard storage pool is used for the type as described in 13.11. 6.a/3 Ramification: {AI05-0190-1} {AI05-0229-1} Default_Storage_Pool is the only way to specify the storage pool for an anonymous access type. 6.b/3 {AI05-0190-1} {AI05-0229-1} Note that coextensions should be allocated in the same pool (or on the stack) as the outer object (see 13.11); the Storage_Pool of the access discriminant (and hence the Default_Storage_Pool) is supposed to be ignored for coextensions. This matches the required finalization point for coextensions. 6.b.1/3 {AI05-0190-1} The default storage pool for an allocator that occurs within an instance of a generic is defined by the Default_Storage_Pool aspect of the instantiation (if specified), or by the Default_Storage_Pool pragma that applied to the generic; the Default_Storage_Pool pragma that applies to the instantiation is irrelevant. 6.b.2/3 {AI05-0190-1} It is possible to specify the Default_Storage_Pool aspect for an instantiation such that allocations will fail. For example, the generic unit might be expecting a pool that supports certain sizes and alignments, and the one on the instance might be more restrictive. It is the programmer's responsibility to get this right. 6.b.3/3 {AI05-0190-1} The semantics of the Default_Storage_Pool aspect are similar to passing a pool object as a generic formal, and putting pragma Default_Storage_Pool at the top of the generic's visible part, specifying that formal. 7/3 This paragraph was deleted.{AI05-0229-1} Implementation Permissions 8/3 {AI05-0190-1} {AI05-0229-1} An object created by an allocator that is passed as the actual parameter to an access parameter may be allocated on the stack, and automatically reclaimed, regardless of the default pool. 8.a/3 Discussion: {AI05-0190-1} This matches the required finalization point for such an allocated object. NOTES 9/3 32 {AI05-0190-1} Default_Storage_Pool may be used with restrictions No_Coextensions and No_Access_Parameter_Allocators (see H.4) to ensure that all allocators use the default pool. Wording Changes from Ada 83 9.a/3 This paragraph was deleted.{AI05-0229-1} Incompatibilities With Ada 2005 9.b/3 {AI05-0229-1} Pragma Controlled has been dropped from Ada, as it has no effect in any known Ada implementations and it seems to promise capabilities not expected in Ada implementations. This is usually not an incompatibility, as the pragma merely becomes unrecognized (with a warning) and can be implemented as an implementation-defined pragma if desired. However, it is incompatible if it is (now) implemented as an implementation-defined pragma, someone used this pragma in a unit, and they also used restriction No_Implementation_Pragmas on that unit. In that case, the pragma would now violate the restriction; but use of this pragma (which does nothing) should be very rare, so this is not a significant issue. Extensions to Ada 2005 9.c/4 {AI05-0190-1} {AI12-0005-1} The pragma Default_Storage_Pool and aspect Default_Storage_Pool are new. Wording Changes from Ada 2005 9.d/3 {AI05-0229-1} The entire discussion of garbage collection (and especially that of controlled objects) is deleted. Ada 2012 provides subpools (see 13.11.4) for storage management of objects, including controlled objects, a mechanism which is much more predictable than garbage collection. Note that no version of Ada allows early finalization of controlled objects (other than via the use of Unchecked_Deallocation or Unchecked_Deallocate_Subpool), so that garbage collection of such objects would be ineffective in the standard mode anyway. Extensions to Ada 2012 9.e/4 {AI05-0003-1} Corrigendum: The storage_pool_indicator Standard is new. Wording Changes from Ada 2012 9.f/4 {AI12-0136-1} Corrigendum: We now explicitly say that the behavior of language-defined generic units when given the Default_Storage_Pool aspect is implementation-defined. Portable code cannot rely on such a package using a particular pool implementation. 13.11.4 Storage Subpools 1/3 {AI05-0111-3} This subclause defines a package to support the partitioning of a storage pool into subpools. A subpool may be specified as the default to be used for allocation from the associated storage pool, or a particular subpool may be specified as part of an allocator (see 4.8). Static Semantics 2/3 {AI05-0111-3} The following language-defined library package exists: 3/3 package System.Storage_Pools.Subpools is pragma Preelaborate (Subpools); 4/3 type Root_Storage_Pool_With_Subpools is abstract new Root_Storage_Pool with private; 5/3 type Root_Subpool is abstract tagged limited private; 6/3 type Subpool_Handle is access all Root_Subpool'Class; for Subpool_Handle'Storage_Size use 0; 7/3 function Create_Subpool (Pool : in out Root_Storage_Pool_With_Subpools) return not null Subpool_Handle is abstract; 8/3 {AI05-0252-1} -- The following operations are intended for pool implementers: 9/3 function Pool_of_Subpool (Subpool : not null Subpool_Handle) return access Root_Storage_Pool_With_Subpools'Class; 10/3 procedure Set_Pool_of_Subpool ( Subpool : in not null Subpool_Handle; To : in out Root_Storage_Pool_With_Subpools'Class); 11/3 procedure Allocate_From_Subpool ( Pool : in out Root_Storage_Pool_With_Subpools; Storage_Address : out Address; Size_In_Storage_Elements : in Storage_Elements.Storage_Count; Alignment : in Storage_Elements.Storage_Count; Subpool : in not null Subpool_Handle) is abstract with Pre'Class => Pool_of_Subpool(Subpool) = Pool'Access; 12/3 procedure Deallocate_Subpool ( Pool : in out Root_Storage_Pool_With_Subpools; Subpool : in out Subpool_Handle) is abstract with Pre'Class => Pool_of_Subpool(Subpool) = Pool'Access; 13/3 {AI05-0298-1} function Default_Subpool_for_Pool ( Pool : in out Root_Storage_Pool_With_Subpools) return not null Subpool_Handle; 14/3 overriding procedure Allocate ( Pool : in out Root_Storage_Pool_With_Subpools; Storage_Address : out Address; Size_In_Storage_Elements : in Storage_Elements.Storage_Count; Alignment : in Storage_Elements.Storage_Count); 15/3 overriding procedure Deallocate ( Pool : in out Root_Storage_Pool_With_Subpools; Storage_Address : in Address; Size_In_Storage_Elements : in Storage_Elements.Storage_Count; Alignment : in Storage_Elements.Storage_Count) is null; 16/3 {AI05-0298-1} overriding function Storage_Size (Pool : Root_Storage_Pool_With_Subpools) return Storage_Elements.Storage_Count is (Storage_Elements.Storage_Count'Last); 17/3 private ... -- not specified by the language end System.Storage_Pools.Subpools; 18/3 {AI05-0111-3} A subpool is a separately reclaimable portion of a storage pool, identified by an object of type Subpool_Handle (a subpool handle). A subpool handle also identifies the enclosing storage pool, a storage pool that supports subpools, which is a storage pool whose type is descended from Root_Storage_Pool_With_Subpools. A subpool is created by calling Create_Subpool or a similar constructor; the constructor returns the subpool handle. 19/3 {AI05-0111-3} {AI05-0269-1} A subpool object is an object of a type descended from Root_Subpool. [Typically, subpool objects are managed by the containing storage pool; only the handles need be exposed to clients of the storage pool. Subpool objects are designated by subpool handles, and are the run-time representation of a subpool.] 19.a/3 Proof: We know that subpool handles designate subpool objects because the declaration of Subpool_Handle says so. 20/4 {AI05-0111-3} {AI05-0145-1} Each subpool belongs to a single storage pool [(which will always be a pool that supports subpools)]. An access to the pool that a subpool belongs to can be obtained by calling Pool_of_Subpool with the subpool handle. Set_Pool_of_Subpool causes the subpool of the subpool handle to belong to the given pool[; this is intended to be called from subpool constructors like Create_Subpool.] Set_Pool_of_Subpool propagates Program_Error if the subpool already belongs to a pool. If Set_Pool_of_Subpool has not yet been called for a subpool, Pool_of_Subpool returns null. 20.a/3 Discussion: Pool_of_Subpool and Set_Pool_of_Subpool are provided by the Ada implementation and typically will not be overridden by the pool implementer. 21/3 {AI05-0111-3} When an allocator for a type whose storage pool supports subpools is evaluated, a call is made on Allocate_From_Subpool passing in a Subpool_Handle, in addition to the parameters as defined for calls on Allocate (see 13.11). The subpool designated by the subpool_handle_name is used, if specified in an allocator. Otherwise, Default_Subpool_for_Pool of the Pool is used to provide a subpool handle. All requirements on the Allocate procedure also apply to Allocate_from_Subpool. 21.a/3 Discussion: Deallocate_Subpool is expected to do whatever is needed to deallocate all of the objects contained in the subpool; it is called from Unchecked_Deallocate_Subpool (see 13.11.5). 21.b/3 Typically, the pool implementer will not override Allocate. In the canonical definition of the language, it will never be called for a pool that supports subpools (there is an Implementation Permission below that allows it to be called in certain rare cases). Legality Rules 22/3 {AI05-0111-3} If a storage pool that supports subpools is specified as the Storage_Pool for an access type, the access type is called a subpool access type. A subpool access type shall be a pool-specific access type. 23/3 {AI05-0111-3} {AI05-0252-1} The accessibility level of a subpool access type shall not be statically deeper than that of the storage pool object. If the specified storage pool object is a storage pool that supports subpools, then the name that denotes the object shall not denote part of a formal parameter, nor shall it denote part of a dereference of a value of a non-library-level general access type. In addition to the places where Legality Rules normally apply (see 12.3), these rules also apply in the private part of an instance of a generic unit. Dynamic Semantics 24/3 {AI05-0111-3} {AI05-0252-1} When an access type with a specified storage pool is frozen (see 13.14), if the tag of the storage pool object identifies a storage pool that supports subpools, the following checks are made: 25/3 * the name used to specify the storage pool object does not denote part of a formal parameter nor part of a dereference of a value of a non-library-level general access type; and 26/3 * the accessibility level of the access type is not deeper than that of the storage pool object. 27/3 {AI05-0252-1} Program_Error is raised if either of these checks fail. 27.a/3 Reason: This check (and its static counterpart) ensures that the type of the allocated objects exists at least as long as the storage pool object, so that the subpools are finalized (which finalizes any remaining allocated objects) before the type of the objects ceases to exist. The access type itself (and the associated collection) will cease to exist before the storage pool ceases to exist. 27.b/3 We also disallow the use of formal parameters and dereferences of non-library-level general access types when specifying a storage pool object if it supports subpools, because the "apparent" accessibility level is potentially deeper than that of the underlying object. Neither of these cases is very likely to occur in practice. 28/3 {AI05-0111-3} A call to Subpools.Allocate(P, Addr, Size, Align) does the following: 29/3 Allocate_From_Subpool (Root_Storage_Pool_With_Subpools'Class(P), Addr, Size, Align, Subpool => Default_Subpool_for_Pool (Root_Storage_Pool_With_Subpools'Class(P))); 30/3 {AI05-0111-3} An allocator that allocates in a subpool raises Program_Error if the allocated object has task parts. 30.a/3 Reason: This is to ease implementation. We envision relaxing this restriction in a future version of Ada, once implementation experience has been gained. At this time, we are unable to come up with a set of rules for task termination that is both useful, and surely feasible to implement. 31/3 {AI05-0111-3} Unless overridden, Default_Subpool_for_Pool propagates Program_Error. Erroneous Execution 31.1/4 {AI12-0142-1} If Allocate_From_Subpool does not meet one or more of the requirements on the Allocate procedure as given in the Erroneous Execution rules of 13.11, then the program execution is erroneous. Implementation Permissions 32/3 {AI05-0111-3} When an allocator for a type whose storage pool is of type Root_Storage_Pool'Class is evaluated, but supports subpools, the implementation may call Allocate rather than Allocate_From_Subpool. [This will have the same effect, so long as Allocate has not been overridden.] 32.a/3 Reason: This ensures either of two implementation models are possible for an allocator with no subpool_specification. Note that the "supports subpools" property is not known at compile time for a pool of the class-wide type. 32.b/3 * The implementation can dispatch to Storage_Pools.Allocate. If the pool supports subpools, this will call Allocate_From_Subpool with the default subpool so long as Allocate has not been overridden. 32.c/3 * The implementation can declare Allocate_From_Subpool as a primitive of Root_Storage_Pool in the private part of Storage_Pools. This means that the Allocate_From_Subpool for Root_Storage_Pool_With_Subpools overrides that private one. The implementation can thus call the private one, which will call Allocate for non-subpool-supporting pools. The effect of this implementation does not change if Allocate is overridden for a pool that supports subpools. NOTES 33/3 33 {AI05-0111-3} A user-defined storage pool type that supports subpools can be implemented by extending the Root_Storage_Pool_With_Subpools type, and overriding the primitive subprograms Create_Subpool, Allocate_From_Subpool, and Deallocate_Subpool. Create_Subpool should call Set_Pool_Of_Subpool before returning the subpool handle. To make use of such a pool, a user would declare an object of the type extension, use it to define the Storage_Pool attribute of one or more access types, and then call Create_Subpool to obtain subpool handles associated with the pool. 34/3 34 {AI05-0111-3} A user-defined storage pool type that supports subpools may define additional subpool constructors similar to Create_Subpool (these typically will have additional parameters). 35/3 35 {AI05-0111-3} The pool implementor should override Default_Subpool_For_Pool if the pool is to support a default subpool for the pool. The implementor can override Deallocate if individual object reclamation is to be supported, and can override Storage_Size if there is some limit on the total size of the storage pool. The implementor can override Initialize and Finalize if there is any need for nontrivial initialization and finalization for the pool as a whole. For example, Finalize might reclaim blocks of storage that are allocated over and above the space occupied by the pool object itself. The pool implementor may extend the Root_Subpool type as necessary to carry additional information with each subpool provided by Create_Subpool. Extensions to Ada 2005 35.a/3 {AI05-0111-3} {AI05-0252-1} Subpools and the package System.Storage_Pools.Subpools are new. Wording Changes from Ada 2012 35.b/4 {AI12-0142-1} Corrigendum: Clarified that an incorrect implementation of Allocate_From_Subpool causes execution to become erroneous. The wording already said that the requirements of Allocate apply to Allocate_From_Subpool, so we're just confirming the consequences of violating those requirements also apply. 35.c/4 {AI12-0145-1} Corrigendum: Clarified that Pool_of_Subpool returns null if Set_Pool_of_Subpool has not been called. As that can be inferred from the definition, and all known existing implementations return null in this case, we document this as a wording change rather than a possible inconsistency. 13.11.5 Subpool Reclamation 1/3 {AI05-0111-3} A subpool may be explicitly deallocated using Unchecked_Deallocate_Subpool. Static Semantics 2/3 {AI05-0111-3} The following language-defined library procedure exists: 3/3 with System.Storage_Pools.Subpools; procedure Ada.Unchecked_Deallocate_Subpool (Subpool : in out System.Storage_Pools.Subpools.Subpool_Handle); 4/3 {AI05-0111-3} If Subpool is null, a call on Unchecked_Deallocate_Subpool has no effect. Otherwise, the subpool is finalized, and Subpool is set to null. 5/3 {AI05-0111-3} Finalization of a subpool has the following effects: 6/3 * The subpool no longer belongs to any pool; 7/3 * Any of the objects allocated from the subpool that still exist are finalized in an arbitrary order; 7.1/4 * {AI12-0148-1} All of the objects allocated from the subpool cease to exist; 8/3 * The following [dispatching] call is then made: 9/3 Deallocate_Subpool(Pool_of_Subpool(Subpool).all, Subpool); 10/3 {AI05-0111-3} Finalization of a Root_Storage_Pool_With_Subpools object finalizes all subpools that belong to that pool that have not yet been finalized. 10.a/3 Discussion: There is no need to call Unchecked_Deallocation on an object allocated in a subpool. Such objects are deallocated all at once, when Unchecked_Deallocate_Subpool is called. 10.b/3 If Unchecked_Deallocation is called, the object is finalized, and then Deallocate is called on the Pool, which typically will do nothing. If it wants to free memory, it will need some way to get from the address of the object to the subpool. 10.c/3 There is no Deallocate_From_Subpool. There is no efficient way for the implementation to determine the subpool for an arbitrary object, and if the pool implementer can determinate that, they can use that as part of the implementation of Deallocate. 10.d/3 If Unchecked_Deallocation is not called (the usual case), the object will be finalized when Unchecked_Deallocate_Subpool is called. 10.e/3 If that's never called, then the object will be finalized when the Pool_With_Subpools is finalized (by permission - it might happen when the collection of the access type is finalized). Extensions to Ada 2005 10.f/3 {AI05-0111-3} Unchecked_Deallocate_Subpool is new. Wording Changes from Ada 2012 10.g/4 {AI12-0148-1} Corrigendum: Added missing wording to state that the objects cease to exist after the completion of finalization. This is formally an inconsistency (it would be possible to depend on the fact that objects finalized by Unchecked_Deallocate_Subpool still exist), but that violates every sane expectation for a procedure called "Deallocate" something. 13.11.6 Storage Subpool Example Examples 1/3 {AI05-0111-3} The following example is a simple but complete implementation of the classic Mark/Release pool using subpools: 2/3 with System.Storage_Pools.Subpools; with System.Storage_Elements; with Ada.Unchecked_Deallocate_Subpool; package MR_Pool is 3/3 use System.Storage_Pools; -- For uses of Subpools. use System.Storage_Elements; -- For uses of Storage_Count and Storage_Array. 4/3 -- Mark and Release work in a stack fashion, and allocations are not allowed -- from a subpool other than the one at the top of the stack. This is also -- the default pool. 5/3 subtype Subpool_Handle is Subpools.Subpool_Handle; 6/3 type Mark_Release_Pool_Type (Pool_Size : Storage_Count) is new Subpools.Root_Storage_Pool_With_Subpools with private; 7/3 function Mark (Pool : in out Mark_Release_Pool_Type) return not null Subpool_Handle; 8/3 procedure Release (Subpool : in out Subpool_Handle) renames Ada.Unchecked_Deallocate_Subpool; 9/3 private 10/3 type MR_Subpool is new Subpools.Root_Subpool with record Start : Storage_Count; end record; subtype Subpool_Indexes is Positive range 1 .. 10; type Subpool_Array is array (Subpool_Indexes) of aliased MR_Subpool; 11/4 {AI05-0298-1} {AI12-0134-1} type Mark_Release_Pool_Type (Pool_Size : Storage_Count) is new Subpools.Root_Storage_Pool_With_Subpools with record Storage : Storage_Array (0 .. Pool_Size); Next_Allocation : Storage_Count := 0; Markers : Subpool_Array; Current_Pool : Subpool_Indexes := 1; end record; 12/3 {AI05-0298-1} overriding function Create_Subpool (Pool : in out Mark_Release_Pool_Type) return not null Subpool_Handle; 13/3 function Mark (Pool : in out Mark_Release_Pool_Type) return not null Subpool_Handle renames Create_Subpool; 14/3 overriding procedure Allocate_From_Subpool ( Pool : in out Mark_Release_Pool_Type; Storage_Address : out System.Address; Size_In_Storage_Elements : in Storage_Count; Alignment : in Storage_Count; Subpool : not null Subpool_Handle); 15/3 overriding procedure Deallocate_Subpool ( Pool : in out Mark_Release_Pool_Type; Subpool : in out Subpool_Handle); 16/3 {AI05-0298-1} overriding function Default_Subpool_for_Pool (Pool : in out Mark_Release_Pool_Type) return not null Subpool_Handle; 17/3 overriding procedure Initialize (Pool : in out Mark_Release_Pool_Type); 18/3 -- We don't need Finalize. 19/3 end MR_Pool; 20/3 package body MR_Pool is 21/3 {AI05-0298-1} use type Subpool_Handle; 22/3 {AI05-0298-1} procedure Initialize (Pool : in out Mark_Release_Pool_Type) is -- Initialize the first default subpool. begin Pool.Markers(1).Start := 1; Subpools.Set_Pool_of_Subpool (Pool.Markers(1)'Unchecked_Access, Pool); end Initialize; 23/3 function Create_Subpool (Pool : in out Mark_Release_Pool_Type) return not null Subpool_Handle is -- Mark the current allocation location. begin if Pool.Current_Pool = Subpool_Indexes'Last then raise Storage_Error; -- No more subpools. end if; Pool.Current_Pool := Pool.Current_Pool + 1; -- Move to the next subpool 24/3 {AI05-0298-1} return Result : constant not null Subpool_Handle := Pool.Markers(Pool.Current_Pool)'Unchecked_Access do Pool.Markers(Pool.Current_Pool).Start := Pool.Next_Allocation; Subpools.Set_Pool_of_Subpool (Result, Pool); end return; end Create_Subpool; 25/3 {AI05-0298-1} procedure Deallocate_Subpool ( Pool : in out Mark_Release_Pool_Type; Subpool : in out Subpool_Handle) is begin if Subpool /= Pool.Markers(Pool.Current_Pool)'Unchecked_Access then raise Program_Error; -- Only the last marked subpool can be released. end if; if Pool.Current_Pool /= 1 then Pool.Next_Allocation := Pool.Markers(Pool.Current_Pool).Start; Pool.Current_Pool := Pool.Current_Pool - 1; -- Move to the previous subpool else -- Reinitialize the default subpool: Pool.Next_Allocation := 1; Subpools.Set_Pool_of_Subpool (Pool.Markers(1)'Unchecked_Access, Pool); end if; end Deallocate_Subpool; 26/3 {AI05-0298-1} function Default_Subpool_for_Pool (Pool : in out Mark_Release_Pool_Type) return not null Subpool_Handle is begin return Pool.Markers(Pool.Current_Pool)'Unchecked_Access; end Default_Subpool_for_Pool; 27/3 procedure Allocate_From_Subpool ( Pool : in out Mark_Release_Pool_Type; Storage_Address : out System.Address; Size_In_Storage_Elements : in Storage_Count; Alignment : in Storage_Count; Subpool : not null Subpool_Handle) is begin if Subpool /= Pool.Markers(Pool.Current_Pool)'Unchecked_Access then raise Program_Error; -- Only the last marked subpool can be used for allocations. end if; 28/4 {AI12-0080-1} -- Check for the maximum supported alignment, which is the alignment of the storage area: if Alignment > Pool.Storage'Alignment then raise Program_Error; end if; -- Correct the alignment if necessary: Pool.Next_Allocation := Pool.Next_Allocation + ((-Pool.Next_Allocation) mod Alignment); if Pool.Next_Allocation + Size_In_Storage_Elements > Pool.Pool_Size then raise Storage_Error; -- Out of space. end if; Storage_Address := Pool.Storage (Pool.Next_Allocation)'Address; Pool.Next_Allocation := Pool.Next_Allocation + Size_In_Storage_Elements; end Allocate_From_Subpool; 29/3 end MR_Pool; Wording Changes from Ada 2005 29.a/3 {AI05-0111-3} This example of subpools is new. 13.12 Pragma Restrictions and Pragma Profile 1/3 {AI05-0246-1} [A pragma Restrictions expresses the user's intent to abide by certain restrictions. A pragma Profile expresses the user's intent to abide by a set of Restrictions or other specified run-time policies. These may facilitate the construction of simpler run-time environments.] Syntax 2 The form of a pragma Restrictions is as follows: 3 pragma Restrictions(restriction{, restriction}); 4/2 {AI95-00381-01} restriction ::= restriction_identifier | restriction_parameter_identifier => restriction_parameter_argument 4.1/2 {AI95-00381-01} restriction_parameter_argument ::= name | expression Name Resolution Rules 5 Unless otherwise specified for a particular restriction, the expression is expected to be of any integer type. Legality Rules 6 Unless otherwise specified for a particular restriction, the expression shall be static, and its value shall be nonnegative. 7.a/3 This paragraph was deleted. Paragraph 7 was deleted. Post-Compilation Rules 8/3 {AI05-0013-1} A pragma Restrictions is a configuration pragma. If a pragma Restrictions applies to any compilation unit included in the partition, this may impose either (or both) of two kinds of requirements, as specified for the particular restriction: 8.1/3 * {AI05-0013-1} A restriction may impose requirements on some or all of the units comprising the partition. Unless otherwise specified for a particular restriction, such a requirement applies to all of the units comprising the partition and is enforced via a post-compilation check. 8.2/3 * {AI05-0013-1} A restriction may impose requirements on the run-time behavior of the program, as indicated by the specification of run-time behavior associated with a violation of the requirement. 8.a.1/3 Ramification: In this latter case, there is no post-compilation check needed for the requirement. 8.3/1 {8652/0042} {AI95-00130-01} For the purpose of checking whether a partition contains constructs that violate any restriction (unless specified otherwise for a particular restriction): 8.4/1 * {8652/0042} {AI95-00130-01} Generic instances are logically expanded at the point of instantiation; 8.5/1 * {8652/0042} {AI95-00130-01} If an object of a type is declared or allocated and not explicitly initialized, then all expressions appearing in the definition for the type and any of its ancestors are presumed to be used; 8.6/1 * {8652/0042} {AI95-00130-01} A default_expression for a formal parameter or a generic formal object is considered to be used if and only if the corresponding actual parameter is not provided in a given call or instantiation. Implementation Permissions 8.7/3 {AI05-0269-1} An implementation may provide implementation-defined restrictions; the identifier for an implementation-defined restriction shall differ from those of the language-defined restrictions. 8.a.2/3 Implementation defined: Implementation-defined restrictions allowed in a pragma Restrictions. 9 An implementation may place limitations on the values of the expression that are supported, and limitations on the supported combinations of restrictions. The consequences of violating such limitations are implementation defined. 9.a Implementation defined: The consequences of violating limitations on Restrictions pragmas. 9.b Ramification: Such limitations may be enforced at compile time or at run time. Alternatively, the implementation is allowed to declare violations of the restrictions to be erroneous, and not enforce them at all. 9.1/1 {8652/0042} {AI95-00130-01} An implementation is permitted to omit restriction checks for code that is recognized at compile time to be unreachable and for which no code is generated. 9.2/1 {8652/0043} {AI95-00190-01} Whenever enforcement of a restriction is not required prior to execution, an implementation may nevertheless enforce the restriction prior to execution of a partition to which the restriction applies, provided that every execution of the partition would violate the restriction. Syntax 10/3 {AI95-00249-01} {AI05-0246-1} The form of a pragma Profile is as follows: 11/3 pragma Profile (profile_identifier {, profile_pragma_argument_association}); Legality Rules 12/3 {AI95-00249-01} {AI05-0246-1} The profile_identifier shall be the name of a usage profile. The semantics of any profile_pragma_argument_associations are defined by the usage profile specified by the profile_identifier. Static Semantics 13/3 {AI95-00249-01} {AI05-0246-1} A profile is equivalent to the set of configuration pragmas that is defined for each usage profile. Post-Compilation Rules 14/3 {AI95-00249-01} A pragma Profile is a configuration pragma. There may be more than one pragma Profile for a partition. Implementation Permissions 15/3 {AI05-0269-1} An implementation may provide implementation-defined usage profiles; the identifier for an implementation-defined usage profile shall differ from those of the language-defined usage profiles. 15.a.1/3 Implementation defined: Implementation-defined usage profiles allowed in a pragma Profile. NOTES 16/2 36 {AI95-00347-01} Restrictions intended to facilitate the construction of efficient tasking run-time systems are defined in D.7. Restrictions intended for use when constructing high integrity systems are defined in H.4. 17 37 An implementation has to enforce the restrictions in cases where enforcement is required, even if it chooses not to take advantage of the restrictions in terms of efficiency. 17.a Discussion: It is not the intent that an implementation will support a different run-time system for every possible combination of restrictions. An implementation might support only two run-time systems, and document a set of restrictions that is sufficient to allow use of the more efficient and safe one. Extensions to Ada 83 17.b Pragma Restrictions is new to Ada 95. Extensions to Ada 95 17.c/3 {AI95-00249-01} {AI05-0246-1} Pragma Profile is new; it was moved here by Ada 2012 and renamed to a "usage profile" but was otherwise unchanged. Wording Changes from Ada 95 17.d/2 {8652/0042} {AI95-00130-01} Corrigendum: Corrected the wording so that restrictions are checked inside of generic instantiations and in default expressions. Since not making these checks would violate the purpose of restrictions, we are not documenting this as an incompatibility. 17.e/2 {8652/0043} {AI95-00190-01} Corrigendum: Added a permission that restrictions can be enforced at compile-time. While this is technically incompatible, documenting it as such would be unnecessarily alarming - there should not be any programs depending on the runtime failure of restrictions. 17.f/2 {AI95-00381-01} The syntax of a restriction_parameter_argument has been defined to better support restriction No_Dependence (see 13.12.1). Wording Changes from Ada 2005 17.g/3 {AI05-0013-1} Correction: When restrictions are checked has been clarified. 13.12.1 Language-Defined Restrictions and Profiles Static Semantics 1/2 {AI95-00257-01} The following restriction_identifiers are language defined (additional restrictions are defined in the Specialized Needs Annexes): 1.1/3 {AI05-0241-1} No_Implementation_Aspect_Specifications There are no implementation-defined aspects specified by an aspect_specification. This restriction applies only to the current compilation or environment, not the entire partition. 1.a/3 Discussion: {AI05-0241-1} This restriction (as well as others below) applies only to the current compilation, because it is likely that the runtime (and possibly user-written low-level code) will need to use implementation-defined aspects. But a partition-wide restriction applies everywhere, including the runtime. 2/2 {AI95-00257-01} No_Implementation_Attributes There are no implementation-defined attributes. This restriction applies only to the current compilation or environment, not the entire partition. 2.1/3 {AI05-0246-1} {AI05-0269-1} No_Implementation_Identifiers There are no usage names that denote declarations with implementation-defined identifiers that occur within language-defined packages or instances of language-defined generic packages. Such identifiers can arise as follows: 2.2/3 * The following language-defined packages and generic packages allow implementation-defined identifiers: 2.3/3 * package System (see 13.7); 2.4/3 * package Standard (see A.1); 2.5/3 * package Ada.Command_Line (see A.15); 2.6/3 * package Interfaces.C (see B.3); 2.7/3 * package Interfaces.C.Strings (see B.3.1); 2.8/3 * package Interfaces.C.Pointers (see B.3.2); 2.9/3 * package Interfaces.COBOL (see B.4); 2.10/3 * package Interfaces.Fortran (see B.5); 2.11/3 * The following language-defined packages contain only implementation-defined identifiers: 2.12/3 * package System.Machine_Code (see 13.8); 2.13/3 * package Ada.Directories.Information (see A.16); 2.14/3 * nested Implementation packages of the Queue containers (see A.18.28-31); 2.15/3 * package Interfaces (see B.2); 2.16/3 * package Ada.Interrupts.Names (see C.3.2). 2.17/3 For package Standard, Standard.Long_Integer and Standard.Long_Float are considered language-defined identifiers, but identifiers such as Standard.Short_Short_Integer are considered implementation-defined. 2.18/3 This restriction applies only to the current compilation or environment, not the entire partition. 3/2 {AI95-00257-01} No_Implementation_Pragmas There are no implementation-defined pragmas or pragma arguments. This restriction applies only to the current compilation or environment, not the entire partition. 3.1/3 {AI05-0242-1} No_Implementation_Units There is no mention in the context_clause of any implementation-defined descendants of packages Ada, Interfaces, or System. This restriction applies only to the current compilation or environment, not the entire partition. 4/3 {AI95-00368-01} {AI05-0229-1} No_Obsolescent_Features There is no use of language features defined in Annex J. It is implementation defined whether uses of the renamings of J.1 and of the pragmas of J.15 are detected by this restriction. This restriction applies only to the current compilation or environment, not the entire partition. 4.a/2 Reason: A user could compile a rename like 4.b/2 with Ada.Text_IO; package Text_IO renames Ada.Text_IO; 4.c/2 Such a rename must not be disallowed by this restriction, nor should the compilation of such a rename be restricted by an implementation. Many implementations implement the renames of J.1 by compiling them normally; we do not want to require implementations to use a special mechanism to implement these renames. 4.d/3 {AI05-0229-1} The pragmas have the same functionality as the corresponding aspect (unlike the typical obsolescent feature), and rejecting them could be a significant portability problem for existing code. 5/3 {AI95-00381-01} {AI05-0241-1} The following restriction_parameter_identifiers are language defined: 6/2 {AI95-00381-01} No_Dependence Specifies a library unit on which there are no semantic dependences. 6.1/3 {AI05-0241-1} No_Specification_of_Aspect Identifies an aspect for which no aspect_specification, attribute_definition_clause, or pragma is given. 6.2/3 {AI05-0272-1} No_Use_Of_Attribute Identifies an attribute for which no attribute_reference or attribute_definition_clause is given. 6.3/3 {AI05-0272-1} No_Use_Of_Pragma Identifies a pragma which is not to be used. Legality Rules 7/2 {AI95-00381-01} The restriction_parameter_argument of a No_Dependence restriction shall be a name; the name shall have the form of a full expanded name of a library unit, but need not denote a unit present in the environment. 7.a/2 Ramification: This name is not resolved. 7.1/3 {AI05-0241-1} The restriction_parameter_argument of a No_Specification_of_Aspect restriction shall be an identifier; this is an identifier specific to a pragma (see 2.8) and does not denote any declaration. 7.b/3 Ramification: This restriction_parameter_argument is not resolved as it is an identifier specific to a pragma. As for No_Dependence, there is no check that the aspect identifier is meaningful; it might refer to an implementation-defined aspect on one implementation, but nothing at all on another implementation. 7.2/3 {AI05-0272-1} The restriction_parameter_argument of a No_Use_Of_Attribute restriction shall be an identifier or one of the reserved words Access, Delta, Digits, Mod, or Range; this is an identifier specific to a pragma. 7.c/3 Ramification: This restriction_parameter_argument is not resolved as it is an identifier specific to a pragma. There is no check that the attribute identifier refers to a known attribute_designator; it might refer to an implementation-defined attribute on one implementation, but nothing at all on another implementation. 7.3/3 {AI05-0272-1} The restriction_parameter_argument of a No_Use_Of_Pragma restriction shall be an identifier or the reserved word Interface; this is an identifier specific to a pragma. 7.d/3 Ramification: This restriction_parameter_argument is not resolved as it is an identifier specific to a pragma. There is no check that the pragma identifier refers to a known pragma; it might refer to an implementation-defined pragma on one implementation, but nothing at all on another implementation. Post-Compilation Rules 8/3 {AI95-00381-01} {AI05-0241-1} No compilation unit included in the partition shall depend semantically on the library unit identified by the name of a No_Dependence restriction. 8.a/2 Ramification: There is no requirement that the library unit actually exist. One possible use of the pragma is to prevent the use of implementation-defined units; when the program is ported to a different compiler, it is perfectly reasonable that no unit with the name exist. Static Semantics 9/3 {AI05-0246-1} The following profile_identifier is language defined: 10/3 {AI05-0246-1} No_Implementation_Extensions 11/3 {AI05-0246-1} For usage profile No_Implementation_Extensions, there shall be no profile_pragma_argument_associations. 12/3 {AI05-0246-1} The No_Implementation_Extensions usage profile is equivalent to the following restrictions: 13/3 No_Implementation_Aspect_Specifications, No_Implementation_Attributes, No_Implementation_Identifiers, No_Implementation_Pragmas, No_Implementation_Units. Extensions to Ada 95 13.a/2 {AI95-00257-01} {AI95-00368-01} Restrictions No_Implementation_Attributes, No_Implementation_Pragmas, and No_Obsolescent_Features are new. 13.b/2 {AI95-00381-01} Restriction No_Dependence is new. Extensions to Ada 2005 13.c/3 {AI05-0241-1} {AI05-0242-1} {AI05-0246-1} {AI05-0272-1} Restrictions No_Implementation_Aspect_Specifications, No_Implementation_Identifiers, No_Implementation_Units, No_Specification_of_Aspect, No_Use_of_Attribute, and No_Use_of_Pragma are new. 13.d/3 {AI05-0246-1} Profile No_Implementation_Extensions is new. 13.13 Streams 1 A stream is a sequence of elements comprising values from possibly different types and allowing sequential access to these values. A stream type is a type in the class whose root type is Streams.Root_Stream_Type. A stream type may be implemented in various ways, such as an external sequential file, an internal buffer, or a network channel. 1.a Discussion: A stream element will often be the same size as a storage element, but that is not required. 1.a.1/3 Glossary entry: A stream is a sequence of elements that can be used, along with the stream-oriented attributes, to support marshalling and unmarshalling of values of most types. Extensions to Ada 83 1.b Streams are new in Ada 95. 13.13.1 The Package Streams Static Semantics 1 The abstract type Root_Stream_Type is the root type of the class of stream types. The types in this class represent different kinds of streams. A new stream type is defined by extending the root type (or some other stream type), overriding the Read and Write operations, and optionally defining additional primitive subprograms, according to the requirements of the particular kind of stream. The predefined stream-oriented attributes like T'Read and T'Write make dispatching calls on the Read and Write procedures of the Root_Stream_Type. (User-defined T'Read and T'Write attributes can also make such calls, or can call the Read and Write attributes of other types.) 2 package Ada.Streams is pragma Pure(Streams); 3/2 {AI95-00161-01} type Root_Stream_Type is abstract tagged limited private; pragma Preelaborable_Initialization(Root_Stream_Type); 4/1 {8652/0044} {AI95-00181-01} type Stream_Element is mod implementation-defined; type Stream_Element_Offset is range implementation-defined; subtype Stream_Element_Count is Stream_Element_Offset range 0..Stream_Element_Offset'Last; type Stream_Element_Array is array(Stream_Element_Offset range <>) of aliased Stream_Element; 5 procedure Read( Stream : in out Root_Stream_Type; Item : out Stream_Element_Array; Last : out Stream_Element_Offset) is abstract; 6 procedure Write( Stream : in out Root_Stream_Type; Item : in Stream_Element_Array) is abstract; 7 private ... -- not specified by the language end Ada.Streams; 8/2 {AI95-00227-01} The Read operation transfers stream elements from the specified stream to fill the array Item. Elements are transferred until Item'Length elements have been transferred, or until the end of the stream is reached. If any elements are transferred, the index of the last stream element transferred is returned in Last. Otherwise, Item'First - 1 is returned in Last. Last is less than Item'Last only if the end of the stream is reached. 9 The Write operation appends Item to the specified stream. 9.a/2 Discussion: {AI95-00114-01} The index subtype of Stream_Element_Array is Stream_Element_Offset because we wish to allow maximum flexibility. Most Stream_Element_Arrays will probably have a lower bound of 0 or 1, but other lower bounds, including negative ones, make sense in some situations. 9.b/3 {AI95-00114-01} {AI05-0005-1} Note that there are some language-defined subprograms that fill part of a Stream_Element_Array, and return the index of the last element filled as a Stream_Element_Offset. The Read procedures declared here, Streams.Stream_IO (see A.12.1), and System.RPC (see E.5) behave in this manner. These will raise Constraint_Error if the resulting Last value is not in Stream_Element_Offset. This implies that the Stream_Element_Array passed to these subprograms should not have a lower bound of Stream_Element_Offset'First, because then a read of 0 elements would always raise Constraint_Error. A better choice of lower bound is 0 or 1. Implementation Permissions 9.1/1 {8652/0044} {AI95-00181-01} If Stream_Element'Size is not a multiple of System.Storage_Unit, then the components of Stream_Element_Array need not be aliased. 9.b.1/2 Ramification: {AI95-00114-01} If the Stream_Element'Size is less than the size of System.Storage_Unit, then components of Stream_- Element_Array need not be aliased. This is necessary as the components of type Stream_Element size might not be addressable on the target architecture. NOTES 10 38 See A.12.1, "The Package Streams.Stream_IO" for an example of extending type Root_Stream_Type. 11/2 39 {AI95-00227-01} If the end of stream has been reached, and Item'First is Stream_Element_Offset'First, Read will raise Constraint_Error. 11.a/2 Ramification: Thus, Stream_Element_Arrays should start at 0 or 1, not Stream_Element_Offset'First. Extensions to Ada 95 11.b/2 {AI95-00161-01} Amendment Correction: Added pragma Preelaborable_Initialization to type Root_Stream_Type. Wording Changes from Ada 95 11.c/2 {8652/0044} {AI95-00181-01} Corrigendum: Stream elements are aliased presuming that makes sense. 11.d/2 {AI95-00227-01} Fixed the wording for Read to properly define the result in Last when no stream elements are transfered. 13.13.2 Stream-Oriented Attributes 1/3 {8652/0009} {AI95-00137-01} {AI05-0183-1} The type-related operational attributes Write, Read, Output, and Input convert values to a stream of elements and reconstruct values from a stream. Static Semantics 1.1/2 {AI95-00270-01} For every subtype S of an elementary type T, the following representation attribute is defined: 1.2/3 S'Stream_Size {AI95-00270-01} {AI05-0194-1} Denotes the number of bits read from or written to a stream by the default implementations of S'Read and S'Write. Hence, the number of stream elements required per item of elementary type T is: 1.3/2 T'Stream_Size / Ada.Streams.Stream_Element'Size 1.4/2 The value of this attribute is of type universal_integer and is a multiple of Stream_Element'Size. 1.5/2 Stream_Size may be specified for first subtypes via an attribute_definition_clause; the expression of such a clause shall be static, nonnegative, and a multiple of Stream_Element'Size. 1.a/3 Aspect Description for Stream_Size: Size in bits used to represent elementary objects in a stream. 1.b/2 Discussion: Stream_Size is a type-related attribute (see 13.1). 1.c/3 Ramification: {AI05-0194-1} The value of S'Stream_Size is unaffected by the presence or absence of any attribute_definition_clauses or aspect_specifications specifying the Read or Write attributes of any ancestor of S. S'Stream_Size is defined in terms of the behavior of the default implementations of S'Read and S'Write even if those default implementations are overridden. Implementation Advice 1.6/2 {AI95-00270-01} If not specified, the value of Stream_Size for an elementary type should be the number of bits that corresponds to the minimum number of stream elements required by the first subtype of the type, rounded up to the nearest factor or multiple of the word size that is also a multiple of the stream element size. 1.d/2 Implementation Advice: If not specified, the value of Stream_Size for an elementary type should be the number of bits that corresponds to the minimum number of stream elements required by the first subtype of the type, rounded up to the nearest factor or multiple of the word size that is also a multiple of the stream element size. 1.e/2 Reason: {AI95-00270-01} This is Implementation Advice because we want to allow implementations to remain compatible with their Ada 95 implementations, which may have a different handling of the number of stream elements. Users can always specify Stream_Size if they need a specific number of stream elements. 1.7/2 {AI95-00270-01} The recommended level of support for the Stream_Size attribute is: 1.8/2 * {AI95-00270-01} A Stream_Size clause should be supported for a discrete or fixed point type T if the specified Stream_Size is a multiple of Stream_Element'Size and is no less than the size of the first subtype of T, and no greater than the size of the largest type of the same elementary class (signed integer, modular integer, enumeration, ordinary fixed point, or decimal fixed point). 1.f/2 Implementation Advice: The recommended level of support for the Stream_Size attribute should be followed. 1.g/2 Ramification: There are no requirements beyond supporting confirming Stream_Size clauses for floating point and access types. Floating point and access types usually only have a handful of defined formats, streaming anything else makes no sense for them. 1.h/2 For discrete and fixed point types, this may require support for sizes other than the "natural" ones. For instance, on a typical machine with 32-bit integers and a Stream_Element'Size of 8, setting Stream_Size to 24 must be supported. This is required as such formats can be useful for interoperability with unusual machines, and there is no difficulty with the implementation (drop extra bits on output, sign extend on input). Static Semantics 2 For every subtype S of a specific type T, the following attributes are defined. 3 S'Write S'Write denotes a procedure with the following specification: 4/2 {AI95-00441-01} procedure S'Write( Stream : not null access Ada.Streams.Root_Stream_Type'Class; Item : in T) 5 S'Write writes the value of Item to Stream. 6 S'Read S'Read denotes a procedure with the following specification: 7/2 {AI95-00441-01} procedure S'Read( Stream : not null access Ada.Streams.Root_Stream_Type'Class; Item : out T) 8 S'Read reads the value of Item from Stream. 8.1/3 {8652/0040} {AI95-00108-01} {AI95-00444-01} {AI05-0192-1} For an untagged derived type, the Write (resp. Read) attribute is inherited according to the rules given in 13.1 if the attribute is [specified and] available for the parent type at the point where T is declared. For a tagged derived type, these attributes are not inherited, but rather the default implementations are used. 8.a.1/3 Proof: {AI05-0192-1} The inheritance rules of 13.1 say that only specified or inherited aspects are inherited; we mention it again here as a clarification. 8.2/2 {AI95-00444-01} The default implementations of the Write and Read attributes, where available, execute as follows: 9/3 {8652/0040} {AI95-00108-01} {AI95-00195-01} {AI95-00251-01} {AI95-00270-01} {AI05-0139-2} For elementary types, Read reads (and Write writes) the number of stream elements implied by the Stream_Size for the type T; the representation of those stream elements is implementation defined. For composite types, the Write or Read attribute for each component is called in canonical order, which is last dimension varying fastest for an array (unless the convention of the array is Fortran, in which case it is first dimension varying fastest), and positional aggregate order for a record. Bounds are not included in the stream if T is an array type. If T is a discriminated type, discriminants are included only if they have defaults. If T is a tagged type, the tag is not included. For type extensions, the Write or Read attribute for the parent type is called, followed by the Write or Read attribute of each component of the extension part, in canonical order. For a limited type extension, if the attribute of the parent type or any progenitor type of T is available anywhere within the immediate scope of T, and the attribute of the parent type or the type of any of the extension components is not available at the freezing point of T, then the attribute of T shall be directly specified. 9.a/2 Implementation defined: The contents of the stream elements read and written by the Read and Write attributes of elementary types. 9.1/3 {AI05-0023-1} {AI05-0264-1} If T is a discriminated type and its discriminants have defaults, then S'Read first reads the discriminants from the stream without modifying Item. S'Read then creates an object of type T constrained by these discriminants. The value of this object is then converted to the subtype of Item and is assigned to Item. Finally, the Read attribute for each nondiscriminant component of Item is called in canonical order as described above. Normal default initialization and finalization take place for the created object. 9.b Reason: A discriminant with a default value is treated simply as a component of the object. On the other hand, an array bound or a discriminant without a default value, is treated as " descriptor" or "dope" that must be provided in order to create the object and thus is logically separate from the regular components. Such "descriptor" data are written by 'Output and produced as part of the delivered result by the 'Input function, but they are not written by 'Write nor read by 'Read. A tag is like a discriminant without a default. 9.b.1/1 {8652/0040} {AI95-00108-01} For limited type extensions, we must have a definition of 'Read and 'Write if the parent type has one, as it is possible to make a dispatching call through the attributes. The rule is designed to automatically do the right thing in as many cases as possible. 9.b.2/1 {AI95-00251-01} Similarly, a type that has a progenitor with an available attribute must also have that attribute, for the same reason. 9.b.3/3 {AI05-0023-1} The semantics of S'Read for a discriminated type with defaults involves an anonymous object so that the point of required initialization and finalization is well-defined, especially for objects that change shape and have controlled components. The creation of this anonymous object often can be omitted (see the Implementation Permissions below). 9.c/2 Ramification: {AI95-00195-01} For a composite object, the subprogram denoted by the Write or Read attribute of each component is called, whether it is the default or is user-specified. Implementations are allowed to optimize these calls (see below), presuming the properties of the attributes are preserved. 9.2/3 {AI95-00270-01} {AI05-0264-1} Constraint_Error is raised by the predefined Write attribute if the value of the elementary item is outside the range of values representable using Stream_Size bits. For a signed integer type, an enumeration type, or a fixed point type, the range is unsigned only if the integer code for the lower bound of the first subtype is nonnegative, and a (symmetric) signed range that covers all values of the first subtype would require more than Stream_Size bits; otherwise, the range is signed. 10 For every subtype S'Class of a class-wide type T'Class: 11 S'Class'Write S'Class'Write denotes a procedure with the following specification: 12/2 {AI95-00441-01} procedure S'Class'Write( Stream : not null access Ada.Streams.Root_Stream_Type'Class; Item : in T'Class) 13 Dispatches to the subprogram denoted by the Write attribute of the specific type identified by the tag of Item. 14 S'Class'Read S'Class'Read denotes a procedure with the following specification: 15/2 {AI95-00441-01} procedure S'Class'Read( Stream : not null access Ada.Streams.Root_Stream_Type'Class; Item : out T'Class) 16 Dispatches to the subprogram denoted by the Read attribute of the specific type identified by the tag of Item. 16.a Reason: It is necessary to have class-wide versions of Read and Write in order to avoid generic contract model violations; in a generic, we don't necessarily know at compile time whether a given type is specific or class-wide. Paragraph 17 was deleted. Static Semantics 18 For every subtype S of a specific type T, the following attributes are defined. 19 S'Output S'Output denotes a procedure with the following specification: 20/2 {AI95-00441-01} procedure S'Output( Stream : not null access Ada.Streams.Root_Stream_Type'Class; Item : in T) 21 S'Output writes the value of Item to Stream, including any bounds or discriminants. 21.a Ramification: Note that the bounds are included even for an array type whose first subtype is constrained. 22 S'Input S'Input denotes a function with the following specification: 23/2 {AI95-00441-01} function S'Input( Stream : not null access Ada.Streams.Root_Stream_Type'Class) return T 24 S'Input reads and returns one value from Stream, using any bounds or discriminants written by a corresponding S'Output to determine how much to read. 25/3 {8652/0040} {AI95-00108-01} {AI95-00444-01} {AI05-0192-1} For an untagged derived type, the Output (resp. Input) attribute is inherited according to the rules given in 13.1 if the attribute is [specified and] available for the parent type at the point where T is declared. For a tagged derived type, these attributes are not inherited, but rather the default implementations are used. 25.a/3 Proof: {AI05-0192-1} See the note following the inheritance rules for the Write attribute, above. 25.1/2 {AI95-00444-01} The default implementations of the Output and Input attributes, where available, execute as follows: 26/3 * {AI05-0269-1} If T is an array type, S'Output first writes the bounds, and S'Input first reads the bounds. If T has discriminants without defaults, S'Output first writes the discriminants (using the Write attribute of the discriminant type for each), and S'Input first reads the discriminants (using the Read attribute of the discriminant type for each). 27/3 * {AI95-00195-01} {AI05-0023-1} S'Output then calls S'Write to write the value of Item to the stream. S'Input then creates an object of type T, with the bounds or (when without defaults) the discriminants, if any, taken from the stream, passes it to S'Read, and returns the value of the object. If T has discriminants, then this object is unconstrained if and only the discriminants have defaults. Normal default initialization and finalization take place for this object (see 3.3.1, 7.6, and 7.6.1). 27.1/2 {AI95-00251-01} If T is an abstract type, then S'Input is an abstract function. 27.a/2 Ramification: For an abstract type T, S'Input can be called in a dispatching call, or passed to an abstract formal subprogram. But it cannot be used in nondispatching contexts, because we don't allow objects of abstract types to exist. The designation of this function as abstract has no impact on descendants of T, as T'Input is not inherited for tagged types, but rather recreated (and the default implementation of T'Input calls T'Read, not the parent type's T'Input). Note that T'Input cannot be specified in this case, as any function with the proper profile is necessarily abstract, and specifying abstract subprograms in an attribute_definition_clause is illegal. 28 For every subtype S'Class of a class-wide type T'Class: 29 S'Class'Output S'Class'Output denotes a procedure with the following specification: 30/2 {AI95-00441-01} procedure S'Class'Output( Stream : not null access Ada.Streams.Root_Stream_Type'Class; Item : in T'Class) 31/2 {AI95-00344-01} First writes the external tag of Item to Stream (by calling String'Output(Stream, Tags.External_Tag(Item'Tag)) - see 3.9) and then dispatches to the subprogram denoted by the Output attribute of the specific type identified by the tag. Tag_Error is raised if the tag of Item identifies a type declared at an accessibility level deeper than that of S. 31.a/2 Reason: {AI95-00344-01} We raise Tag_Error here for nested types as such a type cannot be successfully read with S'Class'Input, and it doesn't make sense to allow writing a value that cannot be read. 32 S'Class'Input S'Class'Input denotes a function with the following specification: 33/2 {AI95-00441-01} function S'Class'Input( Stream : not null access Ada.Streams.Root_Stream_Type'Class) return T'Class 34/3 {AI95-00279-01} {AI95-00344-01} {AI05-0109-1} First reads the external tag from Stream and determines the corresponding internal tag (by calling Tags.Descendant_Tag(String'Input(Stream), S'Tag) which might raise Tag_Error - see 3.9) and then dispatches to the subprogram denoted by the Input attribute of the specific type identified by the internal tag; returns that result. If the specific type identified by the internal tag is abstract, Constraint_Error is raised. 34.a/3 Ramification: {AI05-0109-1} Descendant_Tag will ensure that the tag it returns is covered by T'Class; Tag_Error will be raised if it would not cover T'Class. 35/3 {AI95-00195-01} {AI05-0228-1} In the default implementation of Read and Input for a composite type, for each scalar component that is a discriminant or that has an implicit initial value, a check is made that the value returned by Read for the component belongs to its subtype. Constraint_Error is raised if this check fails. For other scalar components, no check is made. For each component that is of an access type, if the implementation can detect that the value returned by Read for the component is not a value of its subtype, Constraint_Error is raised. If the value is not a value of its subtype and this error is not detected, the component has an abnormal value, and erroneous execution can result (see 13.9.1). In the default implementation of Read for a composite type with defaulted discriminants, if the actual parameter of Read is constrained, a check is made that the discriminants read from the stream are equal to those of the actual parameter. Constraint_Error is raised if this check fails. 35.a/3 Reason: {AI05-0228-1} The check for scalar components that have an implicit initial value is to preserve our Language Design Principle that all objects that have an implicit initial value do not become "deinitialized". 35.b/3 Ramification: {AI05-0228-1} A scalar component can have an implicit initial value if it has a default_expression, if the component's type has the Default_Value aspect specified, or if the component is that of an array type that has the Default_Component_Value aspect specified. 35.c/3 To be honest: {AI05-0228-1} An implementation should always be able to detect the error for a null value read into a component of an access subtype with a null exclusion; the "if the implementation can detect" is intended to cover nonnull access values. 36/2 {AI95-00195-01} It is unspecified at which point and in which order these checks are performed. In particular, if Constraint_Error is raised due to the failure of one of these checks, it is unspecified how many stream elements have been read from the stream. 37/1 {8652/0045} {AI95-00132-01} In the default implementation of Read and Input for a type, End_Error is raised if the end of the stream is reached before the reading of a value of the type is completed. 38/4 {8652/0040} {AI95-00108-01} {AI95-00195-01} {AI95-00251-01} {AI05-0039-1} {AI12-0106-1} {AI12-0121-1} The stream-oriented attributes may be specified for any type via an attribute_definition_clause. [Alternatively, each of the specific stream-oriented attributes may be specified using an aspect_specification on any type_declaration, with the aspect name being the corresponding attribute name.] Each of the class-wide stream-oriented attributes may be specified using an aspect_specification for a tagged type T using the name of the stream-oriented attribute followed by 'Class; such class-wide aspects do not apply to other descendants of T. 38.a/4 Reason: {AI95-00195-01} {AI12-0106-1} We need the last sentence to override the blanket rule given in 13.1.1 that aspect'Class applies to the type and all descendants. 38.a.1/2 This paragraph was deleted.{8652/0040} {AI95-00108-01} {AI95-00195-01} 38.a.2/4 Proof: {AI12-0121-1} 13.1.1 says that all operational attributes can be specified with an aspect_specification. 38.a.3/4 Aspect Description for Read'Class: Procedure to read a value from a stream for the class-wide type associated with a given type. 38.a.4/4 Aspect Description for Write'Class: Procedure to write a value to a stream for a the class-wide type associated with a given type. 38.a.5/4 Aspect Description for Input'Class: Function to read a value from a stream for a the class-wide type associated with a given type, including any bounds and discriminants. 38.a.6/4 Aspect Description for Output'Class: Procedure to write a value to a stream for a the class-wide type associated with a given type, including any bounds and discriminants. 38.1/4 {AI12-0121-1} The subprogram name given in such an attribute_definition_- clause or aspect_specification shall statically denote a subprogram that is not an abstract subprogram. Furthermore, if a specific stream-oriented attribute is specified for an interface type, the subprogram name given in the attribute_definition_clause or aspect_specification shall statically denote a null procedure. 38.b/2 Discussion: {AI95-00251-01} Stream attributes (other than Input) are always null procedures for interface types (they have no components). We need to allow explicit setting of the Read and Write attributes in order that the class-wide attributes like LI'Class'Input can be made available. (In that case, any descendant of the interface type would require available attributes.) But we don't allow any concrete implementation because these don't participate in extensions (unless the interface is the parent type). If we didn't ban concrete implementations, the order of declaration of a pair of interfaces would become significant. For example, if Int1 and Int2 are interfaces with concrete implementations of 'Read, then the following declarations would have different implementations for 'Read: 38.c/2 type Con1 is new Int1 and Int2 with null record; type Con2 is new Int2 and Int1 with null record; 38.d/2 This would violate our design principle that the order of the specification of the interfaces in a derived_type_definition doesn't matter. 38.e/2 Ramification: The Input attribute cannot be specified for an interface. As it is a function, a null procedure is impossible; a concrete function is not possible anyway as any function returning an abstract type must be abstract. And we don't allow specifying stream attributes to be abstract subprograms. This has no impact, as the availability of Int'Class'Input (where Int is a limited interface) depends on whether Int'Read (not Int'Input) is specified. There is no reason to allow Int'Output to be specified, either, but there is equally no reason to disallow it, so we don't have a special rule for that. 38.f/2 Discussion: {AI95-00195-01} Limited types generally do not have default implementations of the stream-oriented attributes. The rules defining when a stream-oriented attribute is available (see below) determine when an attribute of a limited type is in fact well defined and usable. The rules are designed to maximize the number of cases in which the attributes are usable. For instance, when the language provides a default implementation of an attribute for a limited type based on a specified attribute for the parent type, we want to be able to call that attribute. 38.g/3 Aspect Description for Read: Procedure to read a value from a stream for a given type. 38.h/3 Aspect Description for Write: Procedure to write a value to a stream for a given type. 38.i/3 Aspect Description for Input: Function to read a value from a stream for a given type, including any bounds and discriminants. 38.j/3 Aspect Description for Output: Procedure to write a value to a stream for a given type, including any bounds and discriminants. 39/2 {AI95-00195-01} A stream-oriented attribute for a subtype of a specific type T is available at places where one of the following conditions is true: 40/2 * T is nonlimited. 41/2 * The attribute_designator is Read (resp. Write) and T is a limited record extension, and the attribute Read (resp. Write) is available for the parent type of T and for the types of all of the extension components. 41.a/2 Reason: In this case, the language provides a well-defined default implementation, which we want to be able to call. 42/2 * T is a limited untagged derived type, and the attribute was inherited for the type. 42.a/2 Reason: Attributes are only inherited for untagged derived types, and surely we want to be able to call inherited attributes. 43/2 * The attribute_designator is Input (resp. Output), and T is a limited type, and the attribute Read (resp. Write) is available for T. 43.a/2 Reason: The default implementation of Input and Output are based on Read and Write; so if the implementation of Read or Write is good, so is the matching implementation of Input or Output. 44/2 * The attribute has been specified via an attribute_definition_clause, and the attribute_definition_clause is visible. 44.a/2 Reason: We always want to allow calling a specified attribute. But we don't want availability to break privacy. Therefore, only attributes whose specification can be seen count. Yes, we defined the visibility of an attribute_definition_clause (see 8.3). 45/2 {AI95-00195-01} A stream-oriented attribute for a subtype of a class-wide type T'Class is available at places where one of the following conditions is true: 46/2 * T is nonlimited; 47/2 * the attribute has been specified via an attribute_definition_clause, and the attribute_definition_clause is visible; or 48/2 * the corresponding attribute of T is available, provided that if T has a partial view, the corresponding attribute is available at the end of the visible part where T is declared. 48.a/2 Reason: The rules are stricter for class-wide attributes because (for the default implementation) we must ensure that any specific attribute that might ever be dispatched to is available. Because we require specification of attributes for extensions of limited parent types with available attributes, we can in fact know this. Otherwise, we would not be able to use default class-wide attributes with limited types, a significant limitation. 49/4 {AI95-00195-01} {AI12-0030-1} An attribute_reference for one of the stream-oriented attributes is illegal unless the attribute is available at the place of the attribute_reference. Furthermore, an attribute_reference for T'Input is illegal if T is an abstract type. In addition to the places where Legality Rules normally apply (see 12.3), these rules also apply in the private part of an instance of a generic unit. 49.a/2 Discussion: Stream attributes always exist. It is illegal to call them in some cases. Having the attributes not be defined for some limited types would seem to be a cleaner solution, but it would lead to contract model problems for limited private types. 49.b/2 T'Input is available for abstract types so that T'Class'Input is available. But we certainly don't want to allow calls that could create an object of an abstract type. Remember that T'Class is never abstract, so the above legality rule doesn't apply to it. We don't have to discuss whether the attribute is specified, as it cannot be: any function returning the type would have to be abstract, and we do not allow specifying an attribute with an abstract subprogram. 49.c/4 To be honest: {AI12-0030-1} "These rules apply" refers to just this paragraph and not to the rest of the rules in this section. This rule probably should have been a Legality Rule, but the word "illegal" should key the reader that this is a Legality Rule, no matter under what text heading it occurs. 49.1/4 {AI12-0030-1} Unless inherited from a parent type, if any, for an untagged type having a task, protected, or explicitly limited record part, the default implementation of each of the Read, Write, Input, and Output attributes raises Program_Error and performs no other action. 49.d/4 Discussion: {AI12-0030-1} It might seem that there is no need to specify the behavior of the default implementation of a streaming attribute of, for example, a task type because there is no way that it can be invoked. It is possible, however, to construct an example where such a stream attribute can be invoked. This involves using a formal untagged limited derived type for which some streaming attribute is available (because it was explicitly specified for the ancestor type) and a corresponding actual type for which the attribute is unspecified (because the derivation occurred before the aspect was specified for the ancestor type and the specification was therefore not inherited). 50/3 {AI95-00195-01} {AI05-0192-1} In the parameter_and_result_profiles for the default implementations of the stream-oriented attributes, the subtype of the Item parameter is the base subtype of T if T is a scalar type, and the first subtype otherwise. The same rule applies to the result of the Input attribute. 50.a/3 Discussion: {AI05-0192-1} An inherited stream attribute has a profile as determined by the rules for inheriting primitive subprograms (see 13.1 and 3.4). 51/3 {AI95-00195-01} {AI05-0007-1} For an attribute_definition_clause specifying one of these attributes, the subtype of the Item parameter shall be the first subtype or the base subtype if scalar, and the first subtype if not scalar. The same rule applies to the result of the Input function. 51.a/2 Reason: This is to simplify implementation. 51.b/3 Ramification: The view of the type at the point of the attribute_definition_clause determines whether the base subtype is allowed. Thus, for a scalar type with a partial view (which is never scalar), whether the base subtype is allowed is determined by whether the attribute_definition_clause occurs before or after the full definition of the scalar type. 52/3 {AI95-00366-01} {AI05-0065-1} [A type is said to support external streaming if Read and Write attributes are provided for sending values of such a type between active partitions, with Write marshalling the representation, and Read unmarshalling the representation.] A limited type supports external streaming only if it has available Read and Write attributes. A type with a part that is of a nonremote access type supports external streaming only if that access type or the type of some part that includes the access type component, has Read and Write attributes that have been specified via an attribute_definition_clause, and that attribute_definition_clause is visible. [An anonymous access type does not support external streaming. ]All other types (including remote access types, see E.2.2) support external streaming. 52.a/3 Ramification: A limited type with a part that is of a nonremote access type needs to satisfy both rules. Erroneous Execution 53/2 {AI95-00279-01} {AI95-00344-01} If the internal tag returned by Descendant_Tag to T'Class'Input identifies a type that is not library-level and whose tag has not been created, or does not exist in the partition at the time of the call, execution is erroneous. 53.a/2 Ramification: The definition of Descendant_Tag prevents such a tag from being provided to T'Class'Input if T is a library-level type. However, this rule is needed for nested tagged types. Implementation Requirements 54/1 {8652/0040} {AI95-00108-01} For every subtype S of a language-defined nonlimited specific type T, the output generated by S'Output or S'Write shall be readable by S'Input or S'Read, respectively. This rule applies across partitions if the implementation conforms to the Distributed Systems Annex. 55/3 {AI95-00195-01} {AI05-0092-1} If Constraint_Error is raised during a call to Read because of failure of one the above checks, the implementation shall ensure that the discriminants of the actual parameter of Read are not modified. Implementation Permissions 56/3 {AI95-00195-01} {AI05-0092-1} The number of calls performed by the predefined implementation of the stream-oriented attributes on the Read and Write operations of the stream type is unspecified. An implementation may take advantage of this permission to perform internal buffering. However, all the calls on the Read and Write operations of the stream type needed to implement an explicit invocation of a stream-oriented attribute shall take place before this invocation returns. An explicit invocation is one appearing explicitly in the program text, possibly through a generic instantiation (see 12.3). 56.1/3 {AI05-0023-1} {AI05-0264-1} If T is a discriminated type and its discriminants have defaults, then in two cases an execution of the default implementation of S'Read is not required to create an anonymous object of type T: If the discriminant values that are read in are equal to the corresponding discriminant values of Item, then no object of type T need be created and Item may be used instead. If they are not equal and Item is a constrained variable, then Constraint_Error may be raised at that point, before any further values are read from the stream and before the object of type T is created. 56.2/3 {AI05-0023-1} A default implementation of S'Input that calls the default implementation of S'Read may create a constrained anonymous object with discriminants that match those in the stream. 56.a/3 Implementation Note: This allows the combined executions of S'Input and S'Read to create one object of type T instead of two. If this option is exercised, then: 56.b/3 * The discriminants are read from the stream by S'Input, not S'Read. 56.c/3 * S'Input declares an object of type T constrained by the discriminants read from the stream, not an unconstrained object. 56.d/3 * The discriminant values that S'Read would normally have read from the stream are read from Item instead. 56.e/3 * The permissions of the preceding paragraph then apply and no object of type T need be created by the execution of S'Read. NOTES 57 40 For a definite subtype S of a type T, only T'Write and T'Read are needed to pass an arbitrary value of the subtype through a stream. For an indefinite subtype S of a type T, T'Output and T'Input will normally be needed, since T'Write and T'Read do not pass bounds, discriminants, or tags. 58 41 User-specified attributes of S'Class are not inherited by other class-wide types descended from S. Examples 59 Example of user-defined Write attribute: 60/2 {AI95-00441-01} procedure My_Write( Stream : not null access Ada.Streams.Root_Stream_Type'Class; Item : My_Integer'Base); for My_Integer'Write use My_Write; 60.a Discussion: Example of network input/output using input output attributes: 60.b with Ada.Streams; use Ada.Streams; generic type Msg_Type(<>) is private; package Network_IO is -- Connect/Disconnect are used to establish the stream procedure Connect(...); procedure Disconnect(...); 60.c -- Send/Receive transfer messages across the network procedure Send(X : in Msg_Type); function Receive return Msg_Type; private type Network_Stream is new Root_Stream_Type with ... procedure Read(...); -- define Read/Write for Network_Stream procedure Write(...); end Network_IO; 60.d with Ada.Streams; use Ada.Streams; package body Network_IO is Current_Stream : aliased Network_Stream; . . . procedure Connect(...) is ...; procedure Disconnect(...) is ...; 60.e procedure Send(X : in Msg_Type) is begin Msg_Type'Output(Current_Stream'Access, X); end Send; 60.f function Receive return Msg_Type is begin return Msg_Type'Input(Current_Stream'Access); end Receive; end Network_IO; Inconsistencies With Ada 95 60.g/2 {8652/0040} {AI95-00108-01} Corrigendum: Clarified how the default implementation for stream attributes is determined (eliminating conflicting language). The new wording provides that attributes for type extensions are created by composing the parent's attribute with those for the extension components if any. If a program was written assuming that the extension components were not included in the stream (as in original Ada 95), it would fail to work in the language as corrected by the Corrigendum. 60.h/2 {AI95-00195-01} Amendment Correction: Explicitly provided a permission that the number of calls to the underlying stream Read and Write operations may differ from the number determined by the canonical operations. If Ada 95 code somehow depended on the number of calls to Read or Write, it could fail with an Ada 2005 implementation. Such code is likely to be very rare; moreover, such code is really wrong, as the permission applies to Ada 95 as well. Extensions to Ada 95 60.i/2 {AI95-00270-01} The Stream_Size attribute is new. It allows specifying the number of bits that will be streamed for a type. The Implementation Advice involving this also was changed; this is not incompatible because Implementation Advice does not have to be followed. 60.j/2 {8652/0040} {AI95-00108-01} {AI95-00195-01} {AI95-00444-01} Corrigendum: Limited types may have default constructed attributes if all of the parent and (for extensions) extension components have available attributes. Ada 2005 adds the notion of availability to patch up some holes in the Corrigendum model. Wording Changes from Ada 95 60.k/2 {8652/0009} {AI95-00137-01} Corrigendum: Added wording to specify that these are operational attributes. 60.l/2 {8652/0045} {AI95-00132-01} Corrigendum: Clarified that End_Error is raised by the default implementation of Read and Input if the end of the stream is reached. (The result could have been abnormal without this clarification, thus this is not an inconsistency, as the programmer could not have depended on the previous behavior.) 60.m/2 {AI95-00195-01} Clarified that the default implementation of S'Input does normal initialization on the object that it passes to S'Read. 60.n/2 {AI95-00195-01} Explicitly stated that what is read from a stream when a required check fails is unspecified. 60.o/2 {AI95-00251-01} Defined availability and default implementations for types with progenitors. 60.p/2 {AI95-00279-01} Specified that Constraint_Error is raised if the internal tag retrieved for S'Class'Input is for some type not covered by S'Class or is abstract. We also explicitly state that the program is erroneous if the tag has not been created or does not currently exist in the partition. (Ada 95 did not specify what happened in these cases; it's very unlikely to have provided some useful result, so this is not considered an inconsistency.) 60.q/2 {AI95-00344-01} Added wording to support nested type extensions. S'Input and S'Output always raise Tag_Error for such extensions, and such extensions were not permitted in Ada 95, so this is neither an extension nor an incompatibility. 60.r/2 {AI95-00366-01} Defined supports external streaming to put all of the rules about "good" stream attributes in one place. This is used for distribution and for defining pragma Pure. 60.s/2 {AI95-00441-01} Added the not null qualifier to the first parameter of all of the stream attributes, so that the semantics doesn't change between Ada 95 and Ada 2005. This change is compatible, because mode conformance is required for subprograms specified as stream attributes, and null_exclusions are not considered for mode conformance. 60.t/2 {AI95-00444-01} Improved the wording to make it clear that we don't define the default implementations of attributes that cannot be called (that is, aren't "available"). Also clarified when inheritance takes place. Incompatibilities With Ada 2005 60.u/3 {AI05-0039-1} Correction: Added a requirement that stream attributes be specified by a static subprogram name rather than a dynamic expression. Expressions cannot provide any useful functionality because of the freezing rules, and the possibility of them complicates implementations. Only pathological programs should be affected. Extensions to Ada 2005 60.v/3 {AI05-0007-1} Correction: Stream attributes for scalar types can be specified with subprograms that take the first subtype as well as the base type. This eliminates confusion about which subtype is appropriate for attributes specified for partial views whose full type is a scalar type. It also eliminates a common user error (forgetting 'Base). Wording Changes from Ada 2005 60.w/3 {AI05-0023-1} Correction: Corrected the definition of the default version S'Read and S'Input to be well-defined if S is a discriminated type with defaulted discriminants and some components require initialization and/or finalizations. 60.x/3 {AI05-0065-1} Correction: Defined remote access types to support external streaming, since that is their purpose. 60.y/3 {AI05-0109-1} Correction: Removed a misleading phrase which implies that Constraint_Error is raised for internal tags of the wrong type, when Tag_Error should be raised for such tags. 60.z/3 {AI05-0139-2} Clarified that arrays with convention Fortran are written in column-major order, rather then row-major order. This is necessary in order that streaming of Fortran arrays is efficient. 60.aa/3 {AI05-0192-1} Correction: Clarified that the profile of an inherited stream attribute is as defined for an inherited primitive subprogram, while the default implementation of the same attribute might have a different profile. 60.bb/3 {AI05-0194-1} Correction: Clarified that Stream_Size has no effect on and is not effected by user-defined stream attributes. Extensions to Ada 2012 60.cc/4 {AI12-0106-1} Corrigendum: Defined how to specify a class-wide stream-oriented attribute using an aspect_specification. It was always intended that this was possible, but the method was not clear, as a class-wide type never has an explicit declaration. Wording Changes from Ada 2012 60.dd/4 {AI12-0030-1} Corrigendum: Defined the runtime effect of stream attributes for untagged limited types, as there is a weird corner case where they can be called. We don't specify this as an inconsistency, as it doesn't make semantic sense to stream a task, and nothing useful could have been done with that, so it should not exist in any programs. 60.ee/4 {AI12-0106-1} Corrigendum: Clarified that the same Legality Rules apply when a stream-oriented attribute is specified via an aspect_specification as applied when it is specified via an attribute_definition_clause. 13.14 Freezing Rules 1/3 {AI05-0299-1} [This subclause defines a place in the program text where each declared entity becomes "frozen." A use of an entity, such as a reference to it by name, or (for a type) an expression of the type, causes freezing of the entity in some contexts, as described below. The Legality Rules forbid certain kinds of uses of an entity in the region of text where it is frozen.] 1.a Reason: This concept has two purposes: a compile-time one and a run-time one. 1.b The compile-time purpose of the freezing rules comes from the fact that the evaluation of static expressions depends on overload resolution, and overload resolution sometimes depends on the value of a static expression. (The dependence of static evaluation upon overload resolution is obvious. The dependence in the other direction is more subtle. There are three rules that require static expressions in contexts that can appear in declarative places: The expression in an attribute_designator shall be static. In a record aggregate, variant-controlling discriminants shall be static. In an array aggregate with more than one named association, the choices shall be static. The compiler needs to know the value of these expressions in order to perform overload resolution and legality checking.) We wish to allow a compiler to evaluate static expressions when it sees them in a single pass over the compilation_unit. The freezing rules ensure that. 1.c The run-time purpose of the freezing rules is called the "linear elaboration model." This means that declarations are elaborated in the order in which they appear in the program text, and later elaborations can depend on the results of earlier ones. The elaboration of the declarations of certain entities requires run-time information about the implementation details of other entities. The freezing rules ensure that this information has been calculated by the time it is used. For example, suppose the initial value of a constant is the result of a function call that takes a parameter of type T. In order to pass that parameter, the size of type T has to be known. If T is composite, that size might be known only at run time. 1.d (Note that in these discussions, words like "before" and " after" generally refer to places in the program text, as opposed to times at run time.) 1.e Discussion: The "implementation details" we're talking about above are: 1.f * For a tagged type, the implementations of all the primitive subprograms of the type - that is (in the canonical implementation model), the contents of the type descriptor, which contains pointers to the code for each primitive subprogram. 1.g * For a type, the full type declaration of any parts (including the type itself) that are private. 1.h * For a deferred constant, the full constant declaration, which gives the constant's value. (Since this information necessarily comes after the constant's type and subtype are fully known, there's no need to worry about its type or subtype.) 1.i * For any entity, representation information specified by the user via representation items. Most representation items are for types or subtypes; however, various other kinds of entities, such as objects and subprograms, are possible. 1.j/3 {AI05-0005-1} Similar issues arise for incomplete types. However, we do not use freezing to prevent premature access; incomplete types have different, more severe, restrictions. Similar issues also arise for subprograms, protected operations, tasks and generic units. However, we do not use freezing to prevent premature access for those, either; 3.11 prevents problems with run-time Elaboration_Checks. Even so, freezing is used for these entities to prevent giving representation items too late (that is, after uses that require representation information, such as calls). Language Design Principles 1.k An evaluable construct should freeze anything that's needed to evaluate it. 1.l However, if the construct is not evaluated where it appears, let it cause freezing later, when it is evaluated. This is the case for default_expressions and default_names. (Formal parameters, generic formal parameters, and components can have default_expressions or default_names.) 1.m The compiler should be allowed to evaluate static expressions without knowledge of their context. (I.e. there should not be any special rules for static expressions that happen to occur in a context that requires a static expression.) 1.n Compilers should be allowed to evaluate static expressions (and record the results) using the run-time representation of the type. For example, suppose Color'Pos(Red) = 1, but the internal code for Red is 37. If the value of a static expression is Red, some compilers might store 1 in their symbol table, and other compilers might store 37. Either compiler design should be feasible. 1.o Compilers should never be required to detect erroneousness or exceptions at compile time (although it's very nice if they do). This implies that we should not require code-generation for a nonstatic expression of type T too early, even if we can prove that that expression will be erroneous, or will raise an exception. 1.p Here's an example (modified from AI83-00039, Example 3): 1.q type T is record ... end record; function F return T; function G(X : T) return Boolean; Y : Boolean := G(F); -- doesn't force T in Ada 83 for T use record ... end record; 1.r AI83-00039 says this is legal. Of course, it raises Program_Error because the function bodies aren't elaborated yet. A one-pass compiler has to generate code for an expression of type T before it knows the representation of T. Here's a similar example, which AI83-00039 also says is legal: 1.s package P is type T is private; function F return T; function G(X : T) return Boolean; Y : Boolean := G(F); -- doesn't force T in Ada 83 private type T is record ... end record; end P; 1.t If T's size were dynamic, that size would be stored in some compiler-generated dope; this dope would be initialized at the place of the full type declaration. However, the generated code for the function calls would most likely allocate a temp of the size specified by the dope before checking for Program_Error. That dope would contain uninitialized junk, resulting in disaster. To avoid doing that, the compiler would have to determine, at compile time, that the expression will raise Program_Error. 1.u This is silly. If we're going to require compilers to detect the exception at compile time, we might as well formulate the rule as a legality rule. 1.v Compilers should not be required to generate code to load the value of a variable before the address of the variable has been determined. 1.w After an entity has been frozen, no further requirements may be placed on its representation (such as by a representation item or a full_type_declaration). 2 The freezing of an entity occurs at one or more places (freezing points) in the program text where the representation for the entity has to be fully determined. Each entity is frozen from its first freezing point to the end of the program text (given the ordering of compilation units defined in 10.1.4). 2.a Ramification: The "representation" for a subprogram includes its calling convention and means for referencing the subprogram body, either a "link-name" or specified address. It does not include the code for the subprogram body itself, nor its address if a link-name is used to reference the body. 2.1/3 {AI05-0019-1} {AI05-0299-1} This subclause also defines a place in the program text where the profile of each declared callable entity becomes frozen. A use of a callable entity causes freezing of its profile in some contexts, as described below. At the place where the profile of a callable entity becomes frozen, the entity itself becomes frozen. 3/4 {8652/0014} {AI05-0017-1} {AI05-0019-1} {AI12-0103-1} The end of a declarative_part, protected_body, or a declaration of a library package or generic library package, causes freezing of each entity and profile declared within it, except for incomplete types. A proper_body, body_stub, or entry_body causes freezing of each entity and profile declared before it within the same declarative_part that is not an incomplete type; it only causes freezing of an incomplete type if the body is within the immediate scope of the incomplete type. 3.a Discussion: This is worded carefully to handle nested packages and private types. Entities declared in a nested package_specification will be frozen by some containing construct. 3.b/3 {AI05-0017-1} An incomplete type declared in the private part of a library package_specification can be completed in the body. For other incomplete types (and in the bodies of library packages), the completion of the type will be frozen at the end of the package or declarative_part, and that will freeze the incomplete view as well. 3.b.1/3 {AI05-0017-1} The reason we have to worry about freezing of incomplete types is to prevent premature uses of the types in dispatching calls. Such uses may need access to the tag of the type, and the type has to be frozen to know where the tag is stored. 3.c/3 Ramification: {AI05-0229-1} The part about bodies does not say immediately within. A renaming-as-body does not have this property. Nor does an imported body 3.d Reason: The reason bodies cause freezing is because we want proper_bodies and body_stubs to be interchangeable - one should be able to move a proper_body to a subunit, and vice-versa, without changing the semantics. Clearly, anything that should cause freezing should do so even if it's inside a proper_body. However, if we make it a body_stub, then the compiler can't see that thing that should cause freezing. So we make body_stubs cause freezing, just in case they contain something that should cause freezing. But that means we need to do the same for proper_bodies. 3.e Another reason for bodies to cause freezing, there could be an added implementation burden if an entity declared in an enclosing declarative_part is frozen within a nested body, since some compilers look at bodies after looking at the containing declarative_part. 3.f/4 {AI05-0177-1} {AI12-0103-1} Note that null_procedure_declarations and expression_function_declarations (even when those are used as completions), as well as generic_instantiations and renames-as-bodies do not necessarily cause freezing; each have their own specific rules. 3.g/4 Ramification: {AI12-0103-1} Note that the rule about proper bodies being freezing only applies in declarative_parts. All of the kinds of bodies (see 3.11.1 - keep in mind the difference from bodys) that are allowed in a package specification have their own freezing rules, so they don't need to be covered by the above rule. 4/1 {8652/0046} {AI95-00106-01} A construct that (explicitly or implicitly) references an entity can cause the freezing of the entity, as defined by subsequent paragraphs. At the place where a construct causes freezing, each name, expression, implicit_dereference[, or range] within the construct causes freezing: 4.a Ramification: Note that in the sense of this paragraph, a subtype_mark "references" the denoted subtype, but not the type. 5/3 * {AI05-0213-1} The occurrence of a generic_instantiation causes freezing, except that a name which is a generic actual parameter whose corresponding generic formal parameter is a formal incomplete type (see 12.5.1) does not cause freezing. In addition, if a parameter of the instantiation is defaulted, the default_expression or default_name for that parameter causes freezing. 5.a/3 Ramification: {AI05-0213-1} Thus, an actual parameter corresponding to a formal incomplete type parameter may denote an incomplete or private type which is not completely defined at the point of the generic_instantiation. 5.1/4 * {AI12-0103-1} {AI12-0157-1} At the occurrence of an expression_function_declaration that is a completion, the return expression of the expression function causes freezing. 5.b/4 Reason: {AI12-0103-1} This rule prevents calls through access values to an expression that might have unfrozen parts. Typically, elaboration checks and other freezing rules prevent this, but in this case the completion is elaborated and since this is not a body it does not by itself freeze anything that precedes it. 5.2/4 * {AI12-0132-1} {AI12-0157-1} At the occurrence of a renames-as-body whose callable_entity_name denotes an expression function, the return expression of the expression function causes freezing. 6 * The occurrence of an object_declaration that has no corresponding completion causes freezing. 6.a Ramification: Note that this does not include a formal_object_declaration. 7 * The declaration of a record extension causes freezing of the parent subtype. 7.a Ramification: This combined with another rule specifying that primitive subprogram declarations shall precede freezing ensures that all descendants of a tagged type implement all of its dispatching operations. 7.b/2 {AI95-00251-01} The declaration of a private extension does not cause freezing. The freezing is deferred until the full type declaration, which will necessarily be for a record extension, task, or protected type (the latter only for a limited private extension derived from an interface). 7.1/2 * {AI95-00251-01} The declaration of a record extension, interface type, task unit, or protected unit causes freezing of any progenitor types specified in the declaration. 7.b.1/2 Reason: This rule has the same purpose as the one above: ensuring that all descendants of an interface tagged type implement all of its dispatching operations. As with the previous rule, a private extension does not freeze its progenitors; the full type declaration (which must have the same progenitors) will do that. 7.b.2/2 Ramification: An interface type can be a parent as well as a progenitor; these rules are similar so that the location of an interface in a record extension does not have an effect on the freezing of the interface type. 7.2/3 * {AI05-0183-1} At the freezing point of the entity associated with an aspect_specification, any expressions or names within the aspect_specification cause freezing. Any static expressions within an aspect_specification also cause freezing at the end of the immediately enclosing declaration list. 8/4 {8652/0046} {AI95-00106-01} {AI05-0177-1} {AI05-0183-1} {AI05-0157-1} A static expression (other than within an aspect_specification) causes freezing where it occurs. An object name or nonstatic expression causes freezing where it occurs, unless the name or expression is part of a default_expression, a default_name, the return expression of an expression function, an aspect_specification, or a per-object expression of a component's constraint, in which case, the freezing occurs later as part of another construct or at the freezing point of an associated entity. 8.1/3 {8652/0046} {AI95-00106-01} {AI05-0019-1} An implicit call freezes the same entities and profiles that would be frozen by an explicit call. This is true even if the implicit call is removed via implementation permissions. 8.2/1 {8652/0046} {AI95-00106-01} If an expression is implicitly converted to a type or subtype T, then at the place where the expression causes freezing, T is frozen. 9 The following rules define which entities are frozen at the place where a construct causes freezing: 10 * At the place where an expression causes freezing, the type of the expression is frozen, unless the expression is an enumeration literal used as a discrete_choice of the array_aggregate of an enumeration_- representation_clause. 10.a Reason: We considered making enumeration literals never cause freezing, which would be more upward compatible, but examples like the variant record aggregate (Discrim => Red, ...) caused us to change our mind. Furthermore, an enumeration literal is a static expression, so the implementation should be allowed to represent it using its representation. 10.b Ramification: The following pathological example was legal in Ada 83, but is illegal in Ada 95: 10.c package P1 is type T is private; package P2 is type Composite(D : Boolean) is record case D is when False => Cf : Integer; when True => Ct : T; end case; end record; end P2; X : Boolean := P2."="( (False,1), (False,1) ); private type T is array(1..Func_Call) of Integer; end; 10.d In Ada 95, the declaration of X freezes Composite (because it contains an expression of that type), which in turn freezes T (even though Ct does not exist in this particular case). But type T is not completely defined at that point, violating the rule that a type shall be completely defined before it is frozen. In Ada 83, on the other hand, there is no occurrence of the name T, hence no forcing occurrence of T. 10.1/4 * {AI05-0019-1} {AI05-0177-1} {AI12-0157-1} At the place where a function call causes freezing, the profile of the function is frozen. Furthermore, if a parameter of the call is defaulted, the default_expression for that parameter causes freezing. If the function call is to an expression function, the return expression of the expression function causes freezing. 10.e/3 Reason: {AI05-0019-1} This is the important rule for profile freezing: a call freezes the profile. That's because generating the call will need to know how the parameters are passed, and that will require knowing details of the types. Other uses of subprograms do not need to know about the parameters, and thus only freeze the subprogram, and not the profile. 10.f/3 Note that we don't need to consider procedure or entry calls, since a body freezes everything that precedes it, and the end of a declarative part freezes everything in the declarative part. 10.g/4 Ramification: {AI05-0177-1} {AI12-0103-1} {AI12-0157-1} Freezing of the return expression of an expression function only needs to be considered when the expression function is in the same compilation unit and there are no intervening bodys; the end of a declarative_part or library package freezes everything in it, and a body freezes everything declared before it. 10.2/4 * {AI05-0019-1} {AI05-0177-1} {AI05-0296-1} {AI12-0157-1} At the place where a generic_instantiation causes freezing of a callable entity, the profile of that entity is frozen unless the formal subprogram corresponding to the callable entity has a parameter or result of a formal untagged incomplete type; if the callable entity is an expression function, the return expression of the expression function causes freezing. 10.h/3 Reason: Elaboration of the generic might call the actual for one of its formal subprograms, so we need to know the profile and (for an expression function) expression. 10.3/4 * {AI05-0177-1} {AI12-0157-1} At the place where a use of the Access or Unchecked_Access attribute whose prefix denotes an expression function causes freezing, the return expression of the expression function causes freezing. 10.i/3 Reason: This is needed to avoid calls to unfrozen expressions. Consider: 10.j/3 package Pack is 10.k/3 type Flub is range 0 .. 100; 10.l/3 function Foo (A : in Natural) return Natural is (A + Flub'Size); -- The expression is not frozen here. 10.m/4 {AI12-0005-1} type Bar is access function (A : in Natural) return Natural; 10.n/3 P : Bar := Foo'Access; -- (A) 10.o/3 Val : Natural := P.all(5); -- (B) 10.p/3 end Pack; 10.q/3 If point (A) did not freeze the expression of Foo (which freezes Flub), then the call at point (B) would be depending on the aspects of the unfrozen type Flub. That would be bad. 11 * At the place where a name causes freezing, the entity denoted by the name is frozen, unless the name is a prefix of an expanded name; at the place where an object name causes freezing, the nominal subtype associated with the name is frozen. 11.a/2 Ramification: {AI95-00114-01} This only matters in the presence of deferred constants or access types; an object_declaration other than a deferred constant declaration causes freezing of the nominal subtype, plus all component junk. 11.b/1 This paragraph was deleted.{8652/0046} {AI95-00106-01} 11.1/1 * {8652/0046} {AI95-00106-01} At the place where an implicit_dereference causes freezing, the nominal subtype associated with the implicit_dereference is frozen. 11.c/2 Discussion: This rule ensures that X.D freezes the same entities that X.all.D does. Note that an implicit_dereference is neither a name nor expression by itself, so it isn't covered by other rules. 12 * [ At the place where a range causes freezing, the type of the range is frozen.] 12.a Proof: This is consequence of the facts that expressions freeze their type, and the Range attribute is defined to be equivalent to a pair of expressions separated by "..".} 13 * At the place where an allocator causes freezing, the designated subtype of its type is frozen. If the type of the allocator is a derived type, then all ancestor types are also frozen. 13.a Ramification: Allocators also freeze the named subtype, as a consequence of other rules. 13.b The ancestor types are frozen to prevent things like this: 13.c type Pool_Ptr is access System.Storage_Pools.Root_Storage_Pool'Class; function F return Pool_Ptr; 13.d package P is type A1 is access Boolean; type A2 is new A1; type A3 is new A2; X : A3 := new Boolean; -- Don't know what pool yet! for A1'Storage_Pool use F.all; end P; 13.e This is necessary because derived access types share their parent's pool. 14/3 * {AI05-0019-1} At the place where a profile is frozen, each subtype of the profile is frozen. If the corresponding callable entity is a member of an entry family, the index subtype of the family is frozen. 14.a/3 This paragraph was deleted. 15 * At the place where a subtype is frozen, its type is frozen. At the place where a type is frozen, any expressions or names within the full type definition cause freezing; the first subtype, and any component subtypes, index subtypes, and parent subtype of the type are frozen as well. For a specific tagged type, the corresponding class-wide type is frozen as well. For a class-wide type, the corresponding specific type is frozen as well. 15.a Ramification: Freezing a type needs to freeze its first subtype in order to preserve the property that the subtype-specific aspects of statically matching subtypes are the same. 15.b Freezing an access type does not freeze its designated subtype. 15.1/3 * {AI95-00341-01} {AI05-0019-1} At the place where a specific tagged type is frozen, the primitive subprograms of the type are frozen. At the place where a type is frozen, any subprogram named in an attribute_definition_clause for the type is frozen. 15.c/2 Reason: We have a language design principle that all of the details of a specific tagged type are known at its freezing point. But that is only true if the primitive subprograms are frozen at this point as well. Late changes of Import and address clauses violate the principle. 15.d/2 Implementation Note: This rule means that no implicit call to Initialize or Adjust can freeze a subprogram (the type and thus subprograms would have been frozen at worst at the same point). 15.e/3 Discussion: {AI05-0019-1} The second sentence is the rule that makes it possible to check that only subprograms with convention Ada are specified in attribute_definition_clauses without jumping through hoops. Legality Rules 16 [The explicit declaration of a primitive subprogram of a tagged type shall occur before the type is frozen (see 3.9.2).] 16.a Reason: This rule is needed because (1) we don't want people dispatching to things that haven't been declared yet, and (2) we want to allow tagged type descriptors to be static (allocated statically, and initialized to link-time-known symbols). Suppose T2 inherits primitive P from T1, and then overrides P. Suppose P is called before the declaration of the overriding P. What should it dispatch to? If the answer is the new P, we've violated the first principle above. If the answer is the old P, we've violated the second principle. (A call to the new one necessarily raises Program_Error, but that's beside the point.) 16.b Note that a call upon a dispatching operation of type T will freeze T. 16.c We considered applying this rule to all derived types, for uniformity. However, that would be upward incompatible, so we rejected the idea. As in Ada 83, for an untagged type, the above call upon P will call the old P (which is arguably confusing). 16.d/3 To be honest: {AI05-0222-1} This rule only applies to "original" declarations and not to the completion of a primitive subprogram, even though a completion is technically an explicit declaration, and it may declare a primitive subprogram. 17 [A type shall be completely defined before it is frozen (see 3.11.1 and 7.3).] 18 [The completion of a deferred constant declaration shall occur before the constant is frozen (see 7.4).] 18.a/3 Proof: {AI95-00114-01} {AI05-0299-1} The above Legality Rules are stated "officially" in the referenced subclauses. 19/1 {8652/0009} {AI95-00137-01} An operational or representation item that directly specifies an aspect of an entity shall appear before the entity is frozen (see 13.1). 19.a/1 Discussion: {8652/0009} {AI95-00137-01} From RM83-13.1(7). The wording here forbids freezing within the aspect_clause itself, which was not true of the Ada 83 wording. The wording of this rule is carefully written to work properly for type-related representation items. For example, an enumeration_representation_- clause is illegal after the type is frozen, even though the _clause refers to the first subtype. 19.a.1/2 {AI95-00114-01} The above Legality Rule is stated for types and subtypes in 13.1, but the rule here covers all other entities as well. 19.b/2 This paragraph was deleted.{AI95-00114-01} 19.c Discussion: Here's an example that illustrates when freezing occurs in the presence of defaults: 19.d type T is ...; function F return T; type R is record C : T := F; D : Boolean := F = F; end record; X : R; 19.e Since the elaboration of R's declaration does not allocate component C, there is no need to freeze C's subtype at that place. Similarly, since the elaboration of R does not evaluate the default_expression "F = F", there is no need to freeze the types involved at that point. However, the declaration of X does need to freeze these things. Note that even if component C did not exist, the elaboration of the declaration of X would still need information about T - even though D is not of type T, its default_expression requires that information. 19.f/3 Ramification: {AI05-0299-1} Although we define freezing in terms of the program text as a whole (i.e. after applying the rules of Clause 10), the freezing rules actually have no effect beyond compilation unit boundaries. 19.g/3 Reason: {AI05-0299-1} That is important, because Clause 10 allows some implementation definedness in the order of things, and we don't want the freezing rules to be implementation defined. 19.h Ramification: These rules also have no effect in statements - they only apply within a single declarative_part, package_specification, task_definition, protected_definition, or protected_body. 19.i Implementation Note: An implementation may choose to generate code for default_expressions and default_names in line at the place of use. Alternatively, an implementation may choose to generate thunks (subprograms implicitly generated by the compiler) for evaluation of defaults. Thunk generation cannot, in general, be done at the place of the declaration that includes the default. Instead, they can be generated at the first freezing point of the type(s) involved. (It is impossible to write a purely one-pass Ada compiler, for various reasons. This is one of them - the compiler needs to store a representation of defaults in its symbol table, and then walk that representation later, no earlier than the first freezing point.) 19.j In implementation terms, the linear elaboration model can be thought of as preventing uninitialized dope. For example, the implementation might generate dope to contain the size of a private type. This dope is initialized at the place where the type becomes completely defined. It cannot be initialized earlier, because of the order-of-elaboration rules. The freezing rules prevent elaboration of earlier declarations from accessing the size dope for a private type before it is initialized. 19.k 2.8 overrides the freezing rules in the case of unrecognized pragmas. 19.l/1 {8652/0009} {AI95-00137-01} An aspect_clause for an entity should most certainly not be a freezing point for the entity. Dynamic Semantics 20/2 {AI95-00279-01} The tag (see 3.9) of a tagged type T is created at the point where T is frozen. Incompatibilities With Ada 83 20.a RM83 defines a forcing occurrence of a type as follows: "A forcing occurrence is any occurrence [of the name of the type, subtypes of the type, or types or subtypes with subcomponents of the type] other than in a type or subtype declaration, a subprogram specification, an entry declaration, a deferred constant declaration, a pragma, or a representation_clause for the type itself. In any case, an occurrence within an expression is always forcing." 20.b It seems like the wording allows things like this: 20.c type A is array(Integer range 1..10) of Boolean; subtype S is Integer range A'Range; -- not forcing for A 20.d Occurrences within pragmas can cause freezing in Ada 95. (Since such pragmas are ignored in Ada 83, this will probably fix more bugs than it causes.) Extensions to Ada 83 20.e In Ada 95, generic_formal_parameter_declarations do not normally freeze the entities from which they are defined. For example: 20.f package Outer is type T is tagged limited private; generic type T2 is new T with private; -- Does not freeze T -- in Ada 95. package Inner is ... end Inner; private type T is ...; end Outer; 20.g This is important for the usability of generics. The above example uses the Ada 95 feature of formal derived types. Examples using the kinds of formal parameters already allowed in Ada 83 are well known. See, for example, comments 83-00627 and 83-00688. The extensive use expected for formal derived types makes this issue even more compelling than described by those comments. Unfortunately, we are unable to solve the problem that explicit_generic_actual_parameters cause freezing, even though a package equivalent to the instance would not cause freezing. This is primarily because such an equivalent package would have its body in the body of the containing program unit, whereas an instance has its body right there. Wording Changes from Ada 83 20.h The concept of freezing is based on Ada 83's concept of "forcing occurrences." The first freezing point of an entity corresponds roughly to the place of the first forcing occurrence, in Ada 83 terms. The reason for changing the terminology is that the new rules do not refer to any particular "occurrence" of a name of an entity. Instead, we refer to "uses" of an entity, which are sometimes implicit. 20.i In Ada 83, forcing occurrences were used only in rules about representation_clauses. We have expanded the concept to cover private types, because the rules stated in RM83-7.4.1(4) are almost identical to the forcing occurrence rules. 20.j The Ada 83 rules are changed in Ada 95 for the following reasons: 20.k * The Ada 83 rules do not work right for subtype-specific aspects. In an earlier version of Ada 9X, we considered allowing representation items to apply to subtypes other than the first subtype. This was part of the reason for changing the Ada 83 rules. However, now that we have dropped that functionality, we still need the rules to be different from the Ada 83 rules. 20.l * The Ada 83 rules do not achieve the intended effect. In Ada 83, either with or without the AIs, it is possible to force the compiler to generate code that references uninitialized dope, or force it to detect erroneousness and exception raising at compile time. 20.m * It was a goal of Ada 83 to avoid uninitialized access values. However, in the case of deferred constants, this goal was not achieved. 20.n * The Ada 83 rules are not only too weak - they are also too strong. They allow loopholes (as described above), but they also prevent certain kinds of default_expressions that are harmless, and certain kinds of generic_declarations that are both harmless and very useful. 20.o/2 * {AI95-00114-01} Ada 83 had a case where an aspect_clause had a strong effect on the semantics of the program - 'Small. This caused certain semantic anomalies. There are more cases in Ada 95, because the attribute_definition_clause has been generalized. Incompatibilities With Ada 95 20.p/2 {8652/0046} {AI95-00106-01} {AI95-00341-01} Corrigendum: Various freezing rules were added to fix holes in the rules. Most importantly, implicit calls are now freezing, which make some representation clauses illegal in Ada 2005 that were legal (but dubious) in Ada 95. Amendment Correction: Similarly, the primitive subprograms of a specific tagged type are frozen when the type is frozen, preventing dubious convention changes (and address clauses) after the freezing point. In both cases, the code is dubious and the workaround is easy. Wording Changes from Ada 95 20.q/2 {8652/0009} {AI95-00137-01} Corrigendum: Added wording to specify that both operational and representation attributes must be specified before the type is frozen. 20.r/2 {AI95-00251-01} Added wording that declaring a specific descendant of an interface type freezes the interface type. 20.s/2 {AI95-00279-01} Added wording that defines when a tag is created for a type (at the freezing point of the type). This is used to specify checking for uncreated tags (see 3.9). Incompatibilities With Ada 2005 20.t/3 {AI05-0019-1} Correction: Separated the freezing of the profile from the rest of a subprogram, in order to reduce the impact of the Ada 95 incompatibility noted above. (The effects were much more limiting than expected.) Wording Changes from Ada 2005 20.u/3 {AI05-0017-1} Correction: Reworded so that incomplete types with a deferred completion aren't prematurely frozen. 20.v/3 {AI05-0177-1} Added freezing rules for expression functions; these are frozen at the point of call, not the point of declaration, like default expressions. 20.w/3 {AI05-0183-1} Added freezing rules for aspect_specifications; these are frozen at the freezing point of the associated entity, not the point of declaration. 20.x/3 {AI05-0213-1} Added freezing rules for formal incomplete types; the corresponding actual is not frozen. Wording Changes from Ada 2012 20.y/4 {AI12-0103-1} {AI12-0132-1} Corrigendum: Clarified when and what an expression_function_declaration that is a completion or that is the target of a renames-as-body freezes. This is formally an incompatibility, but as all known implementations freeze expression functions more aggressively than allowed by either the old or new wording, practically this will be an extension.