3.9 Tagged Types and Type Extensions

{AI95-00251-01} The intended implementation model is for the static portion of a tag to be represented as a pointer to a statically allocated and link-time initialized type descriptor. The type descriptor contains the address of the code for each primitive operation of the type. It probably also contains other information, such as might make membership tests convenient and efficient. Tags for nested type extensions must also have a dynamic part that identifies the particular elaboration of the type.

The primitive operations of a tagged type are known at its first freezing point; the type descriptor is laid out at that point. It contains linker symbols for each primitive operation; the linker fills in the actual addresses.

{AI95-00251-01} Primitive operations of type extensions that are declared at a level deeper than the level of the ultimate ancestor from which they are derived can be represented by wrappers that use the dynamic part of the tag to call the actual primitive operation. The dynamic part would generally be some way to represent the static link or display necessary for making a nested call. One implementation strategy would be to store that information in the extension part of such nested type extensions, and use the dynamic part of the tag to point at it. (That way, the “dynamic” part of the tag could be static, at the cost of indirect access.)

{AI95-00251-01} If the tagged type is descended from any interface types, it also will need to include “subtags” (one for each interface) that describe the mapping of the primitive operations of the interface to the primitives of the type. These subtags could directly reference the primitive operations (for faster performance), or simply provide the tag “slot” numbers for the primitive operations (for easier derivation). In either case, the subtags would be used for calls that dispatch through a class-wide type of the interface.

Other implementation models are possible.

The rules ensure that “dangling dispatching” is impossible; that is, when a dispatching call is made, there is always a body to execute. This is different from some other object-oriented languages, such as Smalltalk, where it is possible to get a run-time error from a missing method.

{AI95-00251-01} Dispatching calls should be efficient, and should have a bounded worst-case execution time. This is important in a language intended for real-time applications. In the intended implementation model, a dispatching call involves calling indirect through the appropriate slot in the dispatch table. No complicated "method lookup" is involved although a call which is dispatching on an interface may require a lookup of the appropriate interface subtag.

The programmer should have the choice at each call site of a dispatching operation whether to do a dispatching call or a statically determined call (i.e. whether the body executed should be determined at run time or at compile time).

The same body should be executed for a call where the tag is statically determined to be T'Tag as for a dispatching call where the tag is found at run time to be T'Tag. This allows one to test a given tagged type with statically determined calls, with some confidence that run-time dispatching will produce the same behavior.

All views of a type should share the same type descriptor and the same tag.

The visibility rules determine what is legal at compile time; they have nothing to do with what bodies can be executed at run time. Thus, it is possible to dispatch to a subprogram whose declaration is not visible at the call site. In fact, this is one of the primary facts that gives object-oriented programming its power. The subprogram that ends up being dispatched to by a given call might even be designed long after the call site has been coded and compiled.

Given that Ada has overloading, determining whether a given subprogram overrides another is based both on the names and the type profiles of the operations.

{AI95-00401-01} When a type extension is declared, if there is any place within its immediate scope where a certain subprogram of the parent or progenitor is visible, then a matching subprogram should override. If there is no such place, then a matching subprogram should be totally unrelated, and occupy a different slot in the type descriptor. This is important to preserve the privacy of private parts; when an operation declared in a private part is inherited, the inherited version can be overridden only in that private part, in the package body, and in any children of the package.

If an implementation shares code for instances of generic bodies, it should be allowed to share type descriptors of tagged types declared in the generic body, so long as they are not extensions of types declared in the specification of the generic unit. 

Glossary entry: The objects of a tagged type have a run-time type tag, which indicates the specific type with which the object was originally created. An operand of a class-wide tagged type can be used in a dispatching call; the tag indicates which subprogram body to invoke. Nondispatching calls, in which the subprogram body to invoke is determined at compile time, are also allowed. Tagged types may be extended with additional components.

Ramification: {AI95-00218-03} If a tagged type is declared other than in a package_specification, it is impossible to add new primitive subprograms for that type, although it can inherit primitive subprograms, and those can be overridden. If the user incorrectly thinks a certain subprogram is primitive when it is not, and tries to call it with a dispatching call, an error message will be given at the call site. Similarly, by using an overriding_indicator (see 6.1), the user can declare that a subprogram is intended to be overriding, and get an error message when they made a mistake. The use of overriding_indicators is highly recommended in new code that does not need to be compatible with Ada 95.

This paragraph was deleted.{AI95-00344-01}

Implementation Note: {AI95-00344-01} In most cases, a tag need only identify a particular tagged type declaration, and can therefore be a simple link-time-known address. However, for tag checks (see 3.9.2) it is essential that each descendant (that currently exists) of a given type have a unique tag. Hence, for types declared in shared generic bodies where an ancestor comes from outside the generic, or for types declared at a deeper level than an ancestor, the tag needs to be augmented with some kind of dynamic descriptor (which may be a static link, global display, instance descriptor pointer, or combination). This implies that type Tag may need to be two words, the second of which is normally null, but in these identified special cases needs to include a static link or equivalent. Within an object of one of these types with a two-word tag, the two parts of the tag would typically be separated, one part as the first word of the object, the second placed in the first extension part that corresponds to a type declared more nested than its parent or declared in a shared generic body when the parent is declared outside. Alternatively, by using an extra level of indirection, the type Tag could remain a single-word.

{AI95-00344-01} For types that are not type extensions (even for ones declared in nested scopes), we do not require that repeated elaborations of the same full_type_declaration correspond to distinct tags. This was done so that Ada 2005 implementations of tagged types could maintain representation compatibility with Ada 95 implementations. Only type extensions that were not allowed in Ada 95 require additional information with the tag. 

To be honest: {AI95-00344-01} The wording “is sufficient to uniquely identify the type among all descendants of the same ancestor” only applies to types that currently exist. It is not necessary to distinguish between descendants that currently exist, and descendants of the same type that no longer exist. For instance, the address of the stack frame of the subprogram that created the tag is sufficient to meet the requirements of this rule, even though it is possible, after the subprogram returns, that a later call of the subprogram could have the same stack frame and thus have an identical tag. 

Reason: Tag is a nonlimited, definite subtype, because it needs the equality operators, so that tag checking makes sense. Also, equality, assignment, and object declaration are all useful capabilities for this subtype.

For an object X and a type T, “X'Tag = T'Tag” is not needed, because a membership test can be used. However, comparing the tags of two objects cannot be done via membership. This is one reason to allow equality for type Tag. 

Reason: {AI95-00260-02} This is similar to the requirement that all access values be initialized to null. 

To be honest: This name, as well as each prefix of it, does not denote a renaming_declaration.

Implementation defined: The result of Tags.Wide_Wide_Expanded_Name for types declared within an unnamed block_statement.

Implementation defined: The sequence of characters of the value returned by Tags.Expanded_Name (respectively, Tags.Wide_Expanded_Name) when some of the graphic characters of Tags.Wide_Wide_Expanded_Name are not defined in Character (respectively, Wide_Character).

Reason: It might seem redundant to provide both the function External_Tag and the attribute External_Tag. The function is needed because the attribute can't be applied to values of type Tag. The attribute is needed so that it can be specified via an attribute_definition_clause.

Reason: {AI95-00279-01} {AI05-0005-1} The check for uncreated library-level types prevents a reference to the type before execution reaches the freezing point of the type. This is important so that T'Class'Input or an instance of Tags.Generic_Dispatching_Constructor do not try to create an object of a type that hasn't been frozen (which might not have yet elaborated its constraints). We don't require this behavior for non-library-level types as the tag can be created multiple times and possibly multiple copies can exist at the same time, making the check complex. 

Reason: Descendant_Tag is used by T'Class'Input to identify the type identified by an external tag. Because there can be multiple elaborations of a given type declaration, Internal_Tag does not have enough information to choose a unique such type. Descendant_Tag does not return the tag for types declared at deeper accessibility levels than the ancestor because there could be ambiguity in the presence of recursion or multiple tasks. Descendant_Tag can be used in constructing a user-defined replacement for T'Class'Input.

{AI05-0113-1} Rules for specifying external tags will usually prevent an external tag from identifying more than one type. However, an external tag can identify multiple types if a generic body contains a derivation of a tagged type declared outside of the generic, and there are multiple instances at the same accessibility level as the type. (The Standard allows default external tags to not be unique in this case.) 

Reason: Is_Descendant_At_Same_Level (or something similar to it) is used by T'Class'Output to determine whether the item being written is at the same accessibility level as T. It may be used to determine prior to using T'Class'Output whether Tag_Error will be raised, and also can be used in constructing a user-defined replacement for T'Class'Output. 

Discussion: In other contexts, “descendant” is dependent on visibility, and the particular view a derived type has of its parent type. See 7.3.1. 

Ramification: {AI12-0005-1} The parent type is always the parent of the full type; a private extension appears to define a parent type, but it does not (only the various forms of derivation do that). As this is a run-time operation, ignoring privacy is OK. 

Ramification: The result of Interface_Ancestor_Tags includes the tag of the parent type, if the parent is an interface.

Indirect interface ancestors are included in the result of Interface_Ancestor_Tags. That's because where an interface appears in the derivation tree has no effect on the semantics of the type; the only interesting property is whether the type has an interface as an ancestor. 

Ramification: This attribute is defined for both specific and class-wide subtypes. The definition is such that S'Class'Class is the same as S'Class.

Note that if S is constrained, S'Class is only partially constrained, since there might be additional discriminants added in descendants of T which are not constrained. 

Reason: {AI95-00326-01} The Class attribute is not defined for untagged subtypes (except for incomplete types and private types whose full view is tagged — see J.11 and 7.3.1) so as to preclude implicit conversion in the absence of run-time type information. If it were defined for untagged subtypes, it would correspond to the concept of universal types provided for the predefined numeric classes. 

Reason: S'Class'Tag equals S'Tag, to avoid generic contract model problems when S'Class is the actual type associated with a generic formal derived type.

Reason: X'Tag is not defined if X is of a specific type. This is primarily to avoid confusion that might result about whether the Tag attribute should reflect the tag of the type of X, or the tag of X. No such confusion is possible if X is of a class-wide type. 

Discussion: This specification is designed to make it easy to create dispatching constructors for streams; in particular, this can be used to construct overridings for T'Class'Input.

Note that any tagged type will match T (see 12.5.1). 

Discussion: The tag of a formal parameter of type T is not necessarily the tag of T, if, for example, the actual was a type conversion. 

Discussion: The tag of an object designated by a value of such an access type might not be T, if, for example, the access value is the result of a type conversion.

Ramification: The tag of an object (even a class-wide one) cannot be changed after it is initialized, since a “class-wide” assignment_statement raises Constraint_Error if the tags don't match, and a “specific” assignment_statement does not affect the tag. 

Implementation Note: {AI95-00318-02} For a limited tagged type, the return object is “built in place” in the ultimate result object with the appropriate tag. For a nonlimited type, a new anonymous object with the appropriate tag is created as part of the function return. See 6.5, “Return Statements”. 

Ramification: The tag check checks both that The_Tag is in T'Class, and that it is not abstract. These checks are similar to the ones required by streams for T'Class'Input (see 13.13.2). In addition, there is a check that the tag identifies a type declared on the current dynamic call chain, and not a more nested type or a type declared by another task. This check is not necessary for streams, because the stream attributes are declared at the same dynamic level as the type used. 

Ramification: One reason that a type might not exist in the partition is that the tag refers to a type whose declaration was elaborated as part of an execution of a subprogram_body which has been left (see 7.6.1).

We exclude tags of library-level types from the current execution of the partition, because misuse of such tags should always be detected. T'Tag freezes the type (and thus creates the tag), and Internal_Tag and Descendant_Tag cannot return the tag of a library-level type that has not been created. All ancestors of a tagged type must be frozen no later than the (full) declaration of a type that uses them, so Parent_Tag and Interface_Ancestor_Tags cannot return a tag that has not been created. Finally, library-level types never cease to exist while the partition is executing. Thus, if the tag comes from a library-level type, there cannot be erroneous execution (the use of Descendant_Tag rather than Internal_Tag can help ensure that the tag is of a library-level type). This is also similar to the rules for T'Class'Input (see 13.13.2).

Discussion: {AI95-00344-01} Ada 95 allowed Tag_Error in this case, or expected the functions to work. This worked because most implementations used tags constructed at link-time, and each elaboration of the same type_declaration produced the same tag. However, Ada 2005 requires at least part of the tags to be dynamically constructed for a type derived from a type at a shallower level. For dynamically constructed tags, detecting the error can be expensive and unreliable. To see this, consider a program containing two tasks. Task A creates a nested tagged type, passes the tag to task B (which saves it), and then terminates. The nested tag (if dynamic) probably will need to refer in some way to the stack frame for task A. If task B later tries to use the tag created by task A, the tag's reference to the stack frame of A probably is a dangling pointer. Avoiding this would require some sort of protected tag manager, which would be a bottleneck in a program's performance. Moreover, we'd still have a race condition; if task A terminated after the tag check, but before the tag was used, we'd still have a problem. That means that all of these operations would have to be serialized. That could be a significant performance drain, whether or not nested tagged types are ever used. Therefore, we allow execution to become erroneous as we do for other dangling pointers. If the implementation can detect the error, we recommend that Tag_Error be raised. 

Reason: {AI95-00260-02} {AI95-00279-01} {AI95-00344-01} Locking would be required to ensure that the mapping of strings to tags never returned tags of types which no longer exist, because types can cease to exist (because they belong to another task, as described above) during the execution of these operations. Moreover, even if these functions did use locking, that would not prevent the type from ceasing to exist at the instant that the function returned. Thus, we do not require the overhead of locking; hence the word “may” in this rule. 

Implementation Advice: Tags.Internal_Tag should return the tag of a type, if one exists, whose innermost master is a master of the point of the function call..

Reason: {AI95-00260-02} {AI95-00344-01} It's not helpful if Internal_Tag returns the tag of some type in another task when one is available in the task that made the call. We don't require this behavior (because it requires the same implementation techniques we decided not to insist on previously), but encourage it. 

Discussion: {AI05-0113-1} There is no Advice for the result of Internal_Tag if no such type exists. In most cases, the Implementation Permission can be used to raise Tag_Error, but some other tag can be returned as well. 

Tagged types are a new concept.

{AI95-00279-01} Amendment Correction: Added wording specifying that Internal_Tag must raise Tag_Error if the tag of a library-level type has not yet been created. Ada 95 gave an Implementation Permission to do this; we require it to avoid erroneous execution when streaming in an object of a library-level type that has not yet been elaborated. This is technically inconsistent; a program that used Internal_Tag outside of streaming and used a compiler that didn't take advantage of the Implementation Permission would not have raised Tag_Error, and may have returned a useful tag. (If the tag was used in streaming, the program would have been erroneous.) Since such a program would not have been portable to a compiler that did take advantage of the Implementation Permission, this is not a significant inconsistency.

{AI95-00417-01} We now define the lower bound of the string returned from [[Wide_]Wide_]Expanded_Name and External_Name. This makes working with the returned string easier, and is consistent with many other string-returning functions in Ada. This is technically an inconsistency; if a program depended on some other lower bound for the string returned from one of these functions, it could fail when compiled with Ada 2005. Such code is not portable even between Ada 95 implementations, so it should be very rare. 

{AI95-00260-02} {AI95-00344-01} {AI95-00400-01} {AI95-00405-01} {AI05-0005-1} Constant No_Tag, and functions Parent_Tag, Interface_Ancestor_Tags, Descendant_Tag, Is_Descendant_At_Same_Level, Wide_Expanded_Name, and Wide_Wide_Expanded_Name are added to Ada.Tags. If Ada.Tags is referenced in a use_clause, and an entity E with the same defining_identifier as a new entity in Ada.Tags is defined in a package that is also referenced in a use_clause, the entity E may no longer be use-visible, resulting in errors. This should be rare and is easily fixed if it does occur. 

{AI95-00362-01} Ada.Tags is now defined to be preelaborated.

{AI95-00260-02} Generic function Tags.Generic_Dispatching_Constructor is new. 

{AI95-00318-02} We talk about return objects rather than return expressions, as functions can return using an extended_return_statement.

{AI95-00344-01} Added wording to define that tags for all descendants of a tagged type must be distinct. This is needed to ensure that more nested type extensions will work properly. The wording does not require implementation changes for types that were allowed in Ada 95. 

{AI05-0113-1} Correction: Added wording specifying that Dependent_Tag must raise Tag_Error if there is more than one type which matches the requirements. If an implementation had returned a random tag of the matching types, a program may have worked properly. However, such a program would not be portable (another implementation may return a different tag) and the conditions that would cause the problem are unlikely (most likely, a tagged type extension declared in a generic body with multiple instances in the same scope). 

{AI05-0173-1} Function Is_Abstract is added to Ada.Tags. If Ada.Tags is referenced in a use_clause, and an entity E with the defining_identifier Is_Abstract is defined in a package that is also referenced in a use_clause, the entity E may no longer be use-visible, resulting in errors. This should be rare and is easily fixed if it does occur. 

{AI05-0115-1} Correction: We explicitly define the meaning of "descendant" at runtime, so that it does not depend on visibility as does the usual meaning.