3.7 Discriminants

Glossary entry: {Discriminant} A discriminant is a parameter for a composite type. It can control, for example, the bounds of a component of the type if the component is an array. A discriminant for a task type can be used to pass data to a task of the type upon creation.

Discussion: {AI95-00114-01} {unknown discriminants [partial]} {discriminants (unknown) [partial]} A view of a type, and all subtypes of the view, have unknown discriminants when the number or names of the discriminants, if any, are unknown at the point of the type declaration for the view. A discriminant_part of (<>) is used to indicate unknown discriminants. 

{AI95-00402-01} When an access discriminant is initialized at the time of object creation with an allocator of an anonymous type, the allocated object and the object with the discriminant are tied together for their lifetime. They should be allocated out of the same storage pool, and then at the end of the lifetime of the enclosing object, finalized and reclaimed together. In this case, the allocated object is called a coextension (see 3.10.2).

Discussion: The above principle when applied to a nonlimited type implies that such an object may be copied only to a shorter-lived object, because attempting to assign it to a longer-lived object would fail because the access discriminants would not match. In a copy, the lifetime connection between the enclosing object and the allocated object does not exist. The allocated object is tied in the above sense only to the original object. Other copies have only secondary references to it.

Note that when an allocator appears as a constraint on an access discriminant in a subtype_indication that is elaborated independently from object creation, no such connection exists. For example, if a named constrained subtype is declared via "subtype Constr is Rec(Acc_Discrim => new T);" or if such an allocator appears in the subtype_indication for a component, the allocator is evaluated when the subtype_indication is elaborated, and hence its lifetime is typically longer than the objects or components that will later be subject to the constraint. In these cases, the allocated object should not be reclaimed until the subtype_indication goes out of scope. 

Implementation Note: Discriminants on array types were considered, but were omitted to ease (existing) implementations.

Discussion: Note that the above definition for “discriminated type” does not include types declared with an unknown_discriminant_part. This seems consistent with Ada 83, where such types (in a generic formal part) would not be considered discriminated types. Furthermore, the full type for a type with unknown discriminants need not even be composite, much less have any discriminants.

{8652/0007} {AI95-00098-01} On the other hand, unknown_discriminant_parts cannot be applied to type declarations that cannot have a known_discriminant_part. There is no point in having unknown discriminants on a type that can never have discriminants (for instance, a formal modular type), even when these are allowed syntactically. 

This paragraph was deleted.{AI95-00230-01}

Reason: Note that discriminants of a named access type are not considered “access discriminants.” Similarly, “access parameter” only refers to a formal parameter defined by an access_definition.

Reason: The all-or-none rule is related to the rule that a discriminant constraint shall specify values for all discriminants. One could imagine a different rule that allowed a constraint to specify only some of the discriminants, with the others provided by default. Having defaults for discriminants has a special significance — it allows objects of the type to be unconstrained, with the discriminants alterable as part of assigning to the object.

Defaults for discriminants of tagged types are disallowed so that every object of a tagged type is constrained, either by an explicit constraint, or by its initial discriminant values. This substantially simplifies the semantic rules and the implementation of inherited dispatching operations. For generic formal types, the restriction simplifies the type matching rules. If one simply wants a "default" value for the discriminants, a constrained subtype can be declared for future use. 

Discussion: This rule implies that a type can have a default for an access discriminant if the type is limited, but not if the only reason it's limited is because of a limited component. Compare with the definition of limited type in 7.5. Also, recall that a “descendant’ includes the type itself, so an explicitly limited record type can have defaults. 

Reason: {AI95-00230-01} We considered the following rules for access discriminants: 

Implementation Note: This ensures that the new discriminant can share storage with an existing discriminant.

Reason: This ensures that on conversion (or extension via an extension aggregate) to a distantly related type, if the discriminants satisfy the target type's requirements they satisfy all the intermediate types' requirements as well. 

Ramification: There is no requirement that the new discriminant have the same (or any) default_expression as the parent's discriminant. 

Ramification: This requires convertibility of the designated subtypes. 

Ramification: The correspondence relationship is transitive, symmetric, and reflexive. That is, if A corresponds to B, and B corresponds to C, then A, B, and C each corresponds to A, B, and C in all combinations.

Ramification: A constraint in a task_body is not considered to depend on a discriminant of the task type, even if it names it. It is only the constraints in the type definition itself that are considered dependents. Similarly for protected types. 

Ramification: A component does not depend on a discriminant just because its default_expression refers to the discriminant.

Reason: When the parent subtype depends on a discriminant, the parent part of the derived type is treated like a discriminant-dependent component. 

Ramification: Because of this rule, we don't really need to worry about “corresponding” discriminants, since all the inherited components will be discriminant-dependent if there is a new known_discriminant_part whose discriminants are used to constrain the old discriminants. 

Reason: The concept of discriminant-dependent (sub)components is primarily used in various rules that disallow renaming or 'Access, or specify that certain discriminant-changing assignments are erroneous. The goal is to allow implementations to move around or change the size of discriminant-dependent subcomponents upon a discriminant-changing assignment to an enclosing object. The above definition specifies that all subcomponents of a discriminant-dependent component or parent part are themselves discriminant-dependent, even though their presence or size does not in fact depend on a discriminant. This is because it is likely that they will move in a discriminant-changing assignment if they are a component of one of several discriminant-dependent parts of the same record. 

To be honest: Which values are present might depend on discriminants of some ancestor type that are constrained in an intervening derived_type_definition. That's why we say "values of discriminants" instead of "values of the discriminants" — a subtle point.

Discussion: {AI95-00114-01} An unknown_discriminant_part “(<>)” is only permitted in the declaration of a (generic or nongeneric) private type, private extension, incomplete type, or formal derived type. Hence, only such types, descendants thereof, and class-wide types can have unknown discriminants. An unknown_discriminant_part is used to indicate that the corresponding actual or full type might have discriminants without defaults, or be an unconstrained array subtype. Tagged class-wide types are also considered to have unknown discriminants because discriminants can be added by type extensions, so the total number of discriminants of any given value of a tagged class-wide type is not known at compile time.

{AI95-00287-01} A subtype with unknown discriminants is indefinite, and hence an object of such a subtype needs explicit initialization. A limited private type with unknown discriminants is “extremely” limited; objects of such a type can be initialized only by subprograms (either procedures with a parameter of the type, or a function returning the type) declared in the package. Subprograms declared elsewhere can operate on and even return the type, but they can only initialize the object by calling (ultimately) a subprogram in the package declaring the type. Such a type is useful for keeping complete control over object creation within the package declaring the type.

A partial view of a type might have unknown discriminants, while the full view of the same type might have known, unknown, or no discriminants. 

Ramification: {AI95-00231-01} {AI95-00416-01} The conversion of the expression defining the access discriminant to the anonymous access type raises Program_Error for an object created by an allocator of an access type T, if the initial value is an access parameter that designates a view whose accessibility level is deeper than that of T. 

Discussion: This connection between discriminant defaults and unconstrained variables can be a source of confusion. For Ada 95, we considered various ways to break the connection between defaults and unconstrainedness, but ultimately gave up for lack of a sufficiently simple and intuitive alternative.

{mutable} An unconstrained discriminated subtype with defaults is called a mutable subtype, and a variable of such a subtype is called a mutable variable, because the discriminants of such a variable can change. There are no mutable arrays (that is, the bounds of an array object can never change), because there is no way in the language to define default values for the bounds. Similarly, there are no mutable class-wide subtypes, because there is no way to define the default tag, and defaults for discriminants are not allowed in the tagged case. Mutable tags would also require a way for the maximum possible size of such a class-wide subtype to be known. (In some implementations, all mutable variables are allocated with the maximum possible size. This approach is appropriate for real-time applications where implicit use of the heap is inappropriate.)

Discussion: {AI95-00114-01} An unknown_discriminant_part is permitted only in the declaration of a private type (including generic formal private), private extension, incomplete type, or generic formal derived type. These are the things that will have a corresponding completion or generic actual, which will either define the discriminants, or say there are none. The (<>) indicates that the actual/full subtype might be an indefinite subtype. An unknown_discriminant_part is not permitted in a normal untagged derived type declaration, because there is no separate full type declaration for such a type. Note that (<>) allows unconstrained array bounds; those are somewhat like undefaulted discriminants.

For a derived type, either the discriminants are inherited as is, or completely respecified in a new discriminant_part. In this latter case, each discriminant of the parent type shall be constrained, either to a specific value, or to equal one of the new discriminants. Constraining a parent type's discriminant to equal one of the new discriminants is like a renaming of the discriminant, except that the subtype of the new discriminant can be more restrictive than that of the parent's one. In any case, the new discriminant can share storage with the parent's discriminant. 

{extensions to Ada 83} The syntax for a discriminant_specification is modified to allow an access discriminant, with a type specified by an access_definition (see 3.10).

{AI95-00251-01} Discriminants are allowed on all composite types other than array and interface types.

Discriminants may be of an access type. 

Discriminant_parts are not elaborated, though an access_definition is elaborated when the discriminant is initialized.

{AI95-00230-01} {AI95-00402-01} {AI95-00416-01} Access discriminants (anonymous access types used as a discriminant) can be used on any type allowing discriminants. Defaults aren't allowed on discriminants of non-limited types, however, so that accessibility problems don't happen on assignment.

{AI95-00231-01} null_exclusion can be used in the declaration of a discriminant. 

{8652/0007} {AI95-00098-01} Corrigendum: The wording was clarified so that types that cannot have discriminants cannot have an unknown_discriminant_part.

{AI95-00251-01} Added wording to prevent interfaces from having discriminants. We don't want interfaces to have any components.

{AI95-00254-01} Removed wording which implied or required an access discriminant to have an access-to-object type (anonymous access types can now be access-to-subprogram types as well).

{AI95-00326-01} Fixed the wording of the introduction to this clause to reflect that both incomplete and partial views can have unknown discriminants. That was always true, but for some reason this wording specified partial views.

{AI95-00419-01} Changed the wording to use the new term “explicitly limited record”, which makes the intent much clearer (and eliminates confusion with derived types that happen to contain the reserved word limited.