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12.5.1 Formal Private and Derived Types

1/2
{AI95-00442-01} [In its most general form, the category The class determined for a formal private type is all types, but it can be restricted to only nonlimited types or to only tagged types can be either limited or nonlimited, and either tagged or untagged; no more specific class is known for such a type. The category class determined for a formal derived type is the derivation class rooted at the ancestor type.] 
1.a/2
Proof: {AI95-00442-01} The first rule is given normatively below, and the second rule is given normatively in 12.5; they are repeated here to give a capsule summary of what this subclause is about. 

Syntax

2
formal_private_type_definition ::= [[abstracttagged] [limitedprivate
3/2
{AI95-00251-01} {AI95-00419-01} {AI95-00443-01} formal_derived_type_definition ::= 
     
[abstract[limited | synchronizednew subtype_mark [[and interface_list] with private]

Legality Rules

4
If a generic formal type declaration has a known_discriminant_part, then it shall not include a default_expression for a discriminant. 
4.a
Ramification: Consequently, a generic formal subtype with a known_discriminant_part is an indefinite subtype, so the declaration of a stand-alone variable has to provide a constraint on such a subtype, either explicitly, or by its initial value. 
5/2
{AI95-00401-01} {AI95-00419-01} {AI95-00443-01} {ancestor subtype (of a formal derived type)} {private extension [partial]} The ancestor subtype of a formal derived type is the subtype denoted by the subtype_mark of the formal_derived_type_definition. For a formal derived type declaration, the reserved words with private shall appear if and only if the ancestor type is a tagged type; in this case the formal derived type is a private extension of the ancestor type and the ancestor shall not be a class-wide type. [Similarly, an interface_list or the optional reserved words word abstract or synchronized shall appear only if the ancestor type is a tagged type]. The reserved word limited or synchronized shall appear only if the ancestor type [and any progenitor types] are limited types. The reserved word synchronized shall appear (rather than limited) if the ancestor type or any of the progenitor types are synchronized interfaces.
5.a
Reason: We use the term “ancestor” here instead of “parent” because the actual can be any descendant of the ancestor, not necessarily a direct descendant.
5.b/2
{AI95-00419-01} We require the ancestor type to be limited when limited appears so that we avoid oddies like limited integer types. Normally, limited means “match anything” for a generic formal, but it was felt that allowing limited elementary types to be declared was just too weird. Integer still matches a formal limited private type; it is only a problem when the type is known to be elementary. Note that the progenitors are required to be limited by rules in 3.9.4, thus that part of the rule is redundant.
5.c/2
{AI95-00443-01} We require that synchronized appear if the ancestor or any of the progenitors are synchronized, so that property is explicitly given in the program text – it is not automatically inherited from the ancestors. However, it can be given even if neither the ancestor nor the progenitors are synchronized. 
5.1/2
  {AI95-00251-01} {AI95-00401-01} {AI95-00443-01} The actual type for a formal derived type shall be a descendant of [the ancestor type and] every progenitor of the formal type. If the reserved word synchronized appears in the declaration of the formal derived type, the actual type shall be a synchronized tagged type. 
5.d/2
Proof: The actual type has to be a descendant of the ancestor type, in order that it be in the correct class. Thus, that part of the rule is redundant.
5.e/2
Discussion: For a non-formal private extension, we require the partial view to be synchronized if the full view is synchronized tagged. This does not apply to a formal private extension — it is OK if the formal is not synchronized. Any attempt to extend the formal type will be rechecked in the instance, where the rule disallowing extending a sychronized non-interface type will be enforced. This is consistent with the “no hidden interfaces” rule also applying only to non-formal private extensions, as well as the rule that a limited non-formal private extension implies a limited full type. Formal private extensions are exempted from all these rules to enable the construction of generics that can be used with the widest possible range of types. In particular, an indefinite tagged limited formal private type can match any “concrete” actual tagged type. 
6
If the formal subtype is definite, then the actual subtype shall also be definite. 
6.a
Ramification: On the other hand, for an indefinite formal subtype, the actual can be either definite or indefinite.
7
For a generic formal derived type with no discriminant_part:
8
8.a
Ramification: In other words, any constraint on the ancestor subtype is considered part of the “contract.”
9
9.a
Reason: This rule ensures that if a composite constraint is allowed on the formal, one is also allowed on the actual. If the ancestor subtype is an unconstrained scalar subtype, the actual is allowed to be constrained, since a scalar constraint does not cause further constraints to be illegal. 
10
10.a
Reason: This ensures that if a discriminant constraint is given on the formal subtype, the corresponding constraint in the instance will make sense, without additional run-time checks. This is not necessary for arrays, since the bounds cannot be overridden in a type extension. An unknown_discriminant_part may be used to relax these matching requirements. 
10.1/2
10.b/2
Reason: We require that the “excludes null” property match, because it would be difficult to write a correct generic for a formal access type without knowing this property. Many typical algorithms and techniques will not work for a subtype that excludes null (setting an unused component to null, default-initialized objects, and so on). We want this sort of requirement to be reflected in the contract of the generic.
11
The declaration of a formal derived type shall not have a known_discriminant_part. For a generic formal private type with a known_discriminant_part:
12
13
14
14.a
Reason: We considered defining the first and third rule to be called “subtype conformance” for discriminant_parts. We rejected that idea, because it would require implicit (inherited) discriminant_parts, which seemed like too much mechanism. 
15
[For a generic formal type with an unknown_discriminant_part, the actual may, but need not, have discriminants, and may be definite or indefinite.]

Static Semantics

16/2
 {AI95-00442-01} The category class determined for a formal private type is as follows: 
17/2
Type Definition  Determined Category Class

limited private  the category class of all types
private  the category class of all nonlimited types
tagged limited private  the category class of all tagged types
tagged private  the category class of all nonlimited tagged types
18
[The presence of the reserved word abstract determines whether the actual type may be abstract.]
19
A formal private or derived type is a private or derived type, respectively. A formal derived tagged type is a private extension. [A formal private or derived type is abstract if the reserved word abstract appears in its declaration.]
20/2
 {AI95-00233-01} If the ancestor type is a composite type that is not an array type, the formal type inherits components from the ancestor type (including discriminants if a new discriminant_part is not specified), as for a derived type defined by a derived_type_definition (see 3.4 and 7.3.1).
21/2
 {8652/0038} {AI95-00202} {AI95-00233-01} {AI95-00401-01} For a formal derived type, the predefined operators and inherited user-defined subprograms are determined by the ancestor type and any progenitor types, and are implicitly declared at the earliest place, if any, immediately within the declarative region in which within the immediate scope of the formal type is declared, where the corresponding primitive subprogram of the ancestor or progenitor is visible (see 7.3.1). In an instance, the copy of such an implicit declaration declares a view of the corresponding primitive subprogram of the ancestor or progenitor of the formal derived type, even if this primitive has been overridden for the actual type. When the ancestor or progenitor of the formal derived type is itself a formal type, the copy of the implicit declaration declares a view of the corresponding copied operation of the ancestor or progenitor. [In the case of a formal private extension, however, the tag of the formal type is that of the actual type, so if the tag in a call is statically determined to be that of the formal type, the body executed will be that corresponding to the actual type.] 
21.a/2
Ramification: {AI95-00401-01} The above rule defining the properties of primitive subprograms in an instance applies even if the subprogram has been overridden or hidden for the actual type. This rule is necessary for untagged types, because their primitive subprograms might have been overridden by operations that are not subtype-conformant with the operations defined for the class. For tagged types, the rule still applies, but the primitive subprograms will dispatch to the appropriate implementation based on the type and tag of the operands. Even for tagged types, the formal parameter names and default_expressions are determined by those of the primitive subprograms of the specified ancestor type (or progenitor type, for subprograms inherited from an interface type)
22/1
 For a prefix prefix S that denotes a formal indefinite subtype, the following attribute is defined: 
23
S'Definite
S'Definite yields True if the actual subtype corresponding to S is definite; otherwise it yields False. The value of this attribute is of the predefined type Boolean. 
23.a/2
Discussion: {AI95-00114-01} Whether an actual subtype is definite or indefinite may have a major effect on the algorithm used in a generic. For example, in a generic I/O package, whether to use fixed-length or variable-length records could depend on whether the actual is definite or indefinite. This attribute is essentially a replacement for the Constrained attribute, which is now considered obsolete. 

Dynamic Semantics

23.1/2
   {AI95-00158-01} In the case where a formal type is tagged with unknown discriminants, and the actual type is a class-wide type T'Class:
23.2/2
23.3/2
23.b/2
Discussion: As it states in 6.3.1, the convention of an inherited subprogram of a generic formal tagged type with unknown discriminants is intrinsic.
23.c/2
In the case of a corresponding primitive of T with no controlling formal parameters, the context of the call provides the controlling tag value for the dispatch. If no tag is provided by context, Program_Error is raised rather than resorting to a nondispatching call. For example:
23.d/2
generic
   type NT(<>) is new T with private;
    -- Assume T has operation "function Empty return T;"
package G is
   procedure Test(X : in out NT);
end G;
23.e/2
package body G is
   procedure Test(X : in out NT) is
   begin
      X := Empty;  -- Dispatching based on X'Tag takes
                   -- place if actual is class-wide.
      declare
          Y : NT := Empty;
                   -- If actual is class-wide, this raises Program_Error
                   -- as there is no tag provided by context.
      begin
          X := Y;  -- We never get this far.
      end;
   end Test;
end G;
23.f/2
type T1 is new T with null record;
package I is new G(T1'Class);
NOTES
24/2
9  {AI95-00442-01} In accordance with the general rule that the actual type shall belong to the category class determined for the formal (see 12.5, “Formal Types”): 
25
26
27
10  The actual type can be abstract only if the formal type is abstract (see 3.9.3).
27.a
Reason: This is necessary to avoid contract model problems, since one or more of its primitive subprograms are abstract; it is forbidden to create objects of the type, or to declare functions returning the type. 
27.b
Ramification: On the other hand, it is OK to pass a non-abstract actual to an abstract formal — abstract on the formal indicates that the actual might be abstract. 
28
11  If the formal has a discriminant_part, the actual can be either definite or indefinite. Otherwise, the actual has to be definite. 

Incompatibilities With Ada 83

28.a
{incompatibilities with Ada 83} Ada 83 does not have unknown_discriminant_parts, so it allows indefinite subtypes to be passed to definite formals, and applies a legality rule to the instance body. This is a contract model violation. Ada 95 disallows such cases at the point of the instantiation. The workaround is to add (<>) as the discriminant_part of any formal subtype if it is intended to be used with indefinite actuals. If that's the intent, then there can't be anything in the generic body that would require a definite subtype.
28.b
The check for discriminant subtype matching is changed from a run-time check to a compile-time check. 

Extensions to Ada 95

28.c/2
{AI95-00251-01} {AI95-00401-01} {AI95-00419-01} {AI95-00443-01} {extensions to Ada 95} A generic formal derived type can include progenitors (interfaces) as well as a primary ancestor. It also may include limited to indicate that it is a limited type, and synchronized to indicate that it is a synchronized type. 

Wording Changes from Ada 95

28.d/2
{8652/0038} {AI95-00202-01} Corrigendum: Corrected wording to define the operations that are inherited when the ancestor of a formal type is itself a formal type to avoid anomalies.
28.e/2
{AI95-00158-01} Added a semantic description of the meaning of operations of an actual class-wide type, as such a type does not have primitive operations of its own.
28.f/2
{AI95-00231-01} Added a matching rule for access subtypes that exclude null.
28.g/2
{AI95-00233-01} The wording for the declaration of implicit operations is corrected to be consistent with 7.3.1 as modified by Corrigendum 1.
28.h/2
{AI95-00442-01} We change to “determines a category” as that is the new terminology (it avoids confusion, since not all interesting properties form a class). 

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