Ada Reference Manual (Ada 2022) — Legal Information

# 12.5.1 Formal Private and Derived Types

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In its most general form, the category determined for a formal private type is all types, but the category can be restricted to only nonlimited types or to only tagged types. Similarly, the category for a formal incomplete type is all types but the category can be restricted to only tagged types; unlike other formal types, the actual type can be incompletely defined, and not ready to be frozen (see 13.14). The category determined for a formal derived type is the derivation class rooted at the ancestor type.

#### Syntax

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formal_private_type_definition ::= [[abstract] tagged] [limited] private
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formal_derived_type_definition ::=
[abstract] [limited | synchronized] new subtype_mark [[and interface_list] with private]

#### Legality Rules

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If a generic formal type declaration has a known_discriminant_part, then it shall not include a default_expression for a discriminant.
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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 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. The ancestor type shall be a limited interface if the reserved word synchronized appears.
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The actual type for a formal derived type shall be a descendant of the ancestor type and every progenitor of the formal type. The actual type for a formal derived type shall be tagged if and only if the formal derived type is a private extension. If the reserved word synchronized appears in the declaration of the formal derived type, the actual type shall be a synchronized tagged type.
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If a formal private or derived subtype is definite, then the actual subtype shall also be definite. If the formal type is nonlimited, the actual type shall be nonlimited.
6.1/3
A formal_incomplete_type_declaration declares a formal incomplete type. The only view of a formal incomplete type is an incomplete view. Thus, a formal incomplete type is subject to the same usage restrictions as any other incomplete type — see 3.10.1.
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For a generic formal derived type with no discriminant_part, the actual subtype shall be statically compatible with the ancestor subtype. Furthermore:
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If the ancestor subtype is constrained, the actual subtype shall be constrained;
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If the ancestor subtype is an unconstrained access or composite subtype, the actual subtype shall be unconstrained.
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If the ancestor subtype is an unconstrained discriminated subtype, then the actual shall have the same number of discriminants, and each discriminant of the actual shall correspond to a discriminant of the ancestor, in the sense of 3.7.
10.1/2
If the ancestor subtype is an access subtype, the actual subtype shall exclude null if and only if the ancestor subtype excludes null.
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The declaration of a formal derived type shall not have a known_discriminant_part. For a generic formal private or incomplete type with a known_discriminant_part:
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The actual type shall be a type with the same number of discriminants.
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The actual subtype shall be unconstrained.
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The subtype of each discriminant of the actual type shall statically match the subtype of the corresponding discriminant of the formal type.
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For a generic formal type with an unknown_discriminant_part, the actual may have discriminants, though that is not required, and may be definite or indefinite.
15.1/4
When enforcing Legality Rules, for the purposes of determining within a generic body whether a type is unconstrained in any partial view, a discriminated subtype is considered to have a constrained partial view if it is a descendant of an untagged generic formal private or derived type.

#### Static Semantics

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The category determined for a formal private type is as follows:
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Type Definition Determined Category
limited private the category of all types
private the category of all nonlimited types
tagged limited private the category of all tagged types
tagged private the category of all nonlimited tagged types
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The presence of the reserved word abstract determines whether the actual type may be abstract.
18.1/3
The category determined for a formal incomplete type is the category of all types, unless the formal_type_declaration includes the reserved word tagged; in this case, it is the category of all tagged types.
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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.
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For a formal derived type, the characteristics (including components, but excluding discriminants if there is a new discriminant_part), predefined operators, and inherited user-defined primitive subprograms are determined by its ancestor type and its progenitor types (if any), in the same way that those of a derived type are determined by those of its parent type and its progenitor types (see 3.4 and 7.3.1).
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In an instance, the copy of an implicit declaration of a primitive subprogram of a formal derived type 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 and even if it is never declared 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.
21.1/5
In an instance, the implicitly composed and additive aspects (see 13.1.1) of a formal type are those of the actual; for a nonoverridable aspect, a formal derived type inherits the aspect if the ancestor or any progenitor has the aspect, according to the rules given in 13.1.
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For a prefix S that denotes a formal indefinite subtype, the following attribute is defined:
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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.

#### Dynamic Semantics

23.1/3
In the case where a formal type has unknown discriminants, and the actual type is a class-wide type T'Class:
23.2/2
For the purposes of defining the primitive operations of the formal type, each of the primitive operations of the actual type is considered to be a subprogram (with an intrinsic calling convention — see 6.3.1) whose body consists of a dispatching call upon the corresponding operation of T, with its formal parameters as the actual parameters. If it is a function, the result of the dispatching call is returned.
23.3/2
If the corresponding operation of T has no controlling formal parameters, then the controlling tag value is determined by the context of the call, according to the rules for tag-indeterminate calls (see 3.9.2 and 5.2). In the case where the tag would be statically determined to be that of the formal type, the call raises Program_Error. If such a function is renamed, any call on the renaming raises Program_Error.
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NOTE 1 The actual type can be abstract only if the formal type is abstract (see 3.9.3).
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NOTE 2 If the formal has a discriminant_part, the actual can be either definite or indefinite. Otherwise, the actual can only be definite.

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