- A generic formal subtype can be used to pass to a generic unit a subtype
whose type is in a certain class of types.
type defining_identifier[discriminant_part] is formal_type_definition;
- For a generic formal subtype, the actual shall be a subtype_mark; it
denotes the (generic) actual subtype.
- A formal_type_declaration declares a (generic) formal type, and its first
subtype, the (generic) formal subtype.
- The form of a formal_type_definition determines a class to which the
formal type belongs. For a formal_private_type_definition the reserved words
tagged and limited indicate the class (see 12.5.1). For a formal_derived_type_definition the class is the derivation class rooted at the ancestor
type. For other formal types, the name of the syntactic category indicates
the class; a formal_discrete_type_definition defines a discrete type, and so
- The actual type shall be in the class determined for the formal.
- The formal type also belongs to each class that contains the determined
class. The primitive subprograms of the type are as for any type in the
determined class. For a formal type other than a formal derived type, these
are the predefined operators of the type; they are implicitly declared
immediately after the declaration of the formal type. In an instance, the
copy of such an implicit declaration declares a view of the predefined
operator of the actual type, even if this operator has been overridden for
the actual type. The rules specific to formal derived types are given in
(7) Generic formal types, like all types, are not named. Instead, a
name can denote a generic formal subtype. Within a generic unit, a
generic formal type is considered as being distinct from all other
(formal or nonformal) types.
(8) A discriminant_part is allowed only for certain kinds of types, and
therefore only for certain kinds of generic formal types. See 3.7.
- Examples of generic formal types:
type Item is private;
type Buffer(Length : Natural) is limited private;
type Enum is (<>);
type Int is range <>;
type Angle is delta <>;
type Mass is digits <>;
type Table is array (Enum) of Item;
- Example of a generic formal part declaring a formal integer type:
type Rank is range <>;
First : Rank := Rank'First;
Second : Rank := First + 1; -- the operator "+" of the type Rank
- Formal Private and Derived Types
- Formal Scalar Types
- Formal Array Types
- Formal Access Types
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