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A generic formal subtype can be used to pass to a generic unit a subtype
whose type is in a certain class of types.
Syntax
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formal_type_declaration ::=
type defining_identifier[discriminant_part] is
formal_type_definition;
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formal_type_definition ::=
formal_private_type_definition
| formal_derived_type_definition
| formal_discrete_type_definition
| formal_signed_integer_type_definition
| formal_modular_type_definition
| formal_floating_point_definition
| formal_ordinary_fixed_point_definition
| formal_decimal_fixed_point_definition
| formal_array_type_definition
| formal_access_type_definition
Legality Rules
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For a generic formal subtype, the actual shall be a subtype_mark; it
denotes the (generic) actual subtype.
Static Semantics
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A formal_type_declaration declares a (generic) formal type, and its
first subtype, the (generic) formal subtype.
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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 section 12.5.1 Formal Private and Derived Types. 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 on.
Legality Rules
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The actual type shall be in the class determined for the formal.
Static Semantics
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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 See section 12.5.1 Formal Private and Derived Types.
NOTES
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(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.
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(8) A discriminant_part is allowed only for certain kinds of types, and
therefore only for certain kinds of generic formal types. See section 3.7 Discriminants.
Examples
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Examples of generic formal types:
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type Item is private;
type Buffer(Length : Natural) is limited private;
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type Enum is (<>);
type Int is range <>;
type Angle is delta <>;
type Mass is digits <>;
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type Table is array (Enum) of Item;
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Example of a generic formal part declaring a formal integer type:
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generic
type Rank is range <>;
First : Rank := Rank'First;
Second : Rank := First + 1;
-- the operator "+" of the type Rank