Ada Reference Manual (Ada 2022) — Legal Information

# 12.1 Generic Declarations

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A generic_declaration declares a generic unit, which is either a generic subprogram or a generic package. A generic_declaration includes a generic_formal_part declaring any generic formal parameters. A generic formal parameter can be an object; alternatively (unlike a parameter of a subprogram), it can be a type, a subprogram, or a package.

#### Syntax

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generic_declaration ::=
generic_subprogram_declaration | generic_package_declaration
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generic_subprogram_declaration ::=
generic_formal_part subprogram_specification
[aspect_specification];
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generic_package_declaration ::=
generic_formal_part package_specification;
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generic_formal_part ::=
generic {generic_formal_parameter_declaration | use_clause}
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generic_formal_parameter_declaration ::=
formal_object_declaration
| formal_type_declaration
| formal_subprogram_declaration
| formal_package_declaration
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The only form of subtype_indication allowed within a generic_formal_part is a subtype_mark (that is, the subtype_indication shall not include an explicit constraint). The defining name of a generic subprogram shall be an identifier (not an operator_symbol).

#### Static Semantics

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A generic_declaration declares a generic unit — a generic package, generic procedure, or generic function, as appropriate.
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An entity is a generic formal entity if it is declared by a generic_formal_parameter_declaration. “Generic formal”, or simply “formal”, is used as a prefix in referring to objects, subtypes (and types), functions, procedures and packages, that are generic formal entities, as well as to their respective declarations. Examples: “generic formal procedure” or a “formal integer type declaration”.
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The list of generic_formal_parameter_declarations of a generic_formal_part form a declaration list of the generic unit.

#### Dynamic Semantics

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The elaboration of a generic_declaration has no effect.
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NOTE 1 Outside a generic unit a name that denotes the generic_declaration denotes the generic unit. In contrast, within the declarative region of the generic unit, a name that denotes the generic_declaration denotes the current instance.
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NOTE 2 Within a generic subprogram_body, the name of this program unit acts as the name of a subprogram. Hence this name can be overloaded, and it can appear in a recursive call of the current instance. For the same reason, this name cannot appear after the reserved word new in a (recursive) generic_instantiation.
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NOTE 3 A default_expression or default_name appearing in a generic_formal_part is not evaluated during elaboration of the generic_formal_part; instead, it is evaluated when used. However, the usual visibility rules apply to any name used in a default, with name resolution performed based on the location of the name within the generic_formal_part.

#### Examples

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Examples of generic formal parts:
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generic -- parameterless
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generic
Size : Natural; -- formal object
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generic
Length : Integer := 200; -- formal object with a default expression
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Area : Integer := Length*Length; -- formal object with a default expression
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generic
type Item is private; -- formal type
type Index is (<>); -- formal type
type Row is array(Index range <>) of Item; -- formal type
with function "<"(X, Y : Item) return Boolean; -- formal subprogram
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Examples of generic declarations declaring generic subprograms Exchange and Squaring:
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generic
type Elem is private;
procedure Exchange(U, V : in out Elem);
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generic
type Item (<>) is private;
with function "*"(U, V : Item) return Item is <>;
function Squaring(X : Item) return Item;
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Example of a generic declaration declaring a generic package:
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generic
type Item is private;
type Vector is array (Positive range <>) of Item;
with function Sum(X, Y : Item) return Item;
package On_Vectors is
function Sum (A, B : Vector) return Vector;
function Sigma(A : Vector) return Item;
Length_Error : exception;
end On_Vectors;

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