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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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.”
Dynamic Semantics
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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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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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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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