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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;
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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.” 

Dynamic Semantics

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The elaboration of a generic_declaration has no effect. 
NOTES
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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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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. (The usual visibility rules apply to any name used in a default: the denoted declaration therefore has to be visible at the place of the expression.) 

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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