Genericity in Ada

Generic Subprograms

Generic subprograms are usefull to create a subprograms that have the same structure for several types. For example, to swap two objects:

generic
    type A_Type is private;
procedure Swap (Left, Right : in out A_Type) is
    Temp : A_Type := Left;
begin
    Left := Right;
    Right := Temp;
end Swap;

Generic Packages

In Ada generic package, upon instantiation, data are duplicated; that is, if they contain global variables, each instance will have its own copy of the variable, properly typed and independent from the others.

generic
    type T is private;
package Gen is
    type C is tagged record
        V : T;
    end record;
    G : Integer;
end Gen;

Generic Parameters

Ada offers a wide variety of generic parameters which is difficult to translate into other languages. The parameters used during instantiation and as a consequence those on which the generic unit may rely on may be variables, types, subprograms, or package instances, with certain properties. For example, the following provides a sort algorithm for any kind of array:

generic
    type Component is private;
    type Index is (<>);
    with function "<" (Left, Right : Component) return Boolean;
    type Array_Type is array (Index range <>) of Component;
procedure Sort (A : in out Array_Type);