Object-oriented Software in Ada 95

198 Child libraries
1.1.2 Implementation techniques for a generic package
The implementation of a generic package is usually performed by one of the following mechanisms:
• A new package is generated for each unique instantiation of the generic package. This is sometimes
referred to as the macro implementation.
• A single code body is used which can cater for different formal types.
14.4 Generic formal subprograms
When specifying a generic formal type, the compiler must know how to perform operations on an instance of this
type. For example, if the formal type is specified as private, then any Ada private type can be used. The
implementor of the body of the generic package may wish to use instances of this type in a comparison. For
example, as part of a logical expression:
if Instance_Of_Formal_Type > Another_Instance_Of_Formal_type then
...
end if;
For this to be allowed, the type passed must allow, in this case, for the ">" operation to be performed between
instances of the generic type. To enforce this contract the specification part of the generic procedure, function or
package must include a generic formal parameter for the ">" logical operation. Remember, the type passed may
not have ">" defined between instances of the type. For example, the class Account does not provide any
comparison operators between instances of an Account.
The following generic procedure which orders its formal parameters of type private is defined with a
formal subprogram specification for ">".
generic
--Specification
type T is private; --Any non limited type
with function ">" ( A, B:in T )
return Boolean is ; --Need def for >
procedure G_Order( A,B:in out T ); --Prototype G_Order
procedure G_Order( A,B:in out T ) is --Implementation G_Order
Tmp : T;
begin
if A > B then
--Compare
Tmp := A; A := B; B := Tmp; --Swap
end if;
end G_Order; --
The generic formal subprogram:
with function ">" ( A, B:in T )
return Boolean is ; --Need def for >
specifies that on instantiation of the package, a definition for ">" between instances of the formal parameter T
must be provided. The part of the generic formal subprogram specifies that this formal parameter has as a
default value of the current definition for ">" at the point of instantiation of the generic.
© M A Smith - May not be reproduced without permission