Object-oriented Software in Ada 95

14 Generics
This chapter looks at generics that enable parameterized re-usable code to be written. The degree of reuseability,
however, will depend on the skill and foresight of the originator.
14.1 Generic functions and procedures
The main problem in re-using code of previously written functions or procedures is that they are restricted to
process specific types of values. For example, the function order developed as an exercise in Chapter 5 will only
work for Float values. To be really useful to a programmer, this procedure should work for all objects for which
a ‘greater than’ value can be defined. Ada allows the definition of generic functions or procedures. In this, the
actual type(s) that are to be used are supplied by the user of the function or procedure. This is best illustrated by
an example:
generic
type T is ( );
procedure G_Order( A,B:in out T );
--Specification
--Any discrete type
--Prototype ord
procedure G_Order( A,B:in out T ) is --Implementation ord
Tmp : T;
--Temporary
begin
if A > B then
--Compare
Tmp := A; A := B; B := Tmp; -- Swap
end if;
end G_Order; --
The declaration of a generic function or procedure is split into two components: a specification part that
defines the interface to the outside world, and an implementation part that defines the physical implementation. In
the specification part, the type(s) that are to be used in the procedure are specified between the generic and the
prototype line of the function or procedure. In this example a single type T is to be supplied. The type T must be
one of Ada’s discrete types. The ‘()’ in the declaration ‘type T is ()’ specifies this restriction. A full
list of the restricted types to which a generic parameter can be constrained to are given in Section 14.2.
To use this procedure the user must first instantiate a procedure that will operate on a particular type. This is
accomplished by the declaration:
procedure Order is new G_Order( Natural ); --Instantiate order
which defines a procedure order which will put into ascending order its two Natural parameters. It would, of
course, be an error detected at compile-time to use the procedure order with any parameter of a type other than a
Natural or a subtype of a Natural.
© M A Smith - May not be reproduced without permission