12.8 Example of a Generic Package

1. The following example provides a possible formulation of stacks by means of a generic package. The size of each stack and the type of the stack elements are provided as generic formal parameters.
   Examples
   

1. generic
      Size : Positive;
      type Item is private;
   package Stack is
      procedure Push(E : in  Item);
      procedure Pop (E : out Item);
      Overflow, Underflow : exception;
   end Stack;
   

2. package body Stack is
   

3.    type Table is array (Positive range <>) of Item;
      Space : Table(1 .. Size);
      Index : Natural := 0;
   

4.    procedure Push(E : in Item) is
      begin
         if Index >= Size then
            raise Overflow;
         end if;
         Index := Index + 1;
         Space(Index) := E;
      end Push;
   

5.    procedure Pop(E : out Item) is
      begin
         if Index = 0 then
            raise Underflow;
         end if;
         E := Space(Index);
         Index := Index - 1;
      end Pop;
   

6. end Stack;
   

7. Instances of this generic package can be obtained as follows:

8. package Stack_Int  is new Stack(Size => 200, Item => Integer);
   package Stack_Bool is new Stack(100, Boolean);
   

9. Thereafter, the procedures of the instantiated packages can be called as follows:

10. Stack_Int.Push(N);
    Stack_Bool.Push(True);
    

11. Alternatively, a generic formulation of the type Stack can be given as follows (package body omitted):

12. generic
       type Item is private;
    package On_Stacks is
       type Stack(Size : Positive) is limited private;
       procedure Push(S : in out Stack; E : in  Item);
       procedure Pop (S : in out Stack; E : out Item);
       Overflow, Underflow : exception;
    private
       type Table is array (Positive range <>) of Item;
       type Stack(Size : Positive) is
          record
             Space : Table(1 .. Size);
             Index : Natural := 0;
          end record;
    end On_Stacks;
    

13. In order to use such a package, an instance has to be created and thereafter stacks of the corresponding type can be declared:

14. declare
       package Stack_Real is new On_Stacks(Real); use Stack_Real;
       S : Stack(100);
    begin
       ...
       Push(S, 2.54);
       ...
    end;
    

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