Section 11.3 - Example of a Generic Package

Let's imagine that you want to create a generic package for a Stack that takes operations Push and Pop. Here's one way to define such a Stack; we'll define a Stack package that stores some Item type and has a maximum size:

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

Now a definition needs to be implemented, so here's a sample implementation:

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

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

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

  end Generic_Stack;

Somewhere else you can instantiate the Generic_Stack package. If you wanted to instantiate a new package called ``Stack_Int'' which could hold 200 Integers, you could say:

  package Stack_Int is new Generic_Stack(Size => 200, Item => Integer);

The ``Size =>'' and ``Item =>'' are optional; you could omit them if you wanted to, but including them makes the code clearer. From then on, you could "Push" a new Integer onto Stack_Int by saying:

  Stack_Int.Push(7);

Quiz:

What are the formal parameters for generic package Stack?

1. Size, Positive, Item, and private.
2. Size and Item.
3. Size.

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