12.8 Example of a Generic Package

Contents | Index | Previous | Next

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

This paragraph was deleted.

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;

package body 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 Stack;

Instances of this generic package can be obtained as follows:

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

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

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

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

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;

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

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

Contents | Index | Previous | Next Legal