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12.8 Example of a Generic Package

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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

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This paragraph was deleted.
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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;
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package body Stack is
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   type Table is array (Positive range <>) of Item;
   Space : Table(1 .. Size);
   Index : Natural := 0;
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   procedure Push(E : in Item) is
   begin
      if Index >= Size then
         raise Overflow;
      end if;
      Index := Index + 1;
      Space(Index) := E;
   end Push;
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   procedure Pop(E : out Item) is
   begin
      if Index = 0 then
         raise Underflow;
      end if;
      E := Space(Index);
      Index := Index - 1;
   end Pop;
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end Stack;
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Instances of this generic package can be obtained as follows: 
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package Stack_Int  is new Stack(Size => 200, Item => Integer);
package Stack_Bool is new Stack(100, Boolean);
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Thereafter, the procedures of the instantiated packages can be called as follows: 
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Stack_Int.Push(N);
Stack_Bool.Push(True);
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Alternatively, a generic formulation of the type Stack can be given as follows (package body omitted): 
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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;
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In order to use such a package, an instance has to be created and thereafter stacks of the corresponding type can be declared: 
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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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