12.8 Example of a Generic Package
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.
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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;
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;
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:
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;
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