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12.8 Example of a Generic Package

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Examples

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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.

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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; 4 package body Stack is 5 type Table is array (Positive range <>) of Item; Space : Table(1 .. Size); Index : Natural := 0; 6 procedure Push(E : in Item) is begin if Index >= Size then raise Overflow; end if; Index := Index + 1; Space(Index) := E; end Push; 7 procedure Pop(E : out Item) is begin if Index = 0 then raise Underflow; end if; E := Space(Index); Index := Index - 1; end Pop; 8 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;