Object-oriented Software in Ada 95

Exceptions 195
14.3 Generic stack
The stack illustrated in Section 13.5 can be built as a generic package. First, the specification of the package that
contains the generic components is defined:
generic
type T is private; --Can specify any type
Max_Stack:in Positive := 3; --Has to be typed / not const
package Class_Stack is
type Stack is tagged private;
Stack_Error: exception;
procedure Reset( The:in out Stack);
procedure Push( The:in out Stack; Item:in T );
procedure Pop( The:in out Stack; Item:out T );
private
type Stack_Index is new Integer range 0 .. Max_Stack;
subtype Stack_Range is Stack_Index
range 1 .. Stack_Index(Max_Stack);
type Stack_Array is array ( Stack_Range ) of T;
type Stack is tagged record
Elements: Stack_Array; --Array of elements
Tos : Stack_Index := 0; --Index
end record;
end Class_Stack;
Note: The constant Max_Stack must be given a type because it is passed as a generic parameter.
The implementation of the package follows the same strategy as seen in Section 13.5.2 except that the
constant that defines the size of the stack is now typed. The body of the package takes this into account by
converting the constant object Max_Stack into an object of type Stack_Index. The body of the package is
implemented as follows:
package body Class_Stack is
procedure Push( The:in out Stack; Item:in T ) is
begin
if The.Tos /= Stack_Index(Max_Stack) then
The.Tos := The.Tos + 1; --Next element
The.Elements( The.Tos ) := Item; --Move in
else
raise Stack_Error;
end if;
end Push;
--Failed
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