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From: Gregory Lampshire <gbol@mrj.com>
Subject: Ada95 OOP programming: calling correct superclass procedure
Date: 1996/11/25
Message-ID: <3299F04C.E86@mrj.com>#1/1
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Date: 1996-11-25T00:00:00+00:00
List-Id: <comp.lang.ada>


I am having difficulty getting the right procedure to be called during
a dispatching operation:

package My_Objs is
	type A_Rec is tagged private;
	type A is access all A_Rec'Class;
	procedure Init (This : access A_Rec; I : Integer);
	function Create_A return A;

	type B_Rec is new A_Rec with private;
	type B is access all B_Rec'Class;
	procedure Init (This : access B_Rec; I : Integer);
	function Create_B return B;
end;
package body My_Objs is
	procedure Init (This : access A_Rec; I : Integer) is
	begin
		Do_Something_A_Specific (I);
	end;
	function Create_A return A is
		This : A := new A_Rec;
	begin
		Init (This);
		return This;
	end;


	procedure Init (This : access B_Rec; I : Integer) is
	begin
*		Init (This, I);
		Do_Something_B_Specific (I);
	end;
	function Create_B return B is
		This : B := new B_Rec;
	begin
		Init (This);
		return This;
	end;
end;

The "*" indicates where I have a problem.  If I do the following;

	declare
		Me : B := Create_B;
	begin
		...
	end;


I get stuck in an endless loop (of course) because Init is 
dispatching.  Note that both defs are in the same package so
I cannot prefix the call with the package name.  Also,
I want the user only to see the Create definition, not the 
Init but I had to stick into the package to force it to
be a primitive operation (for some definitions I do not
mind exposing Init).

I've tried casting using A'(This) but, of course, the tag
does the correct dispatching anyway.

How do I cast or resolve this problem within the constraints
above? (same package, same calling args except 1st parameter, 
same procedure name, reference argument controlling formal parameter)
I can always change the Init name to Init_A and Init_B but
I was wondering how to do this in Ada95.  It would be nice
to have something like Parent.Init (This) where Parent
is already defined as A in the above case.  Perhaps I have an
an OOP conceptual problem.