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Date: Mon, 22 Sep 2003 12:26:14 -0400
From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: Is the Writing on the Wall for Ada?
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Date: 2003-09-22T12:26:14-04:00
List-Id: <comp.lang.ada>

Robert I. Eachus wrote:
 > If the states are independent, you can use mix-ins that add state,
 > but only see their own state variables.

The problem with the mix-in approach is that you don't have a standalone
"interface" type which represents the abilities granted by the mix-in.

The problem I am formulating here is that you have multiple existing
interface types, with existing concrete implementations.
You would like to make a class which reuses the implementations and
is of the interface types. I am talking of the Colorable+Resizable
case, not the Polar+Cartesian case.

> But if you want to inherit from two classes  with state, and adjust
 > the state of each based on changes to the other, the na�ve implementation
 > isn't possible in Ada, and causes indefinite recursion in C++.

struct A { int a; virtual void set_a(int aa) { a = aa; } };
struct B { int b; virtual void set_b(int bb) { b = bb; } };

struct AB : A, B {
     void set_a(int aa) { A::set_a(aa); B::set_b(aa + 1); }
     void set_b(int bb) { A::set_a(bb - 1); B::set_b(bb); }
};

What indefinite recursion?