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From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Anybody in US using ADA ? One silly idea..
Date: Tue, 28 Jan 2003 09:27:27 +0100
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Date: 2003-01-28T09:27:27+01:00
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On Mon, 27 Jan 2003 10:08:17 -0500, Hyman Rosen <hyrosen@mail.com>
wrote:

>Dmitry A. Kazakov wrote:
>> If I define a method or a free a procedure on Ellipse:
>> void Foo (Ellipse& A);
>> then the compiler will not substitute a Circle for Ellipse.
>
>Oh, You want to do *that* :-)

Surely, isn't Circle an Ellipse? (:-))

> It's an odd sort of thing to
>want to do, but if you did, you wouldn't do it with copy-in/
>copy-out, at least in C++. If I had to, I would do it like this,
>in typical OO fashion:
>
>struct EllipseInterface
>{
>     virtual void set(double x, double y) = 0;
>     virtual void get(double &x, double &y) const = 0;
>};
>
>class Ellipse : public EllipseInterface
>{
>     double X, Y;
>public:
>     Ellipse(double x, double y) : X(x), Y(y) { }
>     void set(double x, double y) { X = x; Y = y; }
>     void get(double &x, double &y) const { x = X; y = Y; }
>};
>
>class Circle : public EllipseInterface
>{
>     double R;
>public:
>     struct NotCircle { };
>     Circle(double r) : R(r) { }
>     void set(double x, double y) { R = x == y ? x : throw NotCircle(); }
>     void get(double &x, double &y) const { x = R; y = R; }
>};
>
>void Foo (EllipseInterface &A);

But that's another thing. Here, Circle cannot use (inherit) the
methods of Ellipse. It inherits only the abstract methods of
EliipseInterface and thus, have to implement all of them for scratch.
This kills the whole idea of inheritance for code reuse.

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de