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From: jimmaureenrogers@worldnet.att.net (Jim Rogers)
Newsgroups: comp.lang.ada
Subject: Re: Non-philosophical definition of Eiffel?
Date: 29 Jul 2003 14:39:53 -0700
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Hyman Rosen <hyrosen@mail.com> wrote in message news:<1059496557.747795@master.nyc.kbcfp.com>...
> Dmitry A. Kazakov wrote:
> > There is little sense in contravariant arguments
> 
> OK, I just want to make sure that we're talking about
> the same thing, so let me give specific examples. I'll
> write in C++ terms, just because.
> 
>      struct B1 { };
>      struct D1 : B1 { };
> 
>      struct B2
>      {
>          virtual B1 &f1();
>          virtual B1 &f2();
>          virtual void f3(B1 &);
>          virtual void f4(D1 &);
>          virtual void f5(B1 &);
>      };
> 
>      struct D2 : B2
>      {
>          B1 &f1();
>          D1 &f2();
>          void f3(B1 &);
>          void f4(B1 &);
>          void f5(D1 &);
>      };
> 
>      D2 d2;
>      B2 *b2 = &d2;
> 
> f1) This is ordinary overriding, with same return type.
>      Pretty much legal in every language.
> f2) This is covariant return type. Legal in C++. This
>      preserves LSP, since a call b2->f2() must return a
>      B1 &, and a D1 & is a B1 &.

The equivalent in Ada would require that f2 return a class
wide access type. In one case it would be an access to
b2'class, in the other case it would be to d2'class.

In general, neither C++ nor Java consider the function
return type as part of the signature of a function. This
means that a function in C++ or Java cannot be overriden
with only a change in the return type.

Compare this with Ada:

package foo
   type foo_array is array(1..10) of float;
   function total(Item : foo_array) return float;
   function total(Item : foo_array) return integer;
end foo;

Ada allows the overloading of the "total" function based
upon the return type of the function.


Jim Rogers