It sounds to me like you should just drop the virtual function
declaration in the abstract "A" class (or make it non-virtual). It
serves no purpose other than trying to enforce your design on the
subclasses. Since you require the foo() function to take a parameter of
the subclass type, only a B can foo a B, and only a C can foo a C. Let
us not use complex language constructs (templates for example) just to
allow the compiler to check that you're following your own convention.
Put a comment in the base class that defines this requirement on
subclasses and call it a day. OTOH, if generic code is going to tell an
X to foo another X (somehow knowing they are the same type) this might
not work. Hmmmm. I think another option is to declare A:foo(A p1) and
then have the B class explicitly cast the parameter p1 to a B inside
B:foo when using the B-specific functionality of the parameter. I don't
know how to do templates, so this is what I might do until it got too
ugly.
Hope that helps,
Paul
On Sat, 2005-01-15 at 15:12 +0100, Christian Mayer wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Hi,
>
> can someone help me to solve thise problem:
>
> Imagine I've got this class hierachy:
>
> class A
> {
> virtual bool foo( A bar ) = 0;
> }
>
> class B : A
> {
> bool foo( B bar )
> {
> ...
> }
> ....
> }
>
> int main( void )
> {
> B foobar;
> }
>
> this won't compile as my class B is still abstract as I didn't provide a
> "bool foo( A bar )" member.
>
> But I only want class A to be an interface that tells everybody what to
> expect from it's derivated classes. And one of these things is, that
> every child must have a member that is called "foo" and has one
> parameter of the type of the child itself.
>
> How do I achieve that?
>
>
> The only workaround I can come up with is, that I have:
> class B
> {
> friend bool foo( class B, class B );
> }
>
> bool foo( class B bar1, class B bar2 )
> {
> ...
> }
>
> I this doesn't guarantee me, that every child of A must have a foo
> function...
>
> Thanks,
> Christian
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