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Thanks for all the replies.
They brought me on the right track.
The solution I've got now is also known as the "Barton and Nackman Trick".
It's a bit pervert - but totaly legal C++ code:
template<typename leaftype>
class A
{
leaftype& asLeaf()
{ return static_cast<leaftype&>(*this); }
public:
bool foo( leaftype bar )
{ return asLeaf().foo( bar ); }
};
class B : public A<B>
{
public:
bool foo( B bar )
{ return /*...*/; }
}
The Barton and Nackman trick is important to avoid virtual function
where they are too slow (i.e. when the additional pointer lookup hurts
performance)
CU,
Christian
Paul Kahler schrieb:
> 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:
>
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>>
>>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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