Re: Templates and derivation

Lars Jordan <>
14 Jun 2006 18:37:08 -0400
Manfred von Willich wrote:

Lars Jordan wrote:

My question is. Are there other solutions which solve the problem from described
  above? Is there a solution which avoids the usage of such a ImplType-like member?

I have stripped out extras, leaving the minimum. What I have given
compiles successfully, even when using your Holder class. Essentially,
two lines change: the addition of a template operator to template class
MF, and the need for the explicit invocation of this operator to get
the compiler to recognise the conversion.

template<typename T> class MF : public T
   template<typename B> operator MF<B>& () { return *this; }

What was the compiler that you used? VC8 gives me the warning that the operator
is recursive on all control paths - which leads to a stack overflow. This is
something that I do not understand at all, since I can't see why the same
operator should be called again. If you could give me a hint why this happens
this would be great.

class Base
   virtual ~Base() { }

class Derived : public Base
   virtual ~Derived() { }

int main( int, char*[] )
   MF<Derived>* pMFD = new MF<Derived>;
   MF<Base>* pMFB = &(MF<Base>&)*pMFD;
   return 0;

I do not know if it can be done without explicitly invoking a
conversion operator. I hope this is something like what you are
looking for.

I tried to call it without the explicit invocation of the operator but that
didn't work. It seems that this operator is not a viable option for the compiler
to select automatically for conversion in assignment.

I like the idea of using operators though this is a part of c++ that I do not
know enough about.



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