Forcing a template parameter to be derived from another

From: Gabriele Trombetti (nonvalid_at_nowhere.com)
Date: 03/10/04 

Date: Wed, 10 Mar 2004 15:24:05 -0600

Hi all,
Is there a way in C++ to force that a template parameter be derived from
something?

The code below should be clear for everybody. I'm asking that the
template parameter T_or_derived is T or derived publicly from T,
otherwise the compiler should give error. Unfortunately this syntax does
not compile.

Is there a way to enforce this constraint? If not, it seems to me that
this would be a very neat addition to C++.

template<class T>
class something
{
...
   template<class T_or_derived : public T>
   const something<T> & operator =
              (something<T_or_derived> & const copythis) {...}
...
};

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