🚀 go-pugleaf

In article 11m3@melimelo.enst-bretagne.fr, croizier@rennes.enst-bretagne.fr (Valery Croizier) writes:
>Sometimes, I wish I could oblige the user of a template class to
>inherit from a given class: ...
>
> ...
>  o Is there a way I have not noticed to do this ?
>  o Does anybody know the reason why the team working on the
>standard rejected this feature ?

This subject is discussed in D&E, including why C++ templates do not
have an explicit constraint mechanism.

Your particular case can be handled like this:

template< class T > // T must be derived from Shape
class C {
	C() { static_cast<Shape*>(static_cast<T*>(0)); ... }
};

The constructor won't compile unless there is an implicit conversion from
T* to Shape*, meaning that T* must be or be derived from Shape. The
result of the casts isn't used, so no code should be generated. 

If your compiler doesn't support static_cast, you can write
	Shape* p = (T*)0; // requires implicit conversion from T* to Shape*
---
Steve Clamage, stephen.clamage@eng.sun.com



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