Re: upcast<>

From: Dave Abrahams (abrahams_at_[hidden])
Date: 2000-05-30 15:05:59

--- In boost_at_[hidden], scleary_at_j... wrote:

> template <typename Base, typename Derived>
> Base * upcast(Derived * const d)
> { return static_cast<Derived *>(static_cast<Base *>(d)); }
> 
> Should work for casting up hierarchy, fail for casting down or 
across (with
> an error message like "cannot convert 'Derived *' to 'Base *'").  
Will allow
> casting to the same type; a POSTULATE can easily be added to 
prevent this if
> desired.  More complicated tests not done :).

Nice. Who's maintaining casts.hpp? Can we put this in?

BTW, in my experience the hack someone suggested a while back to get 
these things to work with MSVC6 doesn't work:

template <typename Base, typename Derived>
Base * upcast(Derived * const d, Base* = 0)
{ return static_cast<Derived *>(static_cast<Base *>(d)); }

The best thing I've been able to do, which isn't quite seamless, is 
to build a class called upcast<Base> which is convertible to Base* 
and has operator->() as well.

-Dave