[boost] Re: dynamic vs static cast (Was: assert in inline function w/o ODR violation)

From: Kevlin Henney (Kevlin.Henney_at_[hidden])
Date: 1999-09-02 11:59:21

Valentin Bonnard wrote:
Beman Dawes wrote:
> Kevlin Henney wrote:
> >        template<class Derived, class Base>
> >        inline Derived safe_downcast(Base* x)
> >        {
> >            assert(dynamic_cast<Derived>(x) != 0);
> >            if(false)
> >                dynamic_cast<Derived>(x);
> >            return static_cast<Derived>(x);
> >        }

> Kevlin, after wading throught the standard's description of
> dynamic_cast twice, I am still having trouble knowing which errors
> your added code will detect, and which will still get through.  Could
> you enlighten me?

dynamic_cast from a base class w/ no virtual functions
to a derived class is ill-formed, but static_cast in
such case is well-formed.

That's the only difference in static semantic I can
think of for the moment.

     It ensures both that the class is polymorphic and that it is a
     base-derived conversion, otherwise the following would compile w/
     NDEBUG:

        const void *p = safe_downcast<const void *>("oops");
        string s = safe_downcast<string>("oops again");
        // etc

     Kevlin