[boost] Re: assert in inline function w/o ODR violation

From: Kevlin Henney (Kevlin.Henney_at_[hidden])
Date: 1999-09-02 09:27:29

[...]
I thought "safe_downcast" was spelled "dynamic_cast". Seems about as
safe as static_cast, only it happens to be checked during debugging
(only). Safe to me implies what dynamic_cast provides, and nothing
less. Perhaps debug_static_cast (or something along those lines) would
be clearer?

     Not quite: dynamic_cast supports cross casting and casting through
     virtual base classes, whereas this is not the intent of safe_downcast.

     However, as it probably now stands (I'm looking at code from
     polymorphic_cast as was), it is not actually particularly safe. If
     NDEBUG is set, I lose all of the compile time checking as well as the
     runtime checking -- baby has been thrown out w/ the bath water! This
     can be fixed, and at no extra execution cost, by adding a dummy
     dynamic_cast that is parsed and checked, but is not executed and -- on
     a good compiler -- will not have code gen'd for it either:

        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);
        }

     Thoughts?

     Kevlin