Re: boost::polymorphic_upcast suggestion

From: Jean Llorca (yg-boost-users_at_[hidden])
Date: 2002-06-27 09:31:19

"Darin Adler" <darin_at_[hidden]> a écrit dans le message de news:
B586F909-89D7-11D6-9349-0003935B80A2_at_bentspoon.com...
>
> On Thursday, June 27, 2002, at 06:38  AM, Jean Llorca wrote:
>
> > Here upcasting means casting a base class to a derived class, I know
> > it may seem strange, bust boost offers a downcast
> > which does the static_cast the language does implicitly already (check
> > <boost/cast.hpp>).
>
> I think you've got it backwards. Downcasting, in Boost at least, means
> casting a pointer to a base class part of an object to a pointer to a
> derived class part of the same object. The polymorphic_downcast is
> *not* doing a static_cast for a conversion the language already does
> implicitly.

    template <class Target, class Source>
    inline Target polymorphic_downcast(Source* x
BOOST_EXPLICIT_DEFAULT_TARGET)
    {
        assert( dynamic_cast<Target>(x) == x );  // detect logic error
        return static_cast<Target>(x);
    }

Sorry about it, but it's here in the code. This is in the last version of
the CVS. It does a static_cast. The compiler does static_cast implicitly in
some cases.
For instance a C++ compiler replaces the assert line with:
assert( dynamic_cast<Target>(x) == static_cast<Target>(x) );

>
> Could you give a specific complete example (a file that would compile,
> with all the declarations) of how one would use what you're proposing
> to help make this clearer?

I enclosed two files to this reply. Don't scream about chosen class names
and stuff in the example ;)

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`
end

begin 666 nqcast.h
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`
end