Re: [Boost-Users] Re: boost::polymorphic_upcast suggestion

From: Sam Partington (sam.partington_at_[hidden])
Date: 2002-06-27 10:08:58

Sorry, don't know what happened then, I'll continue...

class A	{};

class B	{};

class C : public A, public B	{};

int main(int argc, char* argv[])
{
    C		c;
    C*		pc = &c;
     //B*		pb = boost::polymorphic_downcast<B*>( pc );
     // this cast is unnecessary, the compiler <knows> that C isa B at
compile time,
     // so it can go ahead and upcast it without asking you or the runtime
type info:
     B*		pb = pc;

     // C*		pc2 = polymorphic_upcast<C*>( pb);
     // this is precisely where the boost::polymorphic_downcast is intended
to be used,
     // ie Cast a Base (ptr or ref) to a Derived (ptr or ref)
     // the compiler doesn't know that a B object is definately a valid C,
so it requires you to
     // qualify it, using some kind of cast.
     C*		pc2 = boost::polymorphic_downcast<C*>(pb);

     return 0;
}

So thats downcasting and upcasting, there is no need for a
polymorphic_upcast, as it is implicity done and can be tested for at compile
time.

The only other cast not covered here is cross casting, which is what
boost::polymorphic_cast is designed for, but I find that almost everytime I
come across a use of a cross cast it is due to a design flaw.  (the same
could also be said to a lesser extent about downcasting)

Hope this clears things up a bit.

Sam

> > -----Original Message-----
> > From: news [mailto:news_at_[hidden]]On Behalf Of Jean Llorca
> > Sent: 27 June 2002 15:31
> > To: boost-users_at_[hidden]
> > Subject: [Boost-Users] Re: boost::polymorphic_upcast suggestion
> >
> >
> > "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 ;)