Re: downcasting smart pointers (suggested code)

From: martinschuerch_at_[hidden]
Date: 2001-07-13 02:34:05

...
>  >> Sounds like a plan.
>  >
>  >I would very appreciate some downcast of shared_ptr in boost. At 
the
>  >moment I'm using a hack similar as posted in related mails. But 
for a
>  >proper solution there has to be some support of shared_ptr.
>  >
>  >Is there any work in progress for this subject?
> 
> Why don't you post suggested code?
> 
> --Beman

So here it is:

The template function shared_dynamic_cast<> delegates the work to the 
memberfunction 

template<class Q>
void downcasted_copy_to( shared_ptr<Q>& q ) const

that way no friend is used. VC would not support the partial 
specialization (or is it overloading I never know) of 
polymorphic_downcast<> and I like shared_ptr and the downcast to be 
an unit without dependencies.
The suggested code isn't optimized for speed.

Member-template:

        template<class Q>
        void downcasted_copy_to( shared_ptr<Q>& q ) const
        {
                Q* rawq = dynamic_cast<Q*>(px);
                if( !rawq || !px ) { q = shared_ptr<Q>(); }
                shared_ptr<Q> ptmp;
                ptmp.px = rawq;
                ++*pn;
                ptmp.pn = pn;
                q.swap( ptmp );
        }

"global" function:

template<class Tout,class Tin>
boost::shared_ptr<Tout> shared_dynamic_cast( 
                         const boost::shared_ptr<Tin>& p )
{
        boost::shared_ptr<Tout> pout;
        p.downcasted_copy_to(pout);
        return pout;
}

int test_main( int, char *[] )  // note the name!
{
        using namespace boost;

        shared_ptr<Base> pb( new Base );
        shared_ptr<Base> pb2d( new Derived );
//	shared_ptr<Derived> pd( new Derived );
        shared_ptr<Derived> pd;

        pd = shared_dynamic_cast<Derived>(pb2d);
        pd->print();
        pd = shared_dynamic_cast<Derived>(pb);
        BOOST_CRITICAL_TEST( pd == shared_ptr<Derived>()  );

        return 0;
}

Thanks for feedback
Martin