Re: [Boost-Users] Re: shared_ptr swap problems

From: Peter Dimov (pdimov_at_[hidden])
Date: 2002-08-26 10:33:48

From: "Olaf Petzold" <yg-boost-users_at_[hidden]>
> Thank you,
>
> >> struct C : public BaseC {
> >>   C() : b( new B("C::b") ) { }
> >>   void set(const boost::shared_ptr<A>& ptr) {
> >>     b.swap( boost::shared_static_cast<B>( ptr ) ); // L21
> >>   }
> >>   void print() { std::cout << b->name << std::endl; }
> >>   boost::shared_ptr<B> b;
> >> };
> >
> > Why not simply
> >
> > b = boost::shared_static_cast<B>( ptr );
> >
> > or
> >
> >
> > b = boost::shared_polymorphic_cast<B>( ptr );
>
> I want to swap the pointers inside for further use (for saving the old ptr
> contents and restoring for circumstances - clone() may be one way before
> set() it but has overhead).

You can't swap() two pointers (objects, in general) of different types, and
you can't swap() with a temporary (which is what the cast returns.)

One possible way is to change set() to return the old value:

shared_ptr<A> set(shared_ptr<A> const & p)
{
    shared_ptr<A> tmp(b);
    b = boost::shared_polymorphic_cast<B>(p);
    return tmp;
}

You can keep the old interface (minus the "const" on the parameter) if you
want, but the swapping needs to be done "by hand":

void set(shared_ptr<A> & p)
{
    shared_ptr<A> tmp(b);
    b = boost::shared_polymorphic_cast<B>(p);
    p = tmp;
}