[Boost-users] shared_ptr and covariant return types

From: Bernhard Wagner boost (lists.boost.org_at_[hidden])
Date: 2007-11-08 08:53:49

In this post Peter Dimov shows a solution for the covariant return type
problem when using shared_ptr:
http://lists.boost.org/boost-users/2003/02/2996.php

I'm interested in the free function solution he provides:

> [snip] you can use the usual pattern from the no-covariant-return days:
>
> class B
> {
>     virtual shared_ptr<B> do_clone() const = 0;
> };
>
> class X: public B
> {
>     virtual shared_ptr<B> do_clone() const { return shared_ptr<X>(new
> X(*this)); }
>
> public:
>
>     shared_ptr<X> clone() const
>     {
>         shared_ptr<X> px = dynamic_pointer_cast<X>(this->do_clone());
>         assert(px);
>         return px;
>     }
> };
>
> It buys you some extra safety, too.
>
> You could also use a free function:
>
> template<class X> shared_ptr<X> clone(X const & x)
> {
>
>     shared_ptr<X> px = dynamic_pointer_cast<X>(x.do_clone());
>     assert(px);
>     return px;
> }
>

So, to clone an object using the clone method, I could use:

X* x;
shared_ptr<X> spX(x = new X());
shared_ptr<X> spX2 = spX->clone();
// or:
shared_ptr<X> spX3 = x->clone();

But how is the free function meant to be used?

This provokes the compiler error: '... do_clone() const is private ...':
shared_ptr<X> spX4 = clone(*spX);

Thank you
Bernhard