[Boost-users] shared_ptr and covariant return types

8 Nov 2007

      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

[Boost-users] shared_ptr and covariant return types

Bernhard Wagner boost