Hi Ovanes Thanks for your reply. Yes, I understand that do_clone is private. I wonder what Peter Dimov's idea is. I believe the point of this free function would be that you would only need to write it once and use it for any class that provides the do_clone() method. Which I'd love to see in action. Or be proven wrong. Thanks Bernhard On Thu, 8 Nov 2007, Ovanes Markarian wrote:

Hi!

do_clone is really private ;) Your derived class has public clone member and you should use it from the free function, but no the do_clone.

Regards, Ovanes

On 8 Nov 2007 14:53:49 +0100, Bernhard Wagner boost <lists.boost.org@xmlizer.biz> wrote: 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

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