Re: [Boost-users] shared_ptr and covariant return types

From: lists.boost.org_at_[hidden]
Date: 2007-11-09 04:13:56

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_at_[hidden]> 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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-- 
  Bernhard Wagner                                 bernhard.wagner_at_[hidden]
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