RE: [Boost-Users] covariance of shared_ptr types

From: Vaclav Vesely (vaclav.vesely_at_[hidden])
Date: 2003-02-16 17:32:30

Peter,

thank you for your suggesstion. It's smart.

Wasek

> -----Original Message-----
> From: Peter Dimov [mailto:pdimov_at_[hidden]]
> Sent: Friday, February 14, 2003 12:11 PM
> To: Boost-Users_at_[hidden]
> Subject: Re: [Boost-Users] covariance of shared_ptr types
> 
> 
> Vaclav Vesely wrote:
> >
> > However it is NOT possible to write that with shared_ptr:
> >
> > class X {
> >     shared_ptr<A> f() { ... };
> > };
> >
> > class Y: public X {
> >     virtual shared_ptr<B> f() { ... };    // Error: shared_ptr<B> is
> > NOT covariant from shared_ptr<A>
> > };
> >
> >
> > I don't know C++ standard very well (in fact I've never
> read it). So
> > my question is: it's possible to adjust shared_ptr to allow such
> > constructions?
> 
> No, but 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;
> }
> 
> 
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