Re: [Boost-users] shared_ptr invalid covariant return type

From: John (john_at_[hidden])
Date: 2008-06-24 11:13:03

Thanks, I did see that in the archives before I posted. Please correct me if I'm 
wrong, but the code from 2003 doesn't provide a solution, since the clone() 
method is not virtual. i.e.,  you can't do this with it:

   shared_ptr<A> a = foo();
   shared_ptr<A> a2 = a->clone ();

which is the goal of the pattern.

Frank Mori Hess wrote:
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> On Tuesday 24 June 2008 09:43 am, John wrote:
>> This pattern/idiom is well-established:
>>
>>    struct A {
>>        virtual A*  clone () = 0;
>>    };
>>
>>    struct B : public A {
>>        virtual B*  clone ();
>>    };
>>
>>
>> But the above does not work when the pointers A* and B* are replaced by
>> shared_ptr<A> and shared_ptr<B>:
>>
>>    struct  A {
>>       virtual shared_ptr<A> clone () = 0;
>>    };
>>
>>    struct  B : public A {
>>       virtual shared_ptr<B> clone ();
>>    };
>>
>>
>> Is there a workaround? Another way to solve the same problem? Thanks!
> 
> See:
> 
> http://lists.boost.org/boost-users/2003/02/2996.php
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