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

From: Andy Rushton (quergle-mingw_at_[hidden])
Date: 2008-07-05 07:33:57

John wrote:
> This pattern/idiom is well-established:
> 
>   struct A {
>       virtual A*  clone () = 0;
>   };
> 
>   struct B : public A {
>       virtual B*  clone ();
>   };

Except you've quoted it wrong. The return type of virtual functions must 
always be the same for all classes in the polymorphic hierarchy. This is 
standard C++ polymorphism - always use a pointer to the superclass to 
point to all subclasses. The return type in this case should be A* for 
both cases:

   struct A {
       virtual A*  clone () = 0;
   };

   struct B : public A {
       virtual A*  clone ();
   };
--------------^

Similarly, for the shared_ptr version, do the same, i.e. always use 
shared_ptr<A> to point to objects of types A and B.

See the manual for my own smart pointer classes for more on this topic:

http://stlplus.sourceforge.net/stlplus3/docs/smart_ptr.html#cloning