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