Re: covariance of shared_ptr types

--- In Boost-Users@yahoogroups.com, "Vaclav Vesely" wrote:

Hi,

in C++ it's possible to write this:

class A {};

class B: public A {};

class X { virtual A* f() { ... }; };

class Y: public X { virtual B* f() { ... }; // B* is covariant from A* };

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?

I'm also not a C++ guru, but I think it requires that shared_ptr<A> be the parent of shared_ptr<B>. How it can be achieved? If it could be possible in C++, it then it was easy: template<typename T> class shared_ptr<T>: public T* { ... } but AFAIK it is not possible.