How to accomplish covariant return types when returning a shared_ptr?

Question

using namespace boost;

class A {};
class B : public A {};

class X {
  virtual shared_ptr<A> foo();
};

class Y : public X {
  virtual shared_ptr<B> foo();
};

The return types aren't covariant (nor are they, therefore, legal), but they would be if I was using raw pointers instead. What's the commonly accepted idiom to work around this, if there is one?

Answer 1

I think that a solution is fundamentally impossible because covariance depends on pointer arithmetic which is incompatible with smart pointers.

When Y::foo returns shared_ptr<B> to a dynamic caller, it must be cast to shared_ptr<A> before use. In your case, a B* can (probably) simply be reinterpreted as an A*, but for multiple inheritance, you would need some magic to tell C++ about static_cast<A*>(shared_ptr<B>::get()).