dynamic_cast across a shared_ptr?

If you just want to call a function from B you can use one of these:

std::shared_ptr<A> ap = ...;
dynamic_cast<B&>(*ap).b_function();
if (B* bp = dynamic_cast<B*>(ap.get()) {
    ...
}

In case you actually want to get a std::shared_ptr<B> from the std::shared_ptr<A>, you can use use

std::shared_ptr<B> bp = std::dynamic_pointer_cast<B>(ap);