Came here to say exactly that. You can also do this, thought it looks a little worse it allows for even more flexibility, such as complete decoupling of Impl from A across different files:

  // Forward declaration of Impl. What does Impl do?
  // You're not allowed to know.
  class Impl;

  class A {
  protected:
    Impl* mImpl;
  };

I've been using this trick but for a different reason - to reduce the number of #include statements in header files that are included a lot themselves.