Hiding a C++ class in a header without using the unnamed namespace

Question

I am writing a C++ header in which I define a

class A {
   // ...
};

that I would like to hide from the outside world (because it may chang

Answer 1

Instead of class B holding an A object, have it hold an A* instead (or a shared_ptr, or an unique_ptr, etc.). This way class B only needs a forward declaration of class A and class A can be fully defined inside of class B's source file.