Pointer to Implementation (Pimpl) - Bridge Design Pattern

In the non-C++ world Pimpl is otherwise known as the Bridge design pattern, with the following definition:

Decouple an abstraction from its implementation so that the two can be extended independently.

In the vain of Bridge, in C++, when anything in a header file class definition changes, all users of that class must be recompiled - even if the only change was to the private class members that the users/clients of the class cannot even access. This is because C++'s build model is based on textual inclusion, and because C++ assumes that callers know two main things about a class that can be affected by private members: 1. Size and Layout 2. Functions

Because private data members of a class participate in its object representation, affecting size and layout, and because private member functions of a class participate in overload resolution (which takes place before member access checking), any change to those implementation...