Separating interfaces and implementations with the non-virtual interface idiom

Virtual functions provide customization points for a class, by allowing derived classes to modify implementations from a base class. When a derived class object is handled through a pointer or a reference to a base class, calls to overridden virtual functions end up invoking the overridden implementation from the derived class. On the over hand, a customization is an implementation detail, and a good design separates interfaces from implementation. The non-virtual interface idiom, proposed by Herb Sutter in an article about virtuality in C/C++ Users Journal, promotes the separation of concerns of interfaces and implementations by making (public) interfaces non-virtual and virtual functions private. Public virtual interfaces prevent a class from enforcing pre- and post-conditions on its interface. Users expecting an instance...