CRTP as a delegation pattern

So far, we have used CRTP as a compile-time equivalent of dynamic polymorphism, including virtual-like calls through the base pointer (compile-time, of course, with a template function). This is not the only way CRTP can be used. In fact, more often than not, the function is called directly on the derived class. This is a very fundamental difference—typically, public inheritance expresses the is-a relationship—the derived object is a kind of a base object. The interface and the generic code are in the base class, while the derived class overrides the specific implementation. This relation continues to hold when a CRTP object is accessed through the base class pointer or reference. Such use of CRTP is sometimes also called a static interface.

When the derived object is used directly, the situation is quite different—the base class is no longer the interface, and the derived class is not just the implementation. The derived class expands...