To me the original sin of the example is not to perform the calculation in the constructor. The third version, the one with the "int count() const" method and w

To me the original sin of the example is not to perform the calculation in the constructor. The third version, the one with the "int count() const" method and without the h and w members, is perfectly valid and (correct me if I'm wrong) thread-safe. It is a bit longer than the 'functional' implementation but does handle memoization. This is basically the C++ way to make a 'closure'. The point is certainly valid, but the example could have been more carefully chosen.