Andrzej talks in his recent blog post about class invariants.

Class Invariants

by Andrzej Krzemieński

About the blog post:

The primary motivation for defining a class in C++ is to reflect and maintain a class invariant. In this post we will see what class invariants are and how you deal with them. Class invariants are important part of C++, even though there is no “invariant” keyword in C++.