The Waite Group's C++ Primer Plus, Third Edition

Nested Classes

In C++, you can place a class declaration inside another class. The class declared within another is called a nested class, and it helps avoid name clutter by giving the new type class scope. Member functions of the class containing the declaration can create and use objects of the nested class. The outside world can use the nested class only if the declaration is in the public section and if you use the scope resolution operator. (Older versions of C++, however, don't allow nested classes or else implement the concept incompletely.)

Nesting classes is not the same as containment. Containment, recall, means having a class object as a member of another class. Nesting a class, on the other hand, does not create a class member. Instead, ...