keyword mutable

The mutable specifier indicates that the member may be modified even in a const member function.

Example: class foo { mutable int count; public: int IncrementCount const { ++count; } };

In this example, IncrementCount may be called on a const object. (This is not a particularly good example, IncrementCount would normally be expected to be non-const. A better, and typical, example would be in a threaded synchronization object.  Such objects often allow locking even of const instances.)