Fragile base class problem

A subclass may depend on its superclass's internal calls to overridable object methods. A base implementation change that preserves the base abstraction can then change subclass behavior. For example, a set subclass may count insertion attempts by overriding insertion. If a revised deletion operation rebuilds the set by dynamically dispatched clear and insert calls, it unexpectedly resets and increases that counter. Object composition can isolate the counting wrapper from the component's internal dispatch, provided only wrapper operations can mutate the owned component.