FAQ 24.07 What should be returned by private: and protected: assignment operators?

FAQ 24.07 What should be returned by private: and protected: assignment operators?

Either return a reference to the this object, or make the return type void.

Assignment operators that are private: or protected: needn't return *this because such operators have very few users, and therefore the advantage of returning *this is limited.

Assignment operators are often declared as private: to prevent users from assigning objects of the class (see FAQ 30.13). They are often left undefined to prevent being accidentally called by a member function or a friend function.

Assignment operators are often declared as protected: in abstract base classes to ensure that assignment doesn't occur when the destination is a reference to an abstract class (for example, assigning a circle to a square when both are referenced by a Shape&; see FAQ 24.05).