What is the difference between protected and private in C++?

In C++, both protected and private are access control modifiers used to restrict access to class members.

1. Private members: Private members can only be accessed within the class where they are defined, external classes or derived classes cannot access private members directly. Private members are not visible to class users and can only be accessed by class member functions.
2. Protected members: Protected members can be accessed by the class in which they are defined and by any derived classes of that class. These members are not visible to the users of the class and can only be accessed by the member functions of the class or the derived classes.

In conclusion, private members can only be accessed by the class where they are defined, while protected members can be accessed by the class where they are defined and its derived classes.