15. Access levels for inheritance

class Derived : public Base

    // same as
    struct Derived : Base

Most common: all public members of Base are also public in Derived, so they can be used by everyone.

class Derived : private Base

    // same as
    class Derived : Base

All public and protected members of Base become private within Derived, so they can be used only by members of Derived.

class Derived : protected Base

All public and protected members of Base become protected within Derived, so they can only be used by members of Derived and members of classes derived from Derived.

All non-public inheritance is a suspect - have a good reason before doing it.