E.g., the "car has-a engine" relationship can be expressed using composition:
class Engine { public: Engine(int numCylinders); void start(); //starts this Engine }; class Car { public: Car() : e_(8) { } //initializes this Car with 8 cylinders void start() { e_.start(); } //start this Car by starting its engine private: Engine e_; };The same "has-a" relationship can also be expressed using private inheritance:
class Car : private Engine { public: Car() : Engine(8) { } //initializes this Car with 8 cylinders Engine::start; //start this Car by starting its engine };There are several similarities between these two forms of composition:
There are also several distinctions:
Note that private inheritance is usually used to gain access into the "protected:" members of the base class, but this is usually a short-term solution (translation: a band-aid; see below).