C++ FAQ / Section 24 / FAQ 24.2

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Revised Jul 4, 2012

[24.2] How are "private inheritance" and "composition" similar?

private inheritance is a syntactic variant of composition (AKA aggregation and/or has-a).

E.g., the "Car has-a Engine" relationship can be expressed using simple 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_;                    // Car has-a Engine
};

The "Car has-a Engine" relationship can also be expressed using private inheritance:

class Car : private Engine {    // Car has-a Engine
public:
  Car() : Engine(8) { }         // Initializes this Car with 8 cylinders
  using Engine::start;          // Start this Car by starting its Engine
};

There are several similarities between these two variants:

• In both cases there is exactly one Engine member object contained in every Car object
• In neither case can users (outsiders) convert a Car* to an Engine*
• In both cases the Car class has a start() method that calls the start() method on the contained Engine object.

There are also several distinctions:

• The simple-composition variant is needed if you want to contain several Engines per Car
• The private-inheritance variant can introduce unnecessary multiple inheritance
• The private-inheritance variant allows members of Car to convert a Car* to an Engine*
• The private-inheritance variant allows access to the protected members of the base class
• The private-inheritance variant allows Car to override Engine's virtual functions
• The private-inheritance variant makes it slightly simpler (20 characters compared to 28 characters) to give Car a start() method that simply calls through to the Engine's start() method

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).