Linxutopia - Thinking in C++ - 14: Inheritance & Composition

Thinking in C++
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Combining composition & inheritance

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Of course, you can use composition & inheritance together. The following example shows the creation of a more complex class using both of them.

//: C14:Combined.cpp
// Inheritance & composition

class A {
  int i;
public:
  A(int ii) : i(ii) {}
  ~A() {}
  void f() const {}
};

class B {
  int i;
public:
  B(int ii) : i(ii) {}
  ~B() {}
  void f() const {}
};

class C : public B {
  A a;
public:
  C(int ii) : B(ii), a(ii) {}
  ~C() {} // Calls ~A() and ~B()
  void f() const {  // Redefinition
    a.f();
    B::f();
  }
};

int main() {
  C c(47);
} ///:~

C inherits from B and has a member object (“is composed of”) of type A. You can see the constructor initializer list contains calls to both the base-class constructor and the member-object constructor.

The function C::f( ) redefines B::f( ), which it inherits, and also calls the base-class version. In addition, it calls a.f( ). Notice that the only time you can talk about redefinition of functions is during inheritance; with a member object you can only manipulate the public interface of the object, not redefine it. In addition, calling f( ) for an object of class C would not call a.f( ) if C::f( ) had not been defined, whereas it would call B::f( ).

Thinking in C++
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