MFC Programmer's SourceBook : Thinking in C++

Composition syntax

Actually, you’ve been using composition all along to create classes. You’ve just been composing classes using built-in types. It turns out to be almost as easy to use composition with user-defined types.

Consider an existing class that is valuable for some reason:

//: C14:Useful.h
// A class to reuse
#ifndef USEFUL_H
#define USEFUL_H

class X {
  int i;
  enum { factor = 11 };
public:
  X() { i = 0; }
  void set(int ii) { i = ii; }
  int read() const { return i; }
  int permute() { return i = i * factor; }
};
#endif // USEFUL_H ///:~

The data members are private in this class, so it’s completely safe to embed an object of type X as a public object in a new class, which makes the interface straightforward:

//: C14:Compose.cpp
// Reuse code with composition
#include "Useful.h"

class Y {
  int i;
public:
  X x; // Embedded object
  Y() { i = 0; }
  void f(int ii) { i = ii; }
  int g() const { return i; }
};

int main() {
  Y y;
  y.f(47);
  y.x.set(37); // Access the embedded object
} ///:~

Accessing the member functions of the embedded object (referred to as a subobject) simply requires another member selection.

It’s probably more common to make the embedded objects private, so they become part of the underlying implementation (which means you can change the implementation if you want). The public interface functions for your new class then involve the use of the embedded object, but they don’t necessarily mimic the object’s interface:

//: C14:Compose2.cpp
// Private embedded objects
#include "Useful.h"

class Y {
  int i;
  X x; // Embedded object
public:
  Y() { i = 0; }
  void f(int ii) { i = ii; x.set(ii); }
  int g() const { return i * x.read(); }
  void permute() { x.permute(); }
};

int main() {
  Y y;
  y.f(47);
  y.permute();
} ///:~

Here, the permute( ) function is carried through to the new class interface, but the other member functions of X are used within the members of Y.

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