Composition over inheritance

Composition over inheritance in object-oriented programming is a technique by which classes may achieve polymorphic behavior and code reuse by containing other classes which implement the desired functionality instead of through inheritance.^[1]

This technique is also referred to as "Composite Reuse Principle".

Basics

An implementation of composition over inheritance typically begins with the creation of various interfaces representing the behaviors which the system must exhibit. The use of interfaces allows this technique to support the polymorphic behavior that is so valuable in object-oriented programming. Classes implementing the identified interfaces are built and added to Business Domain classes as needed. Thus system behaviors are realized without inheritance. In fact, business domain classes may all be base classes without any inheritance at all. Alternative implementation of system behaviors are accomplished by providing another class which implements the desired behavior interface. Any business domain class that contains a reference to the interface can easily support any implementation of that interface and the choice can even be delayed until run time.

Example

Inheritance

class Object {
  public:
     virtual void update() {} ;
     virtual void draw() {} ;
     virtual void collide(Object objects[]) {} ;
};
class Visible : public Object {
  public:
     virtual void draw() { /* draw model at position of this object*/ };
  private:
     Model* model;
};
class Solid : public Object {
  public:
     virtual void collide(Object objects[]) { /*check and react to collisions with objects */ };
};
class Movable : public Object {
  public:
     virtual void update() { /* update position */ };
};

Then we have concrete classes:

• class Player - which is Solid, Movable and Visible
• class Cloud - which is Movable and Visible, but not Solid
• class Building - which is Solid and Visible, but not Movable
• class Trap - which is Solid and not Visible nor Movable

Using inheritance we either have to do multiple inheritance, which leads to the diamond problem, or make classes like VisibleAndSolid, VisibleAndMovable, VisibleAndSolidAndMovable, etc. for every needed combination, which leads to a large amount of repetitive code.

Composition (and a little inheritance)

class Object {
  public:
    Object(VisibilityDelegate* v, UpdateDelegate *u, CollisionDelegate *c):_v(v),_u(u),_c(c) {};
 
    void update() { _u->update(); };
    void draw()   { _v->draw(); };
    void collide(Object objects[]) { _c->collide(objects); };
  private:
    VisibilityDelegate* _v;
    UpdateDelegate *_u;
    CollisionDelegate *_c;
};
 
class VisibilityDelegate {
  public:
     virtual void draw() = 0;
};
class Invisible : public VisibilityDelegate {
  public:
     virtual void draw() {};
};
class Model : public VisibilityDelegate {
  public:
     virtual void draw() { /*draw model*/ };
};
 
class CollisionDelegate {
  public:
     virtual void collide(Object objects[]) = 0;
};
class Solid : public CollisionDelegate {
  public:
     virtual void collide(Object objects[]) { /* check collisions with object and react*/ };
};
class NotSolid : public CollisionDelegate {
  public:
    virtual void collide(Object objects[]) {};
};
 
class UpdateDelegate {
  public:
     virtual void update() = 0;
};
class Movable : public UpdateDelegate {
  public:
     virtual void update() { /*move object*/ };
};
class NotMovable : public UpdateDelegate {
  public:
     virtual void update() { };
};

Then concrete classes would look like:

class Player : public Object {
  public:
    Player():Object(new Visible(), new Movable(), new Solid()) {};
...
};
class Smoke : public Object {
  public:
    Smoke():Object(new Visible(), new Movable(), new NotSolid()) {};
...
};

Benefits

Composition over inheritance can simplify the initial design of Business Domain classes and provide a more stable business domain in the long term. Its advantage over inheritance is a more thorough isolation of interests than can be described by a hierarchy of descendant classes. Additionally, inheritance models are often contrived during the definition of business domain classes in order to make sense of the information in the problem domain and do not necessarily reflect the true relationship of various system objects.

Initial design is simplified by identifying system object behaviors in separate interfaces instead of creating a hierarchical relationship to distribute behaviors among business domain classes via inheritance. This approach more easily accommodates future requirements changes that would otherwise require a complete restructuring of business domain classes in the inheritance model. Additionally, it avoids problems often associated with relatively minor changes to an inheritance-based model that includes several generations of classes.

See also

• Object-oriented design
• Strategy pattern
• State pattern
• Liskov substitution principle

References

1. ^ http://books.google.com/books?id=4pjbgVHzomsC&pg=PA17&lpg=PA17&dq=composite+reuse+principle&source=bl&ots=snkbw6h5Te&sig=S3UJrrDH_hU5V0AcWJQqLsE3cSY&hl=en&ei=ppKIS4arIIz4M-WE8KYB&sa=X&oi=book_result&ct=result&resnum=10&ved=0CDAQ6AEwCQ#v=onepage&q=composite%20reuse%20principle&f=false

Further reading

• Java Design: Objects, UML, and Process by Kirk Knoernschild
• Design Patterns: Elements of Reusable Object-Oriented Software