What is composition as it relates to object oriented design?

Question

I hear (and read on this site) a lot about \"favour composition over inheritance\".

But what is Compositon? I understand inheritance from the point of Person : Mamma

Answer 1

class Engine
{

}

class Automobile
{

}


class Car extends Automobile // car "is a" automobile //inheritance here
{ 
 Engine engine; // car "has a" engine //composition here

}

Composition - Functionality of an object is made up of an aggregate of different classes. In practice, this means holding a pointer to another class to which work is deferred.

Inheritance - Functionality of an object is made up of it's own functionality plus functionality from its parent classes.

As to why composition is preferred over inheritance, take a look at the Circle-ellipse problem.