What is the difference between inheritance and composition?

Question

As title says, the meaning of both eludes me.

Solution 1

Inheritance expresses a is-a relationship, while composition expresses a has-a relationship between the two classes.

An example for composition is a polygon. It has a ordered sequence of Points. In C++ terms:

struct Polygon {
  std::vector<Point> points;
};

While an logic_error is a exception:

struct logic_error : public exception {
};

Solution 2

As pmr pointed out, inheritence is a is-a relationship, composition is a has-a relationship.

Composition is usually used for wrapping classes and to express relationships between classes that contain one another.

Inheritance is used for polymorphism, where you have a base class and you want to extend or change its functionality.

Solution 3

Just google Inheritance vs Composition you'll get alot of results.
Using java as an example

public class Banana extends Fruit{ //<---Inheritance Banana is-a Fruit
    private Peel bananaPeel; //<--Composition banana has a Peel
    public Peel getPeel(){
        return bananaPeel;
    }
}

Solution 4

Inheritance means inheriting something from a parent. For example, you may inherit your mother's eyes or inherit your father's build.

It means deriving properties, characteristics and behaviors from a parent class. So you can parent.walk(), parent.sleep(), parent.sleep() or whatever.

Containership or maybe composition is a bit hard to explain.

Or maybe a car has a brake. The car is composed of a brake. But the brake isn't inheriting from a brake..different concepts. I know very weird explanation..but that's how much I can do I guess.

Let's look at this code:

class Parent 
{
   public :
     void sleep() ; void eat() ; void walk()  ; 
     string eyeColor; int height ; 
};

class Child: public Parent
{

}

So the Child class can inherit the functions and attributes of the Parent but of course it may have the eye color of the mother or father.. Even if the childs' attributes are different it can still do whatever the Parent can do.

Now composition is another thing. A Child can have a toy or a Parent can have a child. So I could do:

class Toy
{
  string model ; 
};

class Child
{
   Toy transformersToy ;
};

So the Child has the transformers toy now.. but does Child inherit the transformersToy.model attribute? No, because it isn't inheriting.

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