Question

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

But what is composition? I understand inheritance from the point of Person : Mammal : Animal, but I can’t really see the definition of composition anywhere…

Answer 1

Composition refers to combining simple types to make more complex ones. In your example, composition could be:

Animal:
    Skin animalSkin
    Organs animalOrgans


Mammal::Animal:
    Hair/fur mammalFur
    warm-blooded-based_cirulation_system heartAndStuff

Person::Mammal:
    string firstName
    string lastName

If you wanted to go totally composition (and get rid of all inheritance) it would look like this:

Animal:
    Skin animalSkin
    Organs animalOrgans

Mammal:
    private Animal _animalRef
    Hair/fur mammalFur
    warm-blooded-based_cirulation_system heartAndStuff

Person:
    private Mammal _mammalRef
    string firstName
    string lastName

The advantage to this approach is that the types Mammal and Person do not have to conform to the interface of their previous parent. This could be a good thing because sometimes a change to the superclass can have serious effects on the subclasses.

They still can have access to the properties and behaviours of these classes through their private instances of these classes, and if they want to expose these former-superclass behaviours, they can simply wrap them in a public method.

I found a good link with good examples in Composition versus Inheritance. A Comparative Look at Two Fundamental Ways to Relate Classes.