Composition Over Inheritance

Composition over inheritance (or composite reuse principle) in object-oriented programming (OOP) is the principle that classes should achieve polymorphic behavior and code reuse by their composition (by containing instances of other classes that implement the desired functionality) rather than inheritance from a base or parent class. This is an often-stated principle of OOP, such as in the influential book ''Design Patterns'' (1994).

Basics

An implementation of composition over inheritance typically begins with the creation of various interfaces representing the behaviors that the system must exhibit. Interfaces can facilitate polymorphic behavior. 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 is accomplished by providing another class that implements the desired behavior interface. A class that contains a reference to an interface can support implementations of the interface—a choice that can be delayed until runtime.

Example

Inheritance

An example in C++ follows:

class Object
;

class Visible : public Object
;

class Solid : public Object
;

class Movable : public Object
;

Then, suppose we also have these concrete classes:

* class - which is , and
* class - which is and , but not
* class - which is and , but not
* class - which is , but neither nor

Note that multiple inheritance is dangerous if not implemented carefully because it can lead to the diamond problem. One solution to this is to create classes such as , , , etc. for every needed combination; however, this leads to a large amount of repetitive code. C++ uses virtual inheritance to solve the diamond problem of multiple inheritance.

Composition and interfaces

The C++ examples in this section demonstrate the principle of using composition and interfaces to achieve code reuse and polymorphism. Due to the C++ language not having a dedicated keyword to declare interfaces, the following C++ example uses inheritance from a pure abstract base class. For most purposes, this is functionally equivalent to the interfaces provided in other languages, such as Java and C#.

Introduce an abstract class named , with the subclasses and , which provides a means of drawing an object:

class VisibilityDelegate
;

class NotVisible : public VisibilityDelegate
;

class Visible : public VisibilityDelegate
;

Introduce an abstract class named , with the subclasses and , which provides a means of moving an object:

class UpdateDelegate
;

class NotMovable : public UpdateDelegate
;

class Movable : public UpdateDelegate
;

Introduce an abstract class named , with the subclasses and , which provides a means of colliding with an object:

class CollisionDelegate
;

class NotSolid : public CollisionDelegate
;

class Solid : public CollisionDelegate
;

Finally, introduce a class named with members to control its visibility (using a ), movability (using an ), and solidity (using a ). This class has methods which delegate to its members, e.g. simply calls a method on the :

class Object
;

Then, concrete classes would look like:

class Player : public Object
;

class Smoke : public Object
;

Benefits

To favor composition over inheritance is a design principle that gives the design higher flexibility. It is more natural to build business-domain classes out of various components than trying to find commonality between them and creating a family tree. For example, an accelerator pedal and a steering wheel share very few common traits, yet both are vital components in a car. What they can do and how they can be used to benefit the car is easily defined. Composition also provides a more stable business domain in the long term as it is less prone to the quirks of the family members. In other words, it is better to compose what an object can do (''has-a'') than extend what it is (''is-a'').

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.
Composition relation is more flexible as it may be changed on runtime, while sub-typing relations are static and need recompilation in many languages.

Some languages, notably Go and Rust, use type composition exclusively.

Drawbacks

One common drawback of using composition instead of inheritance is that methods being provided by individual components may have to be implemented in the derived type, even if they are only forwarding methods (this is true in most programming languages, but not all; see ). In contrast, inheritance does not require all of the base class's methods to be re-implemented within the derived class. Rather, the derived class only needs to implement (override) the methods having different behavior than the base class methods. This can require significantly less programming effort if the base class contains many methods providing default behavior and only a few of them need to be overridden within the derived class.

For example, in the C# code below, the variables and methods of the base class are inherited by the and derived subclasses. Only the method needs to be implemented (specialized) by each derived subclass. The other methods are implemented by the base class itself, and are shared by all of its derived subclasses; they do not need to be re-implemented (overridden) or even mentioned in the subclass definitions.

// Base class
public abstract class Employee

// Derived subclass
public class HourlyEmployee : Employee

// Derived subclass
public class SalariedEmployee : Employee

Avoiding drawbacks

This drawback can be avoided by using traits, mixins, (type) embedding, or protocol extensions.

Some languages provide specific means to mitigate this:

* C# provides default interface methods since version 8.0 which allows to define body to interface member.
* D provides an explicit "alias this" declaration within a type can forward into it every method and member of another contained type.
* Dart provides mixins with default implementations that can be shared.
* Go type embedding avoids the need for forwarding methods.
* Java provides default interface methods since version 8. Project Lombok supports delegation using the annotation on the field, instead of copying and maintaining the names and types of all the methods from the delegated field.
* Julia macros can be used to generate forwarding methods. Several implementations exist such as Lazy.jl and TypedDelegation.jl.
* Kotlin includes the delegation pattern in the language syntax.
* PHP supports traits.
* Python supports use of the __getattr__ magic method to forward attribute requests.
* Raku provides a trait to facilitate method forwarding.
* Rust provides traits with default implementations.
* Scala (since version 3) provides an "export" clause to define aliases for selected members of an object.
* Swift extensions can be used to define a default implementation of a protocol on the protocol itself, rather than within an individual type's implementation.

Empirical studies

A 2013 study of 93 open source Java programs (of varying size) found that:

See also

* Delegation pattern
* Liskov substitution principle
* Object-oriented design
*Object composition
* Role-oriented programming
* State pattern
* Strategy pattern

References

{{Reflist

Component-based software engineering
Software architecture
Programming principles
Articles with example C Sharp code