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Composition over inheritance (or composite reuse principle) in object-oriented programming (OOP) is the principle that classes should favor polymorphic behavior and code reuse by their composition (by containing instances of other classes that implement the desired functionality) over inheritance from a base or parent class. [2]
For example, the decorator pattern (as mentioned above) has been proposed to overcome the static nature of inheritance between classes. As a more fundamental solution to the same problem, role-oriented programming introduces a distinct relationship, played-by, combining properties of inheritance and composition into a new concept. [citation needed]
The diagram on top shows Composition between two classes: A Car has exactly one Carburetor, and a Carburetor is a part of one Car. Carburetors cannot exist as separate parts, detached from a specific car. The diagram on bottom shows Aggregation between two classes: A Pond has zero or more Ducks, and a Duck has at most one Pond (at a time).
The ECS uses composition, rather than inheritance trees. An entity will be typically made up of an ID and a list of components that are attached to it. Any game object can be created by adding the correct components to an entity. This allows the developer to easily add features to an entity, without any dependency issues.
Thus in UML, composition has a more narrow meaning than the usual object composition. UML notation for association, composition and aggregation. The graphical notation represents: the property as a typed element in the box of the enclosing class, the association as a plain line between the associated classes,
Inheritance, by contrast, typically targets the type rather than the instances, and is restricted to compile time. On the other hand, inheritance can be statically type-checked, while delegation generally cannot without generics (although a restricted version of delegation can be statically typesafe [7]). Delegation can be termed "run-time ...
Here lies the important difference between the concepts of mixins and inheritance, in that the child class can still inherit all the features of the parent class, but, the semantics about the child "being a kind of" the parent need not be necessarily applied.
In database design, object-oriented programming and design, has-a (has_a or has a) is a composition relationship where one object (often called the constituted object, or part/constituent/member object) "belongs to" (is part or member of) another object (called the composite type), and behaves according to the rules of ownership.