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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]
Language designs that decouple inheritance from subtyping (interface inheritance) appeared as early as 1990; [21] a modern example of this is the Go programming language. Complex inheritance, or inheritance used within an insufficiently mature design, may lead to the yo-yo problem. When inheritance was used as a primary approach to structure ...
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. In simple ...
The doctrine of composition over inheritance advocates implementing has-a relationships using composition instead of inheritance. For example, instead of inheriting from class Person, class Employee could give each Employee object an internal Person object, which it then has the opportunity to hide from external code even if class Person has ...
In software engineering, the delegation pattern is an object-oriented design pattern that allows object composition to achieve the same code reuse as inheritance. In delegation, an object handles a request by delegating to a second object (the delegate). The delegate is a helper object, but with the original context.
OCaml: Traits can be implemented using a variety of language features: module and module type inclusion, functors and functor types, class and class type inheritance, et cetera. Perl: Called roles, they are implemented in Perl libraries such as Moose, Role::Tiny and Role::Basic. Roles are part of the sister language Raku.
If the language supports both delegation and inheritance one can do dual inheritance by utilizing both mechanisms at the same time as in class C extends A { delegationlink D d ; } This calls for additional rules for method lookup, as there are now potentially two methods that can be denoted as the most specific (due to the two lookup paths).
How those other classes gain access to the mixin's methods depends on the language. Mixins are sometimes described as being "included" rather than "inherited". Mixins encourage code reuse and can be used to avoid the inheritance ambiguity that multiple inheritance can cause [ 5 ] (the " diamond problem "), or to work around lack of support for ...