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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]
Still, inheritance is a commonly used mechanism for establishing subtype relationships. [7] Inheritance is contrasted with object composition, where one object contains another object (or objects of one class contain objects of another class); see composition over inheritance.
In general, the further down in the hierarchy a class appears, the more specialized its behavior. When a message is sent to an object, it is passed up the inheritance tree starting from the class of the receiving object until a definition is found for the method. This process is called upcasting.
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.
The composite aggregation (colloquially called composition) relationship is a stronger form of aggregation where the aggregate controls the lifecycle of the elements it aggregates. The graphical representation is a filled diamond shape on the containing class end of the line that connect contained class(es) to the containing class.
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 ...
Some languages like Go do not support inheritance at all. Go states that it is object-oriented, [35] and Bjarne Stroustrup, author of C++, has stated that it is possible to do OOP without inheritance. [36] The doctrine of composition over inheritance advocates implementing has-a relationships using composition instead of inheritance. For ...
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.