Object composition

:{{mergefrom|composite type}}

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In computer science, composition is a way and practice to combine simple objects or data types into more complex ones. Compositions are a critical building block of many basic data structures, including the tagged union, the linked list, and the binary tree, as well as the object used in object-oriented programming.

Related Topics:
Computer science - Object - Data type - Tagged union - Linked list - Binary tree

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Composition is contrasted with subtyping, which is the process of adding detail to a general data type to create a more specific data type. In composition, the composite type "has an" object of a simpler type, while in subtyping, the subtype "is an" instance of its parent type. Composition does not form a subtype but a new type.

Related Topics:
Subtyping - Has an - Is an

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Composited objects are called fields, items, members or attributes, and the resulting composition a structure, record, tuple, or composite type. The terms usually vary across languages. Fields are given a unique name so that each one can be distinguished from the others. Sometimes an issue of ownership arises: when a composition is destroyed, should objects belonging to it be destroyed as well? If not, the case is sometimes called aggregation. For more, see the aggregation section below.

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In UML, composition is depicted as a filled diamond. It always implies a multiplicity of 1, as only one object at a time can have lifetime responsibility for another object. The more general composition is depicted as an un-filled diamond.

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