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In database design, object-oriented programming and design (see object oriented program architecture), 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 words, has-a relationship in an object is called a member field of an object. Multiple has-a relationships will combine to form a possessive hierarchy. This is to be contrasted with an ''is-a'' (''is_a'' or ''is a'') relationship which constitutes a taxonomic hierarchy (subtyping). The decision whether the most logical relationship for an object and its subordinate is not always clearly ''has-a'' or ''is-a''. Confusion over such decisions have necessitated the creation of these metalinguistic terms. A good example of the ''has-a'' relationship is containers in the C++ STL. To summarize the relations, we have * hypernym-hyponym (supertype-subtype) relations between types (classes) defining a taxonomic hierarchy, where * * for a subsumption relation: a hyponym (subtype, subclass) has a ''type-of'' (''is-a'') relationship with its hypernym (supertype, superclass); * holonym-meronym (whole/entity/container-part/constituent/member) relations between types (classes) defining a possessive hierarchy, where * * for an aggregation (i.e. without ownership) relation: * * * a holonym (whole) has a ''has-a'' relationship with its meronym (part), * * for a composition (i.e. with ownership) relation: * * * a meronym (constituent) has a ''part-of'' relationship with its holonym (entity), * * for a containment〔See also Containment (computer programming).〕 relation: * * * a meronym (member) has a ''member-of'' relationship with its holonym (container); * concept-object (type-token) relations between types (classes) and objects (instances), where * * a token (object) has an ''instance-of'' relationship with its type (class). ==Examples== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Has-a」の詳細全文を読む スポンサード リンク
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