When one object is responsible for the lifecycle management of another object, this relationship is known as composition. Composition represents a “has-a” type relationship. 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. In UML, composition is depicted as a filled diamond and a solid line. It always implies a multiplicity of 1 or 0..1, as no more than one object at a time can have lifetime responsibility for another object. In an “is a” relationship, the derived class is clearly a kind of the base class.

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