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In COP-I and COP-II, object is one half of an element where an element is defined as a couple consisting of one reference and one object. Reference is an instance of a reference class and object is an instance of an object class.
In COP-III, an object is defined as a number of functions of a reference. This approach is more general and the previous definition is its particular case. Here concepts produce values as their instances and in this sense they are similar to classes in C++. The duality between references and objects is modeled via incoming and outgoing methods of concepts. Outgoing methods are supposed to model objects. A return value of an outgoing method is interpreted as one field of this object.