has special properties. If the comparison of references indicates that the references are equal, then it's clear that the two objects pointed by the references are the same object. If the references do not compare equal, then it's not necessarily guaranteed that the identity of the objects behind those references is different. The object identity of two objects of the same type is the same, if every change to either object is also a change to the other object. Identity and object-oriented conceptual model Identity allows the construction of a platonic ideal world, the ontology or conceptual model, that is often used as basis of object-oriented thinking. The conceptual model describes the client side view to a domain, terminology or an API. This world contains point-like objects as instances, properties of the objects and links between those objects. The objects in the world can be grouped to form classes. The properties of the objects can be grouped to form roles. The links can be grouped to form associations. All locations in the world together with the links between the locations form the structure of the world. These groups are types of the corresponding instances of the world. Notes See also Meyer: Object-oriented software construction, second edition Domain-driven design: An entity is defined by identity Object-oriented programming