object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called procedures or methods and impl ...

, analysis and design, object identity is the fundamental property of every object that it is distinct from other objects. Objects have identity are distinct even when they are otherwise indistinguishable, i.e. equal. In this way, object identity is closely related to the philosophical meaning.

Identity and references

reference A reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''nam ...

can be used to refer to an object with a specific identity. A reference contains the information that is necessary for the identity property to be realized in the

programming language A programming language is a system of notation for writing computer programs. Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...

, and allows access to the object with the identity. A type of a target of a reference is a role. Typically, references are isomorphic to memory addresses. However, multiple such references can refer to the same object, if some form of address mapping is present ( virtual addresses / page tables / memory segments). Object identity is less useful as a semantic concept in environments or situations in which the structure of objects is not encapsulated, and two objects are considered to be the same object based on having identical properties, even if they are not actually the same physical instance (structural equivalence). However, object identity can nevertheless provide optimization. A function which tests whether two arguments are the same object can quickly short circuit to an affirmative answer if the two arguments have the same identity (are references to the same instance). Only if the argument are distinct objects do the individual properties need to be considered to determine equality, which is a more expensive operation. For instance,

bignum In computer science, arbitrary-precision arithmetic, also called bignum arithmetic, multiple-precision arithmetic, or sometimes infinite-precision arithmetic, indicates that calculations are performed on numbers whose numerical digit, digits of p ...

integers may be heap-allocated objects such that two bignums are considered to be the same if they represent the same number. It might be a waste of machine cycles in the equality function not to take advantage of the discovery that the two arguments being compared are references to the same bignum.

Consequences of identity

Identity of objects allows objects to be treated as black boxes. The object need not expose its internal structure. It can still be referred to, and its other properties can be accessed via its external behaviour associated with the identity. The identity provides a mechanism for referring to such parts of the object that are not exposed in the interface. Thus, identity is the basis for polymorphism in object-oriented programming. Identity allows comparison of references. Two references can be compared whether they are equal or not. Due to the identity property, this comparison 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 Ontology is the philosophical study of existence, being. It is traditionally understood as the subdiscipline of metaphysics focused on the most general features of reality. As one of the most fundamental concepts, being encompasses all of realit ...

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 An application programming interface (API) is a connection between computers or between computer programs. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how to build ...

. 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

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

es. The properties of the objects can be grouped to form

role A role (also rôle or social role) is a set of connected behaviors, rights, obligations, beliefs, and norms as conceptualized by people in a social situation. It is an expected or free or continuously changing behavior and may have a given indi ...

s. 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.

Identity and references

Consequences of identity

Identity and object-oriented conceptual model

See also