Instance (computer science)

In object-oriented programming (OOP), an instance is a concrete occurrence of any object, existing usually during the runtime of a computer program. Formally, "instance" is synonymous with "object" as they are each a particular value (realization), and these may be called an instance object; "instance" emphasizes the distinct identity of the object. The creation of an instance is called instantiation.

In the context of POSIX-oriented operating systems, the term "(program) instance" typically refers to any executing process instantiated from that program (via the fork() and exec() system calls); that is, each executing process in the OS is an instance of some program which it has been instantiated from.[1]

In class-based programming, objects are created from classes by subroutines called constructors, and destroyed by destructors. An object is an instance of a class, and may be called a class instance or class object; instantiation is then also known as construction. Not all classes can be instantiated – abstract classes cannot be instantiated, while classes that can be instantiated are called concrete classes. In prototype-based programming, instantiation is instead done by copying (cloning) a prototype instance.

An object may be varied in a number of ways. Each realized variation of that object is an instance of its class. That is, it is a member of a given class that has specified values rather than variables. In a non-programming context, you could think of "dog" as a type and your particular dog as an instance of that class.[2]

An important distinction is between the data type, which is interface, and the class, which is implementation.

The meaning of the term "type" in computer science is rather similar to the meaning of the word "type" in everyday language. For example, a barman can ask a client what type of beverage does he or she want – coffee, tea or beer? A particular cup of coffee that the client receives is in the role of an instance, while two cups of coffee would form a set of two instances of coffee, determining its type at the same time.

References
  1. Bach, Maurice J. (1986). The Design of the UNIX Operating System. Prentice Hall. pp. 10, 24. ISBN 0-13-201799-7. Archived from the original on 2010-03-15.
  2. "What is instance? – Definition from WhatIs.com". Whatis.techtarget.com. Retrieved February 9, 2014.
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