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 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. Each time a program runs, it is an instance of that program. 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.

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

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