Object computer science: Encyclopedia II - Object computer science - Objects in Object-Oriented Programming

Object computer science - Objects in Object-Oriented Programming

In Object-Oriented Programming (OOP), an instance of a program (i.e. a program running in a computer) is treated as a dynamic set of interacting objects. Objects in OOP extend the more general notion of objects described above to include a very specific kind of typing, which among other things allows for:

1. data members that represent the data associated with the object.
2. methods that access the data members in predefined ways.

In the case of most objects, one can access the data members only through the method members, making it easy to guarantee that the data will always remain in a well-defined state (class invariants will be enforced). Some languages do not make distinctions between data members and methods.

In a language where each object is created from a class, an object is called an instance of that class. If each object has a type, two objects with the same class would have the same datatype. Creating an instance of a class is sometimes referred to as instantiating the class.

A real-world example of an object would be "my dog", which is an instance of a type (a class) called "dog", which is a subclass of a class "animal". In the case of a polymorphic object, some details of its type can be selectively ignored, for example a "dog" object could be used by a function looking for an "animal". So could a "cat", because it too belongs to the class of "animal". While being accessed as an "animal", some member attributes of a "dog" or "cat" would remain unavailable, such as the "tail" attribute, because not all animals have tails.

Three properties characterize objects:

1. identity - the property of an object that distinguishes it from other objects
2. state - describes the data stored in the object
3. behaviour - describes the methods in the object's interface by which the object can be used

Some terms for specialized kinds of objects include:

• Singleton object - An object that is the only instance of its class during the lifetime of the program.
• Functor (function object) - an object with a single method (in C++, this method would be the function operator, "operator()") that acts much like a function (like a C/C++ pointer to a function).
• Immutable object - an object set up with a fixed state at creation time and which does not vary afterward.
• First-class object - an object that can be used without restriction.
• Container object - an object that can contain other objects.
• Factory object - an object whose purpose is to create other objects.
• Metaobject - an object from which other objects can be created (Compare with class, which is not necessarily an object)
• Prototype - a specialized metaobject from which other objects can be created by copying
• God object is an object that knows too much or does too much. The God object is an example of an anti-pattern.

Adapted from the Wikipedia article "Objects in Object-Oriented Programming", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki