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Objects can contain other objects in their instance variables; this is known as object composition. For example, an object in the Employee class might contain (either directly or through a pointer) an object in the Address class, in addition to its own instance variables like "first_name" and "position".
In some languages, for example C++, Java, and Raku this or self is a keyword, and the variable automatically exists in instance methods. In others, for example, Python, Rust, and Perl 5, the first parameter of an instance method is such a reference. It needs to be specified explicitly.
In Python, everything is an object, even classes. Classes, as objects, have a class, which is known as their metaclass. Python also supports multiple inheritance and mixins. The language supports extensive introspection of types and classes. Types can be read and compared—types are instances of type. The attributes of an object can be ...
Instance variables are properties of that object. All instances of a class have their own copies of instance variables, even if the value is the same from one object to another. One class instance can change values of its instance variables without affecting all other instances. A class may have both instance variables and class variables.
In object-oriented computer programming, a null object is an object with no referenced value or with defined neutral (null) behavior.The null object design pattern, which describes the uses of such objects and their behavior (or lack thereof), was first published as "Void Value" [1] and later in the Pattern Languages of Program Design book series as "Null Object".
In computer science, an instance is an occurrence of a software element that is based on a type definition. When created, an occurrence is said to have been instantiated , and both the creation process and the result of creation are called instantiation .
The latter returns true when the particular instance the message was sent to is an instance of a descendant of the class in question. For example, consider the following example code (you can immediately try this with the Interactive Ruby Shell):
As an example, a car object c is an instance of the class Car. In turn, the class Car is again an object and as such an instance of the metaclass of Car called Car class. Note the blank in the name of the metaclass. The name of the metaclass is the Smalltalk expression that, when evaluated, results in the metaclass object.