Class variable

In object-oriented programming with classes, a class variable is a variable defined in a class (i.e. a member variable) of which a single copy exists, regardless of how many instances of the class exist.^[1][2][3][4]

A class variable is the opposite of an instance variable. It is a special type of class attribute (or class property, field, or data member).

In Java, C#, and C++, class variables are declared with the keyword static, and may therefore be referred to as static member variables.

The same dichotomy between instance and class members applies to methods ("member functions") as well; a class may have both instance methods and class methods. Again, Java, C#, and C++ use the keyword static to indicate that a method is a class method ("static member function").

Example

struct Request {
 
static int count;
int number;
 
    Request() {
        number = count; // modifies the instance variable "this->number"
       ++count; // modifies the class variable "Request::count"
    }
 
};
 
int Request::count = 0;

In this C++ example, the class variable Request::count is incremented on each call to the constructor, so that Request::count always holds the number of Requests that have been constructed, and each new Request object is given a number in sequential order. Since count is a class variable, there is only one object Request::count; in contrast, each Request object contains its own distinct number field.

Notes

1. ^ "The Java Tutorial, Variables". http://download.oracle.com/javase/tutorial/java/nutsandbolts/variables.html. Retrieved 2010-10-21. 
2. ^ "The Java Tutorial, Understanding Instance and Class Members". http://download.oracle.com/javase/tutorial/java/javaOO/classvars.html. Retrieved 2010-10-21. 
3. ^ "The Python Language Reference, Compound Statements". http://docs.python.org/reference/compound_stmts.html#class-definitions. Retrieved 2010-10-21. 
4. ^ "Objective-C Runtime Reference". http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/ObjCRuntimeRef/Reference/reference.html. Retrieved 2010-10-21.