Class variable

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title: "Class variable" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["object-oriented-programming", "variable-(computer-science)", "articles-with-example-c++-code"] description: "Variable defined in a class whose objects all possess the same copy" topic_path: "technology/programming-languages" source: "https://en.wikipedia.org/wiki/Class_variable" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Variable defined in a class whose objects all possess the same copy ::

In class-based, object-oriented programming, a class variable is a variable defined in a class of which a single copy exists, regardless of how many instances of the class exist.

A class variable is not an instance variable. It is a special type of class attribute (or class property, field, or data member). 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.

Static member variables and static member functions

In some languages, class variables and class methods are either statically resolved, not via dynamic dispatch, or their memory statically allocated at compile time (once for the entire class, as static variables), not dynamically allocated at run time (at every instantiation of an object). In other cases, however, either or both of these are dynamic. For example, if classes can be dynamically defined (at run time), class variables of these classes are allocated dynamically when the class is defined, and in some languages class methods are also dispatched dynamically.

Thus in some languages, static member variable or static member function are used synonymously with or in place of "class variable" or "class function", but these are not synonymous across languages. These terms are commonly used in Java, C#, and C++, where class variables and class methods are declared with the static keyword, and referred to as static member variables or static member functions.

Examples

C++

::code[lang=cpp] struct Request {

static int count;
int number;

Requestobject() {
    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.

Also note that the variable Request::count is initialized only once.

Python

::code[lang=python] class Dog: vertebrate_group = "mammals" # class variable

dog_1 = Dog print(dog_1.vertebrate_group) # accessing the class variable ::

In the above Python code, it does not provide much information as there is only class variable in the Dog class that provide the vertebrate group of dog as mammals. In instance variable, you could customize your own object (in this case, dog_1) by having one or more instance variables in the Dog class.

This can also be type hinted using . ::code[lang=python] from typing import ClassVar

class Dog: vertebrate_group: ClassVar[str] = "mammals" ::

Notes

References

1. "The Java Tutorial, Variables".
2. "The Java Tutorial, Understanding Instance and Class Members".
3. "The Python Language Reference, Compound Statements".
4. "Objective-C Runtime Reference".
5. "Class Variables in C#".

::callout[type=info title="Wikipedia Source"] This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page. ::