In
class-based,
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
, a class variable is a
variable defined in a
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differently ...
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
method
Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to:
*Scien ...
s ("member functions") as well; a class may have both
instance methods and
class method
A method in object-oriented programming (OOP) is a procedure associated with a message and an object. An object consists of ''state data'' and ''behavior''; these compose an ''interface'', which specifies how the object may be utilized by any of ...
s.
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
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
,
C# [ , and ]C++
C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
, where class variables and class methods are declared with the static keyword, and referred to as static member variables or static member functions.
Example
struct Request ;
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.
Notes
{{DEFAULTSORT:Class Variable
Object-oriented programming
Variable (computer science)
Articles with example C++ code