Class variable

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  keyword, and referred to  as static member variables or static member functions.

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

Also note that the variable  is initialized only once.

Python
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.