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.
classOrder{ private: inlinestaticintnextId=0; intid; public: Order(): id{nextId++}{} };In this C++ example, the class variable Order::nextId is incremented on each call to the constructor, so that Order::nextId always holds the number of Orders that have been constructed, and each new Order object is given a number in sequential order. Since nextId is a class variable, there is only one object Order::nextId; in contrast, each Order object contains its own distinct id field.
Also note that the variable Order::nextId is initialized only once (as static members that are instances of itself must provide the definitions outside the class. Trying to use declare an instance of a class as a static member of itself fails as no definition of an object may be an incomplete type, which the class is incomplete until it is closed.
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