Static variable in c plus plus?

There are two uses for a static variable in C++. When declared outside of a class, a variable is regarded as being global. However a static variable is deemed local to the file in which it is declared. That is, the variable is scoped to the file, and cannot be accessed by code outside of that file. This aspect was inherited from C.

C++ also allows static variables to be declared inside a class. In this case, the variable is local to the class. By contrast, instance variables (non-static member variables) are local to each instance of the class. With static variables, there is only one instance of each variable which can be shared by all instances of the class. It is not unlike a global but it is scoped to the class.

Since all static variables are instantiated at compile time, they exist for the entire duration a program runs. Even if they fall from scope, they never lose their value. Static variables defined within a class are also available even when no instances of the class are instantiated. Their visibility outside of the class is dependent upon whether they are declared public, protected or private.