Instance variables are created when an object is instantiated, and are accessible to all the constructors, methods, or blocks in the class.
Each instance variable lives in memory for the lifetime of the object it is owned by.
An instance variable may also be changed directly, provided access restrictions are set.
[6] In this C++ example, the instance variable Request::number is a copy of the class variable Request::count1 where each instance constructed is assigned a sequential value of count1 before it is incremented.
Since number is an instance variable, each Request object contains its own distinct value; in contrast, there is only one object Request::count1 available to all class instances with the same value.