static Class Members

Each object of a class has its own copy of all the data members of the class. in certain cases, only one copy of a variable should be shared by all objects of a class.

• static class variable
  • "Class-wide" data; Property of class, not specific object of class
  • Efficient when single copy of data is enough; Only the static variable has to be updated
  • May seem like global variables, but have class scope; Only accessible to objects of same class
  • Initialized exactly once at file scope
  • Exist even if no objects of class exist
  • Can be public, private or protected
• Accessing static class variables
  • Accessible through any object of class
  • public static variables
    • Can also be accessed using binary scope resolution operator(::)
    • Employee::count
• private static variables
  • When no class member objects exist
    • Can only be accessed via public static member function
    • To call public static member function combine class name, binary scope resolution operator (::) and function name; Employee::getCount()
• static member functions
  • Cannot access non-static data or functions
  • No this pointer for static functions; static data members and static member functions exist independent of objects

The programs of Figs. 2.25-2.28 demonstrates a private static data member called count and a public static member function called getCount. Figure 2.28 uses function getCount to determine the number of Employee objects currently instantiated.

Figure 2.25: Employee class definition with a static data member to track the number Employee objects in memory.

Figure 2.26: Employee class member-function definitions. (part 1 of 2)

Figure 2.27: Employee class member-function definitions. (part 2 of 2) and static data member tracking the number of objects of a class. (part 1 of 2)

Figure 2.28: static data member tracking the number of objects of a class. (part 2 of 2)