C++ Primer, Fifth Edition

7.6. static Class Members

Classes sometimes need members that are associated with the class, rather than with individual objects of the class type. For example, a bank account class might need a data member to represent the current prime interest rate. In this case, we’d want to associate the rate with the class, not with each individual object. From an efficiency standpoint, there’d be no reason for each object to store the rate. Much more importantly, if the rate changes, we’d want each object to use the new value.