The classes you have seen so far are complete classes: you can create an instance of the class on the free store or the stack. You can do this because the data members of the class have been defined and so it is possible to calculate how much memory is needed for the object, and you have provided the full functionality of the class. These are called concrete classes.
If you have a routine in a class that proves useful and you want to reuse in a new class, you have a few choices. The first is called composition. With composition you add an instance of your utility class as a data member of the classes that will use the routine. A simple example is the string class--this provides all the functionality that you want from a string. It will allocate memory according to how many characters have to be stored and deallocate the memory it uses when the string...