The Art of Assembly Language, 2nd Edition

12.7 Writing Class Methods and Procedures

For each class procedure and method prototype appearing in a class definition, there must be a corresponding procedure or method appearing within the program (for the sake of brevity, this section will use the term routine to mean procedure or method from this point forward). If the prototype does not contain the external option, then the code must appear in the same compilation unit as the class declaration. If the external option does follow the prototype, then the code may appear in the same compilation unit or a different compilation unit (as long as you link the resulting object file with the code containing the class declaration). Like external (non-class) procedures, if you fail to provide the code, ...