We can solve the problem of including a header file twice by defining a unique preprocessor instruction constant that if it is encountered we do not process the body of the header file, i.e.:
#ifndef SOME_ARBITRARY_UNIQUE_CODE #define SOME_ARBITRARY_UNIQUE_CODE //.... body of header file goes here. #endif // end of the block which started at line 2
So how do you make a unique arbitrary code? There is more than one way to solve to solve this problem:
Have convention that maps the file name of the header file to a unique code
Or use something like a Globally Unique IDentifier - GUID.