#define question

[SunnySmile]

1.I see in some classes, there are

#ifndef IPADD_H
#define IPADD_H
#endif

What is that #xxx for ?
2.I also see

#define something somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse...........
Very long, the whole screen
WHy should they define something like that ? When should we do so ?

Thank you

--Sunny Smile

[Lindley]

1.I see in some classes, there are

#ifndef IPADD_H
#define IPADD_H
#endif

What is that #xxx for ?

These are include guards. They prevent a given header from being #included more than once during a given compile. For instance, if A.h #includes both B.h and C.h, and C.h also #includes B.h, we only want one copy of B.h in the final preprocessed file and this ensures we only get one.

Note, include guards cannot protect against linker errors.

#define something somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse-somethingelse...........
Very long, the whole screen
WHy should they define something like that ? When should we do so ?

Some C programmers like to use macros to reduce the amount of repeated code they need to type. However, macros are difficult to debug, and in most cases C++ provides superior compile-time (as opposed to preprocess-time) alternatives such as templates and inline functions.