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