The preprocessor has three main jobs: it allows you to include the contents of other files in your own file (usually header files that declare functions), substitute named expressions, conditionally compile things.
The preprocessor is a blunt tool, as it just substitutes text without any regard to the syntax or conventions of the C language itself. It is a source of lots of subtle errors, which require thought and planning to avoid. It is also virtually invisible at point of use, which makes finding problems more difficult than it should be.
Modern C and C++ development guidelines suggest that you should reduce your reliance on the preprocessor. There are better mechanisms for most usage:Defining constants - use const definitions like const int MAX_ERRORS = 42; instead of #define MAX_ERRORS 42Defining functions - use inline function definitions like inline int max(int a, int b) { return (a>b)?a:b; } instead of #define max(a,b) (a>b)?a:b(Can you spot the deliberate error in the #define of "max", above?)File inclusion and conditional compilation do not have good substitutes for #include and #if - my only advice here is to use common and well-known patterns for file inclusion, and try to keep #if usage to a minimum. If you have lots of #ifs everywhere, it's a bad sign.
