#ifdef v #ifndef

ifdef v ifndef

It is worth noting that this is done before compilation.

#ifdef SOMETHING

// DO if SOMETHING has been defined.

#endif

#ifndef SOMETHING

// SKIP if SOMETHING has been defined.

#endif

 

#ifdef IF DEFINED, DO IT.

#ifndef IF NOT DEFINED, DO IT. 

 

So, in a header file for example, we might see;

#ifndef MY_HEADER

#define MY_HEADER

// If this header has not yet been seen, define it and do it.

// So that the next time a file includes this header, it will be excluded.

#endif