C++

21.7    Summary

For clarity, Table 21.1 lists the most important preprocessor directives once again. I have added a few more with a brief explanation.

Directive	Explanation
#include	Include another file
#define	Define identifier or macro
#undef	Remove identifier definition
#ifdef	Check if a preprocessor identifier is defined
#ifndef	Check if a preprocessor identifier is not defined
#elifdef	Alternative check if an identifier is defined (C++23)
#elifndef	Alternative check if an identifier is not defined (C++23)
#if	Check preprocessor expression
#else	Alternative source code to one of the preprocessor ifs
#elif	Alternative source code and another check
#endif	End of the ...