__has_include ("file-name")

__has_include (<file-name>)

Evaluates as 1 if the file specified by file-name for __has_include exists during preprocessing, and evaluates as 0 if the file does not exist.

If __has_include is evaluated as a macro, the value is set to 1.

__has_include can only be used in a preprocess directive such as #if. If it is used for anything else, an error occurs.

#if defined(__has_include)

#if __has_include("existent_usr_header1.h")

#include "existent_usr_header1.h";

#endif 

#if __has_include(<existent_usr_header1.h>)

#include <existent_usr_header1.h>;

#endif

#endif

The following description will cause an error.

int exists = __has_include("existent_usr_header1.h");