#ifndef checks whether the given token has been #defined earlier in the file or in an included file; if not, it includes the code between it and the closing #else or, if no #else is present, #endif …

Text inside an ifdef/endif or ifndef/endif pair will be left in or removed by the pre-processor depending on the condition. ifdef means "if the following is defined" while ifndef means "if the …

#ifndef = if not defined. #define = define. Recall also that: #include = act as though the contents of the named file had been copied and pasted here. So the net effect is that with code like: …

Aug 26, 2012 · #ifndef Name2 #define Name2 extern bool Test1; extern bool Test2; #endif // single .cpp bool Test1 = false; bool Test2 = false; Although there is some code smell around …

#ifndef HEADER_INCLUDED_ #define HEADER_INCLUDED_ // Actual payload of the header file #endif In addition at a time where the parsing and compilation of the files would take long time …

Well, the preprocessor's #ifdef and #ifndef mean the following: In your example, you used #define to set a constant variable named LINUX_GRAPHICS to be equal to 011x101. So later in your …

Jul 11, 2010 · #ifndef MYFOO_H #define MYFOO_H /* header information for myfoo.h */ #endif belongs in every header-file. The trick is: you can include a header file (accidentally) more …

#ifndef HEADER_H #include "header.h" #define HEADER_H #endif But, as other answers have already said, it's much better to put the guard in the header itself: header.h : #ifndef …

Jul 13, 2012 · #ifndef _WIN32 tells the pre-processor to include the code below it till a corresponding #endif, if _WIN32 is not defined. #ifndef _WIN32 #define STR1 "Some String" …

Conditional Compilation. You can use the defined operator in the #if directive to use expressions that evaluate to 0 or 1 within a preprocessor line.