Why are #ifndef and #define used in C++ header files?
#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 statement. #ifndef is often used to make header files idempotent by defining a token once the file has been included and checking that the token was ...
c preprocessor - The role of #ifdef and #ifndef - Stack Overflow
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 following is not defined".
What is the purpose of using #ifdef and #if in C++?
#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 this could result in speedup since the payload of the header could quickly be skipped (the preprocessing phase processes the source much faster than the actual ...
How to use #ifndef #define #endif properly in C++
2012年8月26日 · #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 the whole thing. You might want to redesign your solution. The use of global variables is in most cases not a good solution.
c++ - #if vs #ifndef vs #ifdef - Stack Overflow
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 program you might want to check if this variable is defined. Then you use #ifdef, when you want to check if this variable is defined and #ifndef if not.
c++ - what does mean #ifndef #define directive - Stack Overflow
#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: #include "A.h" #include "A.h" The first one does a #define and the second then declines to insert a second copy of the same text by virtue of the #ifndef.
#ifndef syntax for include guards in C++ - Stack Overflow
#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 HEADER_H #define HEADER_H /* contents of header.h */ #endif and then the include simply has: #include "header.h" and has one less piece of information to worry about.
What's difference between #pragma and #ifndef? [duplicate]
2010年7月11日 · The #ifndef/#define/#endif trick works on any C compiler, and on some of them it speeds up the compilation process. The #pragma trick is non-standards, and only works on few C compilers, and may result in different semantics in those that do not support it.
c++ - How to use #ifndef with macro argument? - Stack Overflow
2018年1月17日 · I have a simple code in header.h- #define SWAP(a, b) {a ^= b; b ^= a; a ^= b;} This header.h is included in a code.c file, but my requirement is- I want SWAP to be checked first like- #ifndef ...
c - what does #ifndef _WIN32 signify? - Stack Overflow
2012年7月13日 · #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" #endif The macro STR1 will get included if _WIN32 is not defined and will not get included if _WIN32 is defined. Please note that _WIN32 is a system defined macro.