#if defined、#if ！defined用法

大型程序或者修改别人的程序时，当我们需要定义常量(源文件还是头文件 ),我们就必须返回检查原来此常量是否已经定义,

if defined宏 就是用于检测的。

举个例子,如下:

#define ....

#define ....      ....      ....

#define Dataauto 1      ....

要检查Dataauto是否定义,或者我们要给Dataauto一个不同的值，可以添加语句

#if defined a

#undef Dataauto

#define Dataauto 0

#endif   上述语句检验Dataauto是否被定义，如果被定义，则用#undef语句解除定义，并重新定义Dataauto为0

同样，检验Dataauto是否定义：

#ifndef Dataauto    //如果Dataauto没有被定义

#define Dataauto 0

#endif

以上所用的宏中：#undef为解除定义，#ifndef是if not defined的缩写，即如果没有定义。

这就是#if defined 的唯一作用！

1）   #if defined XXX_XXX

#endif   是条件编译，是根据你是否定义了XXX_XXX这个宏，而使用不同的代码。

一般.h文件里最外层的

#if !defined XXX_XXX

#define XXX_XXX

#endif   是为了避免.h头文件被重复include。

2）   #error XXXX   是用来产生编译时错误信息XXXX的，一般用在预处理过程中；

例子：

#if !defined(__Dataauto)

#error C++ compiler required.

#endif

引用别人的英文用法说明：

The special operator defined is used in #if and #elif expressions to test whether a certain name is defined as a macro.

defined name and defined (name) are both expressions whose value is 1 if name is defined as a macro at the current point in the program, and 0 otherwise.

Thus, #if defined MACRO is precisely equivalent to #ifdef MACRO.   defined is useful when you wish to test more than one macro for existence at once.

For example, #if defined (__vax__) || defined (__ns16000__)  would succeed if either of the names __vax__ or __ns16000__ is defined as a macro.   Conditionals written like this: #if defined BUFSIZE && BUFSIZE >= 1024 can generally be simplified to just #if BUFSIZE >= 1024, since if BUFSIZE is not defined, it will be interpreted as having the value zero.

If the defined operator appears as a result of a macro expansion, the C standard says the behavior is undefined. GNU cpp treats it as a genuine defined operator and evaluates it normally. It will warn wherever your code uses this feature if you use the command-line option -pedantic, since other compilers may handle it differently  .