#if defined、#if ！defined用法
(摘自百度文库)
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 defineda
#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 #elifexpressions to test whether a certain name is defined as a macro. defined nameand defined (name) are both expressions whose value is 1 if name is defined asa macro at the current point in the program, and 0 otherwise. Thus, #if definedMACRO is precisely equivalent to #ifdef MACRO.
defined is useful when you wish to test more than onemacro 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 havingthe value zero.
If the defined operator appears as a resultof a macro expansion, the C standard says the behavior is undefined. GNU cpptreats it as a genuine defined operator and evaluates it normally. It will warnwherever your code uses this feature if you use the command-line option-pedantic, since other compilers may handle it differently
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