#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. define的基本用法
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 opti
on -pedantic, since other compilers may handle it differently
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