#if #elif #define typedef 使用时的注意事项

假设有如下代码需要兼容三个版本

这样无法完成任务的

无论

typedef enum{
	FIRST,
	SECOND,
	THIRD,
};

#define NUM  THIRD

#if (NUM == FIRST)
#define  VAR  1
#elif(NUM == SECOND)
#define  VAR  2
#elif(NUM == THIRD)
#define  VAR  3
#endif

int main(void)
{
#if (NUM == FIRST)
printf("VAL = %d\n",VAR);
#elif(NUM == SECOND)
printf("VAL = %d\n",VAR);
#elif(NUM == THIRD)
printf("VAL = %d\n",VAR);
#endif
}

#define NUM  FIRST还是

#define NUM  SECOND还是

#define NUM  THIRD

运行结果都一样

但是这样就可以完成任务了

#define FIRST    1
#define SECOND   2
#define THIRD    3

#define NUM  THIRD

#if (NUM == FIRST)
#define  VAR  1
#elif(NUM == SECOND)
#define  VAR  2
#elif(NUM == THIRD)
#define  VAR  3
#endif

int main(void)
{
#if (NUM == FIRST)
printf("VAL = %d\n",VAR);
#elif(NUM == SECOND)
printf("VAL = %d\n",VAR);
#elif(NUM == THIRD)
printf("VAL = %d\n",VAR);
#endif
}

#define NUM  FIRST还是

#define NUM  SECOND还是

#define NUM  THIRD

都可以得到想要的结果

为什么呢？？？？？