在头文件里写下下面两行相同的代码：
typedef int X;
typedef int X;
gcc编译马上就会报redefinition of typedef ‘X’，换g++居然啥事没有，怪异吧？于是去查文档，发现C++里是这样描述的：“ In a given non-class scope, a typedef specifier can be used to redefine the name of any type declared in that scope to refer to the type to which it already refers. ”嗯，看起来的确是C++针对non-class情况容忍了，像下面这种情况就报错：
class A{
typedef int Y;
typedef int Y;
};
可惜在C语言标准里找来找去，没看到对redefinition的明确说明，不确定是不是其它C编译器都会报错。
我随即想到了宏，于是尝试了下面三种case：
Case1： #define M 1
#define M 1
Case2： #define M(x) x++
#define M(x) x++
Case3: #define M(x) (x+1)
#define M(x) (x + 1)
gcc/g++编译(应该说这里其实是预处理器cc1/cc1plus)的结果是一致的，Case1/2都OK，Case3会报错。查了下标准，是这样描述的
(1) Tw o replacement lists are identical if and only if the preprocessing tokens in both have the same number, ordering, spelling, and white-space separation, where all white-space separations are considered identical.
(2) An identifier currently defined as an object-like macro shall not be redefined by another #define preprocessing directive unless the second definition is an object-like macro definition and the two replacement lists are identical. Likewise, an identifier currently defined as a function-like macro shall not be redefined by another #define preprocessing directive unless the second definition is a function-like macro definition that has the same number and spelling of parameters, and the two replacement lists are
identical.
这里的关键点就是，仅当两者文字（完全一致，包括空白符）
http://hi.baidu.com/zhuxiaoyin/blog/item/f603f58e2e1321e2f11f3656.html