C/C++的redefinition

在標頭檔裡寫下下面兩行相同的代碼：

typedef int X;

typedef int X;

gcc編譯馬上就會報redefinition of typedef 'X'，換g++居然啥事沒有，怪異吧？於是去查文檔，發現C++裡是這樣描述的：“
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，看起來的確是C++針對non-class情況容忍了，像下面這種情況就報錯：

class A{

typedef int Y;

typedef int Y;

};

 

可惜在C語言標準裡找來找去，沒看到對redefinition的明確說明，不確定是不是其它C編譯器都會報錯。

 

 

我隨即想到了宏，於是嘗試了下面三種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++編譯(應該說這裡其實是前置處理器cc1/cc1plus)的結果是一致的，Case1/2都OK，Case3會報錯。查了下標準，是這樣描述的

(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.

這裡的關鍵點就是，僅當兩者文字（完全一致，包括空白符）

 

 

http://hi.baidu.com/zhuxiaoyin/blog/item/f603f58e2e1321e2f11f3656.html