c - #define 多次没有#undef，合法吗？

Question

比如我定义了3次AA，合法吗？:

#include<stdio.h>
#define AA 10
#define AA 20
#define AA 30
int main() {
    printf("AA");
}

Answer 1

这在 C 和 C++ 中都是不合法的。

引自 C 标准草案 N1570 :

6.10.3 Macro replacement

Constraints

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.

引自 C++ 标准草案 N4582 :

16.3 Macro replacement [cpp.replace]

1 Two 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 may be redefined by another #define preprocessing directive provided that the second definition is an object-like macro definition and the two replacement lists are identical, otherwise the program is ill-formed. Likewise, an identifier currently defined as a function-like macro may be redefined by another #define preprocessing directive provided that 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, otherwise the program is ill-formed.