c - 当 C 预处理器宏被定义两次时会发生什么？

Question

我定义了两次宏如下:

#define a 2  
#define a 3   

我认为代码中出现的任何 a 都会被替换为 2，而当遇到 #define a 3 时，没有代码中有更多的 a 可用 3 替换，因此 2 将优先。

但是我执行的时候a被3代替了，为什么？

Answer 1

如果你像这样定义一个宏两次，编译器至少应该给你警告，如果不是错误的话。这是一个错误。

§6.10.3/2 : 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.

您可以通过显式删除以前的定义来重新定义宏:

#define a 2
/* In this part of the code, a will be replaced with 2 */
...

#undef a
#define a 3
/* From here on, a will be replaced with 3 */
...

宏替换在文件被读取时发生，使用文件中此时处于事件状态的宏定义，除了在(大多数)预处理指令内。

§6.10/7: The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated.

§6.10.3.5/1: A macro definition lasts (independent of block structure) until a corresponding #undef directive is encountered or (if none is encountered) until the end of the preprocessing translation unit.