c - C 中的多个宏

Question

#define a 10
#define a 20
{

    printf("%d",a);
}

为什么答案是 20 而不是 10？ 由于/* #define a 10 */是第一个宏，所以不是应该用它来代替 'a' 吗？

Answer 1

你的问题的前提(应该使用第一个定义)是错误的。

您的编译器已决定使用第二个定义。它真的可以做任何它想做的事，因为你的程序格式不正确/损坏/错误:

[C99: 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. [..]

这里，在 the documentation ，我们看到 GCC 的非标准行为被明确定义:

If a macro is redefined with a definition that is not effectively the same as the old one, the preprocessor issues a warning and changes the macro to use the new definition. If the new definition is effectively the same, the redefinition is silently ignored. This allows, for instance, two different headers to define a common macro. The preprocessor will only complain if the definitions do not match.

简而言之:不要这样做。