c-preprocessor - 为什么预处理器给了一个空间？

Question

我想使用预处理器注释一行:

#define open /##*

#define close */

main()
{
        open commented line close
}

当我这样做时 $gcc -E filename.c我期望

/* commented line */ 

但我得到了

/ * commented line */ 

以便编译器显示错误

为什么它会给出不需要的空间？

Answer 1

来自 GNU C 预处理器文档:

However, two tokens that don't together form a valid token cannot be pasted together. For example, you cannot concatenate x with + in either order. If you try, the preprocessor issues a warning and emits the two tokens. Whether it puts white space between the tokens is undefined. It is common to find unnecessary uses of '##' in complex macros. If you get this warning, it is likely that you can simply remove the '##'.

在这种情况下，“*”和“/”不会形成有效的 C 或 C++ 标记。所以它们被发射出来，它们之间有一个空格。

(另外:即使您设法将“注释”插入到 C 预处理器的输出中，您也可能会遇到 C 编译错误。那里不应该有任何注释。)