不是所有的token在宏定义中都可以使用##连接

我定义了下面几个宏:

#define CONNAT(a, b) CONNAT_IMPL(a, b)
#define CONNAT_IMPL(a, b) a##b
#define STR(s) STR_IMPL(s)
#define STR_IMPL(s) #s

然后再代码中使用:

std::string str = STR(CONNAT(/home/boy/, zz));

我的本意是想得到一个字符串"/home/boy/zz"

但是GCC编译器给出了编译错误:

error: pasting "/" and "zz" does not give a valid preprocessing token

 如果将 std::string str = STR(CONNAT(/home/boy/, zz)); 改成std::string str = STR(CONNAT(/home/boy_, zz)); 就没有问题。

看了一下GCC对宏连接处理的说明，发现有些符号是不能用于宏连接的，譬如上面的'/'。

Keep in mind that the C preprocessor converts comments to whitespace before macros are even considered. Therefore, you cannot create a comment by concatenating `/' and `*'. You can put as much whitespace between `##' and its operands as you like, including comments, and you can put comments in arguments that will be concatenated. However, it is an error if `##' appears at either end of a macro body.

 GCC文档链接:Concatenation - The C Preprocessor

注意:这种问题和编译器有关系，使用MSVC编译就不会报错。