c++ - 基本说明符中的全局限定

Question

作为“What is this crazy C++11 syntax ==> struct : bar {} foo {};?”的后续，我希望编译以下 C++0x 代码:

struct x {};
struct :::x {} y {};

但是，GCC 4.7.0 20110731 告诉我:

error: global qualification of class name is invalid before ':' token

当我恢复理智并给第二个 UDT 命名时:

struct x {};
struct a:::x {} y{}; // remember, identical to `a::: x` or `a: ::x` or `a:: :x` etc

错误是:

error: 'a' has not been declared

---

这三个冒号似乎被词法化为 <::> <:>而不是 <:> <::> ，但这是否可以从[最终草案]标准中清楚地推导出来？

与问题“Global qualification in a class declarations class-head”是否相关？

Answer 1

这只是与解析有关。来自§2.5.3

If the input stream has been parsed into preprocessing tokens up to a given character, the next preprocessing token is the longest sequence of characters that could constitute a preprocessing token, even if that would cause further lexical analysis to fail.

基本上，它必须取最长的字符序列，所以 ::: 总是被解析为 :: : x+++y 总是被解析为 x ++ + y.

这称为 Maximal Munch解析。