c++ - 基类嵌套类的外联定义

Question

下面的代码是否有效？

struct A 
{
    struct nested;
};

struct B : public A {};

struct B::nested {};

gcc 接受它，但 clang 拒绝它并出现以下错误:

test.cpp:8:14: error: no struct named 'nested' in 'B'
    class B::nested {};
          ~~~^

Answer 1

是的，它是无效的。这是引述。

第 9 节第 11 段，强调我的:

If a class-head-name contains a nested-name-specifier, the class-specifier shall refer to a class that was previously declared directly in the class or namespace to which the nested-name-specifier refers, or in an element of the inline namespace set (7.3.1) of that namespace (i.e., not merely inherited or introduced by a using-declaration), and the class-specifier shall appear in a namespace enclosing the previous declaration. In such cases, the nested-name-specifier of the class-head-name of the definition shall not begin with a decltype-specifier.

在您的示例中，class-head-name 是标记 B::nested，它包含 nested-name-specifier B:: 和类名 嵌套。 class-specifier 是整个 struct B::nested { ... }。