c++ - 定义特殊类的不完整结构

Question

我在类特化中声明不完整的结构并稍后定义它时遇到问题。

struct Foo {
    template <bool Y, typename D>
    struct Bar {};

    template <typename D>
    struct Bar<true, D> {
        struct Qux;
    };

    template <typename D>
    struct Bar<true, D>::Qux { int x; };
};

此代码在 gcc 中有效，但在 clang 3.3 中失败:

r.cpp:42:26: error: non-friend class member 'Qux' cannot have a qualified name
    struct Bar<true, D>::Qux { int x; };
           ~~~~~~~~~~~~~~^

如果代码是在命名空间范围内编写的(没有 struct Foo)，它也可以在 clang 中工作。

另一方面，如果 struct Foo 变成一个模板，如下所示，代码在 gcc-4.9(未发布)中中断，尽管它在 gcc-4.7 中继续工作。

template <typename X>
struct Foo {
    template <bool Y, typename D>
    struct Bar {};

    template <typename D>
    struct Bar<true, D> {
        struct Qux;
    };

    template <typename D>
    struct Bar<true, D>::Qux { int x; };
};

Clang 失败:

r.cpp:43:26: error: template specialization or definition requires a template parameter list corresponding to the nested type 'Bar<true, type-parameter-1-0>'
    struct Bar<true, D>::Qux { int x; };
                         ^
r.cpp:43:26: error: non-friend class member 'Qux' cannot have a qualified name
    struct Bar<true, D>::Qux { int x; };
           ~~~~~~~~~~~~~~^
2 errors generated.

Gcc-4.9 失败并出现类似错误:

r.cpp:43:26: error: too few template-parameter-lists
     struct Bar<true, D>::Qux { int x; };
                          ^

Answer 1

看起来您别无选择，只能将该定义放在命名空间范围内(或 Bar 内)。第 9/1 段 (n3337) 说你的代码是非法的:

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.