嵌套的 class 名称在封闭的 class 模板中使用时是否被视为当前实例化

Question

根据规则：
temp.res#3

When a qualified-id is intended to refer to a type that is not a member of the current instantiation ([temp.dep.type]) and its nested-name-specifier refers to a dependent type, it shall be prefixed by the keyword typename, forming a typename-specifier. If the qualified-id in a typename-specifier does not denote a type or a class template, the program is ill-formed.

这意味着，当当前实例化的成员用作类型说明符时，它没有必要以关键字 typename.

作为前缀

请考虑以下代码：

#include <iostream>
template<class T>
struct A{
    struct B{
        struct C{
           A::B* p; //#5
        };
      B::C c;  //#3
      A::B::C d; //#4
    };
    A::B a;  //#1
    /*B::C b;*/  //#2
};
int main(){

}

当前实例化的定义是： temp.dep.type#1

A name refers to the current instantiation if it is

in the definition of a class template, a nested class of a class template, a member of a class template, or a member of a nested class of a class template, the injected-class-name of the class template or nested class,

in the definition of a primary class template or a member of a primary class template, the name of the class template followed by the template argument list of the primary template (as described below) enclosed in <> (or an equivalent template alias specialization),

in the definition of a nested class of a class template, the name of the nested class referenced as a member of the current instantiation, or

由于 current instantiation 的第一个项目符号，#1 处的声明格式正确。也就是说，在 #1，名称 A 是 current instantiation 的名称，类似于 #3。

#4 格式正确，因为它遵循 current instantiation 的第三个项目符号，因此对于 A::B，其中 B 是当前实例化的名称。

但是，我无法理解为什么 #2 格式错误，换句话说，它需要关键字 typename，根据第一个项目符号，不是nested class 被认为是当前实例化的名称？

坦率地说，我不知道如何理解第一颗子弹。至少，我觉得这句话含糊不清。这是否意味着只要在句子中列出的定义之一中，这些 injected-class-names(primary class name or nested class name) 都被视为当前实例化的名称？ #2似乎不支持这种解释。

我可以从这个例子中推断出，嵌套的class中的enclosing class name可以被认为是当前实例化的名称，而不是相反。如果这样理解句子，就可以理解为什么#5处的代码是合式的了。

那么，如何正确读出第一个项目符号呢？你同意这句话含糊吗？

Answer 1

Is a nested class name be considered as a current instantiation when it is used in a enclosing class template?

没有

然而，与标准的许多其他段落一样，第 [temp.dep.type]/1.1 段确实很冗长：

A name refers to the current instantiation if it is

(1.1) in the definition of a class template, a nested class of a class template, a member of a class template, or a member of a nested class of a class template, the injected-class-name of the class template or nested class, [...]

我们可能需要将其分解为不同的情况，以了解它不适用于 class 模板定义中的嵌套-class-名称，其中，依次，嵌套的class定义为：

A name refers to the current instantiation if it is,

in the definition of a class template, the injected-class-name of the class template ["or nested class" does not apply for this case]

in the definition of a nested class of a class template, the injected-class-name of the class template or [the] nested class

in the definition of a member of a class template, the injected-class-name of the class template ["or nested class" does not apply for this case]

in the definition of a member of a nested class of a class template, the injected-class-name of the class template or [the] nested class

我强调的联系”如果它在定义的X中（ of a Y), the X 的注入-class-名称 (or [the] Y)"。特别注意，当定义是 class 模板时（当 X 是 class 模板时），没有 "a 嵌套 class" 到回头参考后面的部分，在描述“injected-class-name of the ...”时。

相反，无法将 [temp.dep.type]/1.1 解释为

" in the definition of a class template, the injected-class-name of the class template or any nested class of the class template "

因此，在你的例子中，#2，我们在中定义了一个class模板（即A），意思是只有class模板本身的名称，即A，指的是当前实例化。

Do you agree the sentence is vague?

鉴于上述情况，我不会说它是模糊的（因为这意味着一些歧义，我看不到），但可能不必要地复杂。如果将 [temp.dep.type]/1.1 分成两个单独的段落，可以说会更清楚：

A name refers to the current instantiation if it is

(1.1a) in the definition of a class template, or a member of a class template, the injected-class-name of the class template,

(1.1b) in the definition of a nested class of a class template, or a member of a nested class of a class template, the injected-class-name of the class template or nested class, [...]

嵌套的 class 名称在封闭的 class 模板中使用时是否被视为当前实例化

Is a nested class name be considered as a current instantiation when it is used in a enclosing class template

c++

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