C++ 嵌套名称说明符中的不完整类型
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Incomplete type in nested name specifier
提问by Filip Roséen - refp
I tried to use incomplete type in nested name specifier as the following:
我尝试在嵌套名称说明符中使用不完整的类型，如下所示：
class A;
int b= A::c; // error: incomplete type ‘A' used in nested name specifier
class A {
static const int c=5;
};
There is says nothing about it in the 3.4.3/1 of N3797 working draft:
N3797工作草案的3.4.3/1中没有提及：
The name of a class or namespace member or enumerator can be referred to after the :: scope resolution operator (5.1) applied to a nested-name-specifier that denotes its class, namespace, or enumeration
类或命名空间成员或枚举器的名称可以在 :: 作用域解析运算符 (5.1) 之后引用，该运算符应用于表示其类、命名空间或枚举的嵌套名称说明符
So is that behavior implementation dependent?
那么这种行为实现是否依赖？
回答by Filip Roséen - refp
Introduction
介绍
There are several places in the standard that implicitly implies that your code is ill-formed, but the below quotation speaks for itself:
标准中有几个地方暗示您的代码格式不正确，但下面的引用不言自明：
3.3.2p6Point of declaration[basic.scope.pdecl]After the point of declaration of a class member, the member name can be looked up in the scope of its class.
3.3.2p6申报点[basic.scope.pdecl]在类成员声明点之后，可以在其类的范围内查找成员名称。
The problem with your code isn't that you try to reach inside the body of an incomplete type, the problem is that you can only refer to a class member name afterit has been declared.
您的代码的问题不在于您试图访问不完整类型的主体，问题在于您只能在声明后引用类成员名称。
Since your forward-declaration (of course) doesn't introduce any member named c, it is ill-formed to refer to such name.
由于您的前向声明（当然）没有引入任何名为c 的成员，因此引用此类名称是不正确的。
The misleading diagnostic...
误导性的诊断...
The diagnostic issued by both gccand clangwhen being fed your code is somewhat misleading, and honestly I feel a bug report is in order.
gcc和clang在输入您的代码时发出的诊断信息有些误导，老实说，我觉得应该提交错误报告。
foo.cpp:3:8: error: incomplete type 'A' named in nested name specifier
We areallowed to name an incomplete type in a nested-name-specifier, but as said; we are not allowed to refer to a member that has not yet been declared.
我们被允许的名字在一个不完整的类型嵌套名称说明符，但说。我们不允许引用尚未声明的成员。
ill-formed:
格式错误：
class X {
static int a[X::x]; // ill-formed, `X::x` has not yet been declared
static int const x = 123;
};
legal:
合法的：
class X {
int const x = 123;
int a[X::x]; // legal, `X` is incomplete (since we are still defining it)
// but we can still refer to a _declared_ member of it
};
