c++ - 成员函数 Outer::f() 不是类 Outer::Inner 的 friend 。为什么？

Question

根据clang , gcc 和 vs2013，函数 Outer::f 是 not 类 Outer::Inner 的 friend 。

struct Outer {
    void f() {}
    class Inner {
        friend void f();
        static const int i = 0;
    };
};

void f() { int i = Outer::Inner::i; }

从 [namespace.memdef]/3 我希望函数 Outer::f 成为 Outer::Inner 的 friend ，而不是 : :f，因为 friend 声明不是其 namespace 中包含名称 f 的第一个。

[namespace,memdef]/3(重点是我的):

Every name first declared in a namespace is a member of that namespace. If a friend declaration in a non-local class first declares a class, function, class template or function template⁹⁷ the friend is a member of the innermost enclosing namespace. The friend declaration does not by itself make the name visible to unqualified lookup (3.4.1) or qualified lookup (3.4.3). [ Note: The name of the friend will be visible in its namespace if a matching declaration is provided at namespace scope (either before or after the class definition granting friendship). — end note ] If a friend function or function template is called, its name may be found by the name lookup that considers functions from namespaces and classes associated with the types of the function arguments (3.4.2). If the name in a friend declaration is neither qualified nor a template-id and the declaration is a function or an elaborated-type-specifier, the lookup to determine whether the entity has been previously declared shall not consider any scopes outside the innermost enclosing namespace.

Answer 1

您引用的标准的第一部分说(强调我的):

Every name first declared in a namespace is a member of that namespace. If a friend declaration in a nonlocal class first declares a class or function the friend class or function is a member of the innermost enclosing namespace.

您假设一个类与命名空间相同，这是不正确的。

namespace Outer {
    void f();
    class Inner {
        friend void f();
        static const int i = 0;
    };
}

void Outer::f() { int i = Outer::Inner::i; }

应该可以。要将类成员函数用作 friend ，您必须使用:

struct Outer {
    void f();
    class Inner {
        friend void Outer::f();
        static const int i = 0;
    };
};

void Outer::f() { int i = Outer::Inner::i; }