访问类的嵌套私有类

通常,如果嵌套类是在中声明的,则不可能访问它 private

#include <iostream>
#include <vector>

class Outer
{
    private:
        class Inner
        {
            public:
                void sing() const { std::cout << "la la la\n"; }
                void laugh() const { std::cout << "ha ha ha\n"; }
        };

    private:
        std::vector<Inner> inners;

    public:
        void add() { inners.push_back(Inner()); }
};

int main()
{
    auto o = Outer();
    auto i = Outer::Inner();  // error: 'class Outer::Inner' is private within this context
}

Example on Wandbox

:

#include <iostream>
#include <vector>

class Outer
{
    private:
        class Inner
        {
            public:
                void sing() const { std::cout << "la la la\n"; }
                void laugh() const { std::cout << "ha ha ha\n"; }
        };

    private:
        std::vector<Inner> inners;

    public:
        Inner & add() { inners.push_back(Inner()); return inners.back(); }
};

int main()
{
    auto o = Outer();
    auto i = o.add();
    i.sing();
    i.laugh();
}

Example on Wandbox

如果这是有效的,哪个规则允许此访问?

天真的,我会认为我不能坚持什么 Outer::add() 返回,或者最多可以保存它,但不能访问其成员。