C++类中的类方法继承

Question

我有以下结构，我想从 D 调用 foo，这怎么可能？我收到了错误消息，我在下面的行中发表了评论。

class A
{
protected:
    class B
    {
    public:
        B(x)
        {
            //...
        }

    protected:
        virtual void foo()
        {
            //...
        }
    };
};

class C : public A
{
protected:
    class D : public A::B
    {
    public:
        D(x) : B(x)
        {
            //empty
        }
    };

    void bar()
    {
        D var = D(x);
        var.foo(); //cant access protected member in class A::B
    }
};

Answer 1

foo()是B的protected成员函数，意味着foo()只允许从它的子类(child class)中调用。

bar()是C的成员函数，C继承自A，所以C不是B的子类。

如果把bar放在D里就OK了，如下代码:

class A
{
protected:
    class B
    {
    public:
        B(int x)
        {
            //...
        }

    protected:
        virtual void foo()
        {
            //...
        }
    };
};

class C : public A
{
protected:
    class D : public A::B
    {
    public:
        D(int x) : B(x)
        {
            //empty
        }

        // Here is OK, while D is subclass of B
        void bar()
        {
            int x;
            D var = D(x);
            var.foo(); //cant access protected member in class A::B
        }
    };

    // void bar()
    // {
    //     int x;
    //     D var = D(x);
    //     var.foo(); //cant access protected member in class A::B
    // }
};


int main(){


    return 0;
}