c++ - 为什么类的方法不能访问我类的某些字段？

Question

我尝试执行 linked_ptr .这是一项学习任务。这是我的代码的一部分:

template <class T>
class linked_ptr 
{
public:
    //***************
    linked_ptr<T>(linked_ptr<T> const& other)
    {
        p = other.p;

        left_ptr = &other;
        right_ptr = other.right_ptr;

        if (other.right_ptr != nullptr)
        {
            (other.right_ptr)->left_ptr = this;
        }

        other.right_ptr = this;
    }

    template <class U>
    linked_ptr<T>(linked_ptr<U> const& other)
    {
        p = other.p;

        left_ptr = &other;
        right_ptr = other.right_ptr;

        if (other.right_ptr != nullptr)
        {
            (other.right_ptr)->left_ptr = this;
        }

        other.right_ptr = this;
    }

private:
    T *p;
    mutable linked_ptr const* left_ptr;
    mutable linked_ptr const* right_ptr;
};


class A
{
public:
    int a = 0;
    A(int aa)
    {
        a = aa;
    }
};

class B : public A
{
public:
    B(int bb)
    {
        a = bb;
    }
};

int main()
{
    linked_ptr<B> a(new B(5));
    linked_ptr<A> b(a);

    return 0;
}

我有一些错误:

cannot access private member declared in class 'smart_ptr::linked_ptr<B>'
cannot access private member declared in class 'smart_ptr::linked_ptr<B>'   
cannot access private member declared in class 'smart_ptr::linked_ptr<B>'
cannot access private member declared in class 'smart_ptr::linked_ptr<B>'
cannot access private member declared in class 'smart_ptr::linked_ptr<B>'
cannot access private member declared in class 'smart_ptr::linked_ptr<B>'
ptr::linked_ptr<B> *' to 'const smart_ptr::linked_ptr<A> *'
ptr::linked_ptr<B> *' to 'const smart_ptr::linked_ptr<A> *'
linked_ptr<A> *const ' to 'const smart_ptr::linked_ptr<B> *'
linked_ptr<A> *const ' to 'const smart_ptr::linked_ptr<B> *'

我不知道这些错误有什么关系。有趣的是，linked_ptr<T>(linked_ptr<T> const& other)效果很好，但是 linked_ptr<T>(linked_ptr<U> const& other)没有。

我该如何解决这些问题？我可以将两个复制构造函数合并为一个吗？

附言当然，U是 T 的 child .

Answer 1

当 T和 U是不同的类型，那么 linked_ptr<T>和 linked_ptr<U>是不同的类型。这意味着他们看不到对方的私有(private)成员。

您需要制作其他linked_ptr friend 们:

template <class T>
class linked_ptr {
    // The rest...

    template<class U>
    friend class linked_ptr;
};