c++ - 如何在我的自定义列表迭代器类中将迭代器转换为 const_iterator？

Question

我正在尝试用 C++ 实现我自己的双向链表 (my_list) 代码，尤其是我的列表的迭代器类。我的问题是我想要从迭代器到 const_iterator 的隐式转换，例如代码 my_list::iterator it = l.begin(); 其中 l 是一个my_list 的实例编译。但是，如果我的编译器没有提示，我无法找到执行此操作的方法。

下面是实现列表节点和迭代器类的代码:

template<class T> class node {
    node(const T& t = T()):data(t),next(0),prev(0) {}
    T data;
    node* next;
    node* prev;

    friend class my_list<T>;
    friend class my_list_iterator<T>;
};

template<class T> class my_list_iterator {
    public:
            // increment and decrement operators
            my_list_iterator operator++();
            my_list_iterator operator++(int);
            my_list_iterator operator--();
            my_list_iterator operator--(int);

            // bool comparison iterators
            bool operator==(const my_list_iterator& other) const {return pos_==other.pos_;}
            bool operator!=(const my_list_iterator& other) const {return pos_!=other.pos_;}

            // member access
            T& operator*() const {return pos_->data;}
            T* operator->() const {return &(pos_->data);}

            // implicit conversion to const iterator
            operator my_list_iterator<const T>() {return my_list_iterator<const T>(pos_);}
    private:
            node<T>* pos_;
            explicit my_list_iterator(node<T>* p=0):pos_(p) {}
            friend class my_list<T>;
};

我省略了 my_list 实现，但如果您认为相关，我可以将其包括在内。当我测试这段代码时，它不会在 GCC 上编译并出现以下错误:

In file included from test.cpp:2:
my_list.h: In instantiation of ‘my_list_iterator<T>::operator my_list_iterator<const T>() [with T = int]’:
test.cpp:12:49:   required from here
my_list.h:37:48: error: no matching function for call to ‘my_list_iterator<const int>::my_list_iterator(node<int>*&)’
   operator my_list_iterator<const T>() {return my_list_iterator<const T>(pos_);}
                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
my_list.h:40:12: note: candidate: ‘my_list_iterator<T>::my_list_iterator(node<T>*) [with T = const int]’
   explicit my_list_iterator(node<T>* p=0):pos_(p) {}
            ^~~~~~~~~~~~~~~~
my_list.h:40:12: note:   no known conversion for argument 1 from ‘node<int>*’ to ‘node<const int>*’
my_list.h:20:25: note: candidate: ‘constexpr my_list_iterator<const int>::my_list_iterator(const my_list_iterator<const int>&)’
 template<class T> class my_list_iterator {
                         ^~~~~~~~~~~~~~~~
my_list.h:20:25: note:   no known conversion for argument 1 from ‘node<int>*’ to ‘const my_list_iterator<const int>&’
my_list.h:20:25: note: candidate: ‘constexpr my_list_iterator<const int>::my_list_iterator(my_list_iterator<const int>&&)’
my_list.h:20:25: note:   no known conversion for argument 1 from ‘node<int>*’ to ‘my_list_iterator<const int>&&’

有人可以帮我解决我做错的事吗？

Answer 1

my_list.h:40:12: note: candidate: ‘my_list_iterator<T>::my_list_iterator(node<T>*) [with T = const int]’
   explicit my_list_iterator(node<T>* p=0):pos_(p) {}
            ^~~~~~~~~~~~~~~~
my_list.h:40:12: note:   no known conversion for argument 1 from ‘node<int>*’ to ‘node<const int>*’

A node<int>和一个 node<const int>是不相关的类型。您不能将指针传递给 node<int>到一个需要指向 node<const int> 指针的函数.

您可以移动 const 而不是在包含的类型上模板化您的迭代器类向上一级并在节点类型上模板化您的迭代器:

template<class Node> class my_list_iterator {
public:

    //...

    // member access
    auto& operator*() const {return pos_->data;}
    auto* operator->() const {return &(pos_->data);}

    // implicit conversion to const iterator
    operator my_list_iterator<const Node>() {return my_list_iterator<const Node>{pos_};}
private:
    Node* pos_;
    explicit my_list_iterator(Node* p=0):pos_(p) {}
    friend class my_list<type>;
};

template <class T> class my_list {
public:
    using iterator = my_list_iterator<node<T>>;
    using const_iterator = my_list_iterator<const node<T>>;
    //...
};

Live Demo

现在您要传递一个指向 node<int> 的指针到一个需要指向 const node<int> 指针的函数，没关系。