為二叉樹 C++ 實現 const_iterator 時遇到問題

Question

我正在嘗試為我的二叉樹實現一個 const_iterator 但是當我嘗試編譯這個 main 時：

#include "rbtree.hpp"
#include <iostream>

int main(void) {
    rbtree< int >   t;
    t.insert(9);
    t.insert(8);
    t.insert(7);
    t.insert(6);
    t.insert(5);
    t.insert(1);
    t.insert(2);
    t.insert(3);
    t.insert(4);
    for (rbtree< int >::const_iterator it = t.begin(); it != t.end())
        std::cout << *it << std::endl;
    return 0;
}

編譯器（clang++）輸出：

test.cpp:27:37: error: no viable conversion from 'base_iterator<rbtree<int, std::allocator<int>, unsigned long, long>::rbnode>' to 'base_iterator<const rbtree<int, std::allocator<int>, unsigned long, long>::rbnode>'
        for (rbtree< int >::const_iterator it = t.begin(); it != t.end())
                                           ^    ~~~~~~~~~
./rbtree.hpp:172:5: note: candidate constructor not viable: no known conversion from 'rbtree<int, std::allocator<int>, unsigned long, long>::iterator' (aka 'base_iterator<rbtree<int, std::allocator<int>, unsigned long, long>::rbnode>') to 'const rbtree<int, std::allocator<int>, unsigned long, long>::rbnode *' for 1st argument
                                base_iterator(node_type *from) : _ptr(from) { }
                                ^
./rbtree.hpp:173:5: note: candidate constructor not viable: no known conversion from 'rbtree<int, std::allocator<int>, unsigned long, long>::iterator' (aka 'base_iterator<rbtree<int, std::allocator<int>, unsigned long, long>::rbnode>') to 'const rbtree<int, std::allocator<int>, unsigned long, long>::base_iterator<const rbtree<int, std::allocator<int>, unsigned long, long>::rbnode> &' for 1st argument
                                base_iterator(const base_iterator &other) : _ptr(other._ptr) { }
                                ^
1 error generated.

base_iterator< node_type > class 是rbtree class 的公共類型（ rbnode也是rbtree的公共類型）這是我的base_iterator ZA2F2ED4F8EBC2CBB4C21A29DC40 的代碼：

template <
    typename node_type
> class base_iterator : public std::iterator<
    std::bidirectional_iterator_tag,
    node_type,
    difference_type
> {
    public:
        base_iterator() : _ptr(NULL) { }
        base_iterator(node_type *from) : _ptr(from) { }
        base_iterator(const base_iterator &other) : _ptr(other._ptr) { }
        ~base_iterator() { }

        base_iterator   &operator =(const base_iterator &other) {
            _ptr = other._ptr;
        }
        T               &operator *(void) { return *_ptr->_data; }
        T               *operator ->(void) { return _ptr->_data; }
        base_iterator   &operator ++(void) {
            if (_ptr) {
                if (_ptr->_right) {
                    _ptr = _ptr->_right;
                    while (_ptr->_left) _ptr = _ptr->_left;
                } else {
                    while (
                        _ptr->_parent
                        && _ptr->_parent->_right == _ptr
                    ) _ptr = _ptr->_parent;
                    if (!_ptr->_parent) return base_iterator();
                }
            }
            return _ptr;
        }
        base_iterator   operator ++(int) {
            base_iterator   backup(_ptr);
            ++this;
            return backup;
        }
        base_iterator   &operator --(void) {
            if (_ptr) {
                if (_ptr->_left) {
                    _ptr = _ptr->_left;
                    while (_ptr->_right) _ptr = _ptr->_right;
                } else {
                    while (
                        _ptr->_parent
                        && _ptr->_parent->_left == _ptr
                    ) _ptr = _ptr->_parent;
                    if (!_ptr->_parent) return base_iterator();
                }
            }
            return _ptr;
        }
        base_iterator   operator --(int) {
            base_iterator   backup(_ptr);
            --this;
            return backup;
        }
    private:
        node_type   *_ptr;
};

以及iterator和const iterator的 typedef，以及我的rbtree class 中的迭代器函數

typedef base_iterator< rbnode >                 iterator;
typedef base_iterator< const rbnode >           const_iterator;
typedef ft::reverse_iterator< iterator >        reverse_iterator;
typedef ft::reverse_iterator< const_iterator >  const_reverse_iterator;

iterator                begin(void) { return _begin; }
const_iterator          begin(void) const {
    return const_iterator(_begin);
}
iterator                end(void) { return iterator(); }
const_iterator          end(void) const { return const_iterator(); }
reverse_iterator        rbegin(void) { return _rbegin; }
const_reverse_iterator  rbegin(void) const {
    return const_iterator(_rbegin);
}
reverse_iterator        rend(void) { return iterator(); }
const_reverse_iterator  rend(void) const { return const_iterator(); }

我不知道出了什么問題，這就是為什么我需要你的幫助，謝謝。

Answer 1

您必須記住，模板類型是完全獨立的。

 base_iterator<X>
 base_iteraotr<const X>

是完全不同的類型。

您的編譯器抱怨base_iteraotr<const X>中沒有接受base_iteraotr<X> （完全不同的類型）的構造函數（或轉換運算符）。