为什么我不能将反向迭代器转换为前向迭代器？ [英] Why can I not convert a reverse iterator to a forward iterator?

问题描述

嗯，我知道为什么，因为没有转换，但为什么没有转换？为什么可以转发迭代器转向反向迭代器，但不是反过来？更重要的是，如果我想这样做，我该怎么办？是否有一些适配器允许您使用向前迭代器向后迭代？

Well, I know why, it's because there isn't a conversion, but why isn't there a conversion? Why can forward iterators be turned to reverse iterators but not the other way round? And more importantly, what can I do if I want to do this? Is there some adapter that allows you to iterate backwards using a forward iterator?

std::vector<int> buffer(10);
std::vector<int>::iterator forward = buffer.begin();
std::vector<int>::reverse_iterator backward = buffer.rbegin();
++forward;
++backward;
std::vector<int>::iterator forwardFromBackward = std::vector<int>::iterator(backward); // error! Can't convert from reverse_iterator to iterator!
std::vector<int>::reverse_iterator backwardFromForward = std::vector<int>::reverse_iterator(forward); // this is fine

推荐答案

。 reverse_iterator 的一个特殊性是 base（）给出了一个向前迭代器，它是从反向迭代器取消引用至。 这是因为反向迭代器实际上指向之后的元素 / em>指向 。因此，要使向前迭代器与您的reverse_iterator具有相同的项目，您需要将 base（）的结果减1，或您可以先增加反向迭代器 ，然后选择 .base（）。

You could write a helper function. One particularity of reverse_iterator is that base() gives a forward iterator that is next from the value that the reverse iterator dereferences to. This is because a reverse iterator physically points to the element after the one it logically points to. So to have the forward iterator to the same item as your reverse_iterator, you'll need to decrement the result of base() by one, or you could increment the reverse iterator first, then take the .base() of that.

这两个示例如下所示：

#include <iostream>
#include <vector>
#include <iterator>

//result is undefined if passed container.rend()
template <class ReverseIterator>
typename ReverseIterator::iterator_type make_forward(ReverseIterator rit)
{
    return --(rit.base()); // move result of .base() back by one.
    // alternatively
    // return (++rit).base() ;
    // or
    // return (rit+1).base().
}

int main()
{
    std::vector<int> vec(1, 1);
    std::vector<int>::reverse_iterator rit = vec.rbegin();
    std::vector<int>::iterator fit = make_forward(rit);
    std::cout << *fit << ' ' << *rit << '\n';
} 

警告：此行为与 reverse_iterator（iterator）构造函数。

Warning: this behavior is different from that of the reverse_iterator(iterator) constructor.

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