202504301210反向迭代器转正向迭代器

反向迭代器有一个坑点。正向迭代器，指向的位置和解引用对应的元素是一致的；反向迭代器指向的位置和解引用对应的元素差了一个。从前往后看（正向角度），反向迭代器指向位置i，那么解引用会得到i-1位置上的元素。

cpprefrence指出，

For a reverse iterator r constructed from an iterator i, the relationship &*r == &*(i - 1) is always true (as long as r is dereferenceable); thus a reverse iterator constructed from a one-past-the-end iterator dereferences to the last element in a sequence.

This is due to the fact that a reverse iterator has a slightly different referencing logic than a regular iterator: it points to an element, but when dereferenced, it yields a reference to the previous element.

更多参考：

• c++ - Why does removing the first element of a list invalidate .rend()? - Stack Overflow
• c++ - Why can I not convert a reverse iterator to a forward iterator? - Stack Overflow
• c++ - Can I convert a reverse iterator to a forward iterator? - Stack Overflow

反向迭代器的base成员函数，返回底层所持有的正向迭代器，我们转正向迭代器的时候尤其要注意上面的坑点，

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>
 
int main() { 
    int i[] = {1,2,3,4,5,6};
    std::vector<int> v(i, i+6);
 
    auto ri = std::find(std::rbegin(v), std::rend(v), 3);
    std::cout << *ri << std::endl; // 3
    auto fi = ri.base()-1;
    std::cout << *fi << std::endl; // 3
    fi = (++ri).base();
    std::cout << *fi << std::endl; // 3
}

即，

• 要么调用base之前将反向迭代器增一
• 要么调用base之后将得到的正向迭代器减一