c++ - vector 迭代器不可递减？

Question

有人能解释一下为什么这个代码片段会在循环中运行一次，然后给出断言失败表达式: vector 迭代器不可递减吗？

for (auto an = a.rbegin(); an != a.rend(); ++an, indexA--) //first number 
{
        for (auto bn = b.rbegin(); bn != b.rend(); ++bn) //second number
        {
            if (*an - *bn >= 0)
            {
                returnVal.push_back(*an - *bn);
                a.pop_back();
                b.pop_back();
            }
            else
            {
                brrow = *an + 10;
                a.at(indexA - 1) = a.at(indexA - 1) - 1; // remove 1 from the spot ahead of current digit
                returnVal.push_back(brrow - *bn);
                a.pop_back();
                b.pop_back();
            }
        }   
}

Answer 1

根据您的评论，您似乎想串联移动两个迭代器范围 - 只需要一个 for 循环，如下所示:

auto an = a.rbegin();
for (auto bn = b.rbegin();
     an != a.rend() && bn != b.rend();
     ++an, ++bn, indexA--)
    if (*an - *bn >= 0)
        returnVal.push_back(*an - *bn);
    else
    {
        brrow = *an + 10;
        --(a.at(indexA - 1)); // remove 1 from the spot ahead of current digit
                              // shouldn't you check for rend() first????
        returnVal.push_back(brrow - *bn);
    }

请注意，两个迭代器都可以声明为 for-loop local if 你知道它们将属于完全相同的类型:

for (auto an = a.rbegin(), bn = b.rbegin(); ...