C++-一道OJ算法题中有关迭代器错误的原因求解

Question

#include<iostream>
#include<map>
#include<vector>

using namespace std;

int main()
{
    int n,a,b;
    int count1 = 0,count2 = 0;
    map<int ,int> p;
    vector<int> xm;
    vector<int> xy;
    while(scanf("%d",&n) != EOF)
    {
        for(int i = 0;i < n;++i)
        {
            scanf("%d%d",&a,&b);
            p[a] = b;
        }
        vector<int>::iterator iter1;
        xm.push_back(p[1]); 
        iter1 = xm.begin();
        while(p.count(*iter1))
        {
            xm.push_back(p[*iter1]);
            ++iter1;
        }
        vector<int>::iterator iter2;
        xy.push_back(p[2]);
        iter2 = xy.begin();
        while(p.count(*iter2))
        {
            xy.push_back(p[*iter2]);
            ++iter2;
        }
        iter1 = xm.begin();
        iter2 = xy.begin();
        int flag = 0;
        for(;iter1 != xm.end();++iter1,++count1)
        {
            if(flag)
                break;
            for(;iter2 != xy.end();++iter2,++count2)
            {
                if(*iter1 == *iter2)
                {
                    if(count1 == count2)
                    {
                        printf("%sn","You are my brother");
                        flag = 1;
                    }
                    else if(count1 > count2)
                    {
                        printf("%sn","You are my elder");
                        flag = 1;
                    }
                    else
                    {
                        printf("%sn","You are my younger");
                        flag = 1;
                    }
                }
            }
            count2 = 0;
        }
    }
    
    return 0;
}

Answer 1

C++ 标准容器中有很多的“潜规则”，其中之一就是：在容器大小变化之后，之前取得的迭代器不可用。对于 vector 容器尤其如此。

设想一下，iter 指向 vector 中某元素，然后给 vector 追加一个元素，导致重新分配内存，vector 的内容全部“搬家”，然后再来使用 iter ……

事实上，出错信息已经很明显了：迭代器没有对应的容器。