如何替换std multimap中的<key,value>

Question

重新提出问题 - 问题改写 - 我有一个要求,我需要用新的密钥和值替换一对.考虑一下 -

#include <map>
#include <string>
#include <iostream>

using namespace std;

int main()
{
    std::multimap<unsigned int, std::string> mymap;

    mymap.insert(std::multimap<unsigned int, std::string>::value_type(0, "A1"));
    mymap.insert(std::multimap<unsigned int, std::string>::value_type(0, "A2"));
    mymap.insert(std::multimap<unsigned int, std::string>::value_type(2, "C1"));
    mymap.insert(std::multimap<unsigned int, std::string>::value_type(2, "C2"));
    mymap.insert(std::multimap<unsigned int, std::string>::value_type(1, "B1"));
    mymap.insert(std::multimap<unsigned int, std::string>::value_type(1, "B2"));
    mymap.insert(std::multimap<unsigned int, std::string>::value_type(1, "B3"));

    std::pair<std::multimap<unsigned int, std::string>::iterator, std::multimap<unsigned int, std::string>::iterator> pr = mymap.equal_range(1);

    std::multimap<unsigned int, std::string>::iterator it;
    for (it=pr.first; it!=pr.second; ++it)
    {
        unsigned int key = it->first;
    key = key+10;

        std::string val = it->second;
        val = "X" + val;
        mymap.erase(it);
        mymap.insert(std::multimap<unsigned int, std::string>::value_type(key, val));

    }

    for ( it=mymap.begin() ; it != mymap.end(); it++ )
    {
        cout << (*it).first << " => " << (*it).second << endl;
    }

    return 0;

}

由于迭代器无效,该程序在visual studio 2008上崩溃.

我希望它是:

0 => A1
0 => A2
2 => C1
2 => C2
11 => XB1
11 => XB2
11 => XB3

我的想法是想用新条目替换地图中的现有条目.

我究竟做错了什么？任何帮助非常感谢.

Answer 1

诀窍是首先推进迭代器,然后擦除迭代器的副本.

std::multimap<unsigned int, std::string>::iterator it = pr.first;
while (it != pr.second)
{
    unsigned int key = it->first;
    key = key+10;

    std::string val = it->second;
    val = "X" + val;

    std::multimap<unsigned int, std::string>::iterator itCopy = it;
    ++it;
    mymap.erase(itCopy);
    mymap.insert(std::multimap<unsigned int, std::string>::value_type(key, val));
}

在C++ 11中,您可以这样做:

std::multimap<unsigned int, std::string>::iterator it = pr.first;
while (it != pr.second)
{
    unsigned int key = it->first;
    key = key+10;

    std::string val = it->second;
    val = "X" + val;

    it = mymap.erase(it);
    mymap.insert(std::multimap<unsigned int, std::string>::value_type(key, val));
}

顺便说一下,因为这段代码每次都在增加密钥,所以它会一遍又一遍地处理每个元素.