Question

C++中->second 是什么意思

std::multimap<string, string>mtpphone;
mtpphone.insert(std::make_pair("xiaozhu", "123456"));
mtpphone.insert(std::make_pair("xiaozhu", "1234567"));
std::multimap<string, string>::size_type count = mtpphone.count("xiaozhu");
std::multimap<string, string>::iterator it = mtpphone.find("xiaozhu");
for (std::multimap<string, string>::size_type i = 0; i != count;i++,it++)
{
cout << it->second<<endl;
}
second不知道是干什么的？

Answer 1

在这里输出所有键值为xiaozhu的string字符串，it first是迭代器指向键值，it second是迭代器指向对应的值！

追问

我其实 知道 我这个操作 是干什么用的 但我对于这个second 不太理解 他的作用是什么？

追答

　　if you have a std::map, what it actually stores is a whole bunch of std::pairs. That's exactly what a map is - it pairs together the keys and the associated values.
　　When you iterate over a std::map, you're iterating over all of these std::pairs. When you dereference one of these iterators, you get a std::pair containing the key and its associated value.
　　Now the type std::pair gives you access to its elements through two members: first and second. So if you have a std::pair called p, p.first is an X object and p.secondis a Y object.
　　So now you know that dereferencing a std::map iterator gives you a std::pair, you can then access its elements with first and second. For example, (*it).first will give you the key and (*it).second will give you the value. These are equivalent to it->first and it->second.
　　上面是我拷贝老外的解释，人家还是说的比较清楚的，迭代器的这两个方法就是为了获得std pair的key和keyvalue！百度字数不允许删了一点。'