unordered_multimap - 迭代 find() 的结果会产生具有不同值的元素 [英] unordered_multimap - iterating the result of find() yields elements with different value

问题描述

C++ 中的多重映射似乎很奇怪，我想知道为什么

The multimap in C++ seems to work really odd, i would like to know why

#include <iostream>
#include <unordered_map>

using namespace std;

typedef unordered_multimap<char,int> MyMap;

int main(int argc, char **argv)
{
    MyMap map;
    map.insert(MyMap::value_type('a', 1));
    map.insert(MyMap::value_type('b', 2));
    map.insert(MyMap::value_type('c', 3));
    map.insert(MyMap::value_type('d', 4));
    map.insert(MyMap::value_type('a', 7));
    map.insert(MyMap::value_type('b', 18));

    for(auto it = map.begin(); it != map.end(); it++) {
        cout << it->first << '	';
        cout << it->second << endl;
    }

    cout << "all values to a" << endl;
    for(auto it = map.find('a'); it != map.end(); it++) {
        cout << it->first << '	' << it->second << endl;
    }

}

这是输出:

c   3
d   4
a   1
a   7
b   2
b   18
all values to a
a   1
a   7
b   2
b   18

为什么当我明确要求'a'时，输出仍然包含以b为键的任何内容?这是编译器还是 stl 错误?

why does the output still contain anything with b as the key when I am explicitly asking for 'a'? Is this a compiler or stl bug?

推荐答案

find 实现后，返回与 multimap 中的键匹配的第一个元素的迭代器(与任何其他 map 一样).您可能正在寻找 equal_range:

find, as implemented, returns an iterator for the first element which matches the key in the multimap (as with any other map). You're likely looking for equal_range:

// Finds a range containing all elements whose key is k.
// pair<iterator, iterator> equal_range(const key_type& k)
auto its = map.equal_range('a');
for (auto it = its.first; it != its.second; ++it) {
    cout << it->first << '	' << it->second << endl;
}

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