排序std :: unordered_map <std::string, std::atomic<unsigned int> &gt;按值

Question

I've got std::unordered_map<std::string, std::atomic<unsigned int>> .

I would like to print the keys and values, sorted by the values.
The best solution I've encountered is to create a vector of pairs and sort that

But as one cannot copy std::atomic<unsigned int> , what is the most efficient solution?

Answer 1

Making a copy of your data into a vector will work, but you would need to provide a custom operation that calls load() on your atomic<unsigned> to make it a plain unsigned . Since you are forced to do it anyway, you might as well reverse the order of terms in the pair:

std::vector<pair<unsigned int,std::string>> copy;
std::transform(
    m.begin()
,   m.end()
,   back_inserter(copy)
,   [](const pair<const std::string, std::atomic<unsigned int>>& p) {
        return make_pair(p.second.load(), p.first);
    }
);

Now that the value is first, the call of std::sort no longer requires a custom comparator:

std::sort(copy.begin(), copy.end());

Demo.

Answer 2

Another solution is to copy unordered_map into multimap with values being the keys and then print it out:

#include <iostream>
#include <string>
#include <map>
#include <unordered_map>
#include <atomic>
#include <algorithm>
#include <iterator>

int main()
{
    std::unordered_map<std::string, std::atomic<unsigned int>> m;
    for (auto p : std::initializer_list<std::pair<std::string, unsigned int>>{{ "a", 32},{ "b" , 22 },{ "c" , 32 },{ "d" , 22 },{ "e" , 55 } })
        m.emplace(p);

    std::multimap<unsigned int, std::string> printmap;

    std::transform(m.begin(), m.end(), std::inserter(printmap, printmap.end()),
        [](auto const &p) { return std::make_pair(p.second.load(), p.first); });

    for (auto const &p : printmap)
        std::cout << p.first << " - " << p.second << std::endl;

    return 0;
}

Demo: https://ideone.com/MgtMY8

22 - d
22 - b
32 - c
32 - a
55 - e