sn_stl_map_sort_by_value.cpp

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

template<typename A, typename B>
std::pair<B,A> flip_pair(const std::pair<A,B> &p)
{
    return std::pair<B,A>(p.second, p.first);
}

template<typename A, typename B>
std::map<B,A> flip_map(const std::map<A,B> &src)
{
    std::map<B,A> dst;
    std::transform(src.begin(), src.end(), std::inserter(dst, dst.begin()), 
                   flip_pair<A,B>);
    return dst;
}

int main(void)
{
    std::map<std::string, int> src;

    src["a"] = 3;
    src["b"] = 1;
    src["c"] = 2;

    std::map<int, std::string> dst = flip_map(src);
    
    for( std::pair<int, std::string> item : dst)
        std::cout << item.first << "," << item.second << std::endl;
}

from: http://stackoverflow.com/questions/5056645/sorting-stdmap-using-value