Boost provides a map with random-access iterators, so you can std::sort it:

#include <boost/container/flat_map.hpp>
#include <algorithm>

std::sort(your_map.begin(), your_map.end(), 
    [](const auto& a, const auto& b) { 
    return a.second != b.second?  a.second < b.second : a.first < b.first;  
});