Question

Is there really no guaranteed order in an unordered_map? I ask this because I would like to specify an order for an unorderded_map, so that it is possible to iterate the container from begin() to end() according to the specified order (while preserving the efficiency of an hashed access to single elements, globally speaking).

Answer 1

You know, it has its name for a reason…

To actually give it an order, you’d need to implement your own hash that somehow gives you the wanted order.

Now, for a solution to your actual problem, you can just create a std::map from your std::unordered_map, and even with minimal overhead for the insert (no copies):

#include <iostream>
#include <unordered_map>
#include <map>
#include <functional>

int main()
{
    std::unordered_map<int, int> m;
    m[5] = 1;
    m[4] = 2;
    m[3] = 3;
    m[2] = 4;
    m[1] = 5;
    typedef std::reference_wrapper<const int> cref_int;
    typedef std::reference_wrapper<int> ref_int;
    std::map<cref_int, ref_int> ordered(m.begin(), m.end());
    for(auto it=ordered.begin(), ite=ordered.end(); it != ite; ++it){
        std::cout << it->second << '\n';
    }
}