Keep the order of unordered_map as we insert a new key

Question

I inserted the elements to the unordered_map with this code:

    myMap.insert(std::make_pair("A", 10));
    myMap.insert(std::make_pair("B", 11));
    myMap.insert(std::make_pair("C", 12));
    myMap.insert(std::make_pair("D", 13));

But when I used this command to print the keys

for (const auto i : myMap)
{
    cout  << i.first << std::endl;
}

they are not in the same order as I inserted them.

Is it possible to keep the order?

Answer 1

No, it is not possible.

Usage of std::unordered_map doesn't give you any guarantee on element order.

If you want to keep elements sorted by map keys (as seems from your example) you should use std::map.

If you need to keep list of ordered pairs you can use std::vector<std::pair<std::string,int>>.

Answer 2

Not with an unordered associative data structure. However, other data structures preserve order, such as std::map which keeps the data sorted by their keys. If you search Stackoverflow a little, you will find many solutions for a data structure with fast key-based lookup and ordered access, e.g. using boost::multi_index.

If it is just about adding values to a container, and taking them out in the order of insertion, then you can go with something that models a queue, e.g. std::dequeue. Just push_back to add a new value, and pop_front to remove the oldest value. If there is no need to remove the values from the container then just go with a std::vector.