a C++ hash map that preserves the order of insertion

Question

I have the following code:

#include <iostream>
#include "boost/unordered_map.hpp"

using namespace std;
using namespace boost;

int main()
{

    typedef unordered_map<int, int> Map;
    typedef Map::const_iterator It;

    Map m;
    m[11] = 0;
    m[0]  = 1;
    m[21] = 2;

    for (It it (m.begin()); it!=m.end(); ++it)
     cout << it->first << " " << it->second << endl;

    return 0;
}

However, I am looking for something that preserves the order so that later I can iterate over the elements in the same order in which they were inserted. On my computer the above code does not preserve the order, and prints the following:

 0 1
11 0
21 2

I thought maybe I could use a boost::multi_index_container

typedef multi_index_container<
    int,
    indexed_by<
     hashed_unique<identity<int> >,
     sequenced<>
    >
> Map;

Can somebody show me how to implement my original code using this container (or any other appropriate container) so that the iterator follows the order of insertion?

Answer 1

#include <iostream>
#include "boost/unordered_map.hpp"

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/member.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/sequenced_index.hpp>

using namespace std;
using namespace boost;
using namespace boost::multi_index;


struct key_seq{};
struct key{};

struct Data_t
{
    int key_;
    int data_;
    Data_t (int key_v, int data_v) : key_(key_v), data_(data_v) {}
};

int main()
{
    typedef multi_index_container<
        Data_t,
        indexed_by<
            hashed_unique<tag<key>,  BOOST_MULTI_INDEX_MEMBER(Data_t,int,key_)>,
            sequenced<tag<key_seq> >
        >
    > Map;

    typedef Map::const_iterator It;

    typedef index<Map,key>::type Map_hashed_by_key_index_t;
    typedef index<Map,key>::type::const_iterator  Map_hashed_by_key_iterator_t;

    typedef index<Map,key_seq>::type Map_sequenced_by_key_index_t;
    typedef index<Map,key_seq>::type::const_iterator  Map_sequenced_by_key_iterator_t;

    Map m;
    m.insert(Data_t(11,0));
    m.insert(Data_t(0,1));
    m.insert(Data_t(21,1));

    {
        cout << "Hashed values\n";
        Map_hashed_by_key_iterator_t i = get<key>(m).begin();
        Map_hashed_by_key_iterator_t end = get<key>(m).end();
        for (;i != end; ++i) {
            cout << (*i).key_ << " " << (*i).data_ << endl;
        }
    }

    {
        cout << "Sequenced values\n";
        Map_sequenced_by_key_iterator_t i = get<key_seq>(m).begin();
        Map_sequenced_by_key_iterator_t end = get<key_seq>(m).end();
        for (;i != end; ++i) {
            cout << (*i).key_ << " " << (*i).data_ << endl;
        }
    }

    return 0;
}

Answer 2

You can try creating an ordered map using the combination of map and the vector.

• Vector can hold the pair of key and value.
• Vector iterator can be used as iterator to traverse ordered map.
• map can be used access the elements faster.