C++ STL map I don't want it to sort!

This is my code

map<string,int> persons;

persons["B"] = 123;
persons["A"] = 321;

for(map<string,int>::iterator i = persons.begin();
    cout<< (*i).first << ":"<<(*i).second<<endl;

Expected output:


But output it gives is:


I want it to maintain the order in which keys and values were inserted in the map<string,int>.

Is it possible? Or should I use some other STL data structure? Which one?

There is no standard container that does directly what you want. The obvious container to use if you want to maintain insertion order is a vector. If you also need look up by string, use a vector AND a map. The map would in general be of string to vector index, but as your data is already integers you might just want to duplicate it, depending on your use case.

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Like Matthieu has said in another answer, the Boost.MultiIndex library seems the right choice for what you want. However, this library can be a little tough to use at the beginning especially if you don't have a lot of experience with C++. Here is how you would use the library to solve the exact problem in the code of your question:

struct person {
    std::string name;
    int id;
    person(std::string const & name, int id) 
    : name(name), id(id) { 

int main() {

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

    // define a multi_index_container with a list-like index and an ordered index
    typedef multi_index_container<
      person,        // The type of the elements stored
      indexed_by<    // The indices that our container will support
        sequenced<>,                           // list-like index
        ordered_unique<member<person, string, 
                              &person::name> > // map-like index (sorted by name)
    > person_container;

    // Create our container and add some people
    person_container persons;
    persons.push_back(person("B", 123));
    persons.push_back(person("C", 224));
    persons.push_back(person("A", 321));

    // Typedefs for the sequence index and the ordered index
    enum { Seq, Ord };
    typedef person_container::nth_index<Seq>::type persons_seq_index;
    typedef person_container::nth_index<Ord>::type persons_ord_index;

    // Let's test the sequence index
    persons_seq_index & seq_index = persons.get<Seq>();
    for(persons_seq_index::iterator it = seq_index.begin(), 
                                    e = seq_index.end(); it != e; ++it)
        cout << it->name << ":"<< it->id << endl;
    cout << "\n";

    // And now the ordered index
    persons_ord_index & ord_index = persons.get<Ord>();
    for(persons_ord_index::iterator it = ord_index.begin(), 
                                    e = ord_index.end(); it != e; ++it)
        cout << it->name << ":"<< it->id << endl;
    cout << "\n";

    // Thanks to the ordered index we have fast lookup by name:
    std::cout << "The id of B is: " << ord_index.find("B")->id << "\n";

Which produces the following output:



The id of B is: 123

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Map is definitely not right for you:

"Internally, the elements in the map are sorted from lower to higher key value following a specific strict weak ordering criterion set on construction."

Quote taken from here.

Unfortunately there is no unordered associative container in the STL, so either you use a nonassociative one like vector, or write your own :-(

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Besides Neil's recommendation of a combined vector+map if you need both to keep the insertion order and the ability to search by key, you can also consider using boost multi index libraries, that provide for containers addressable in more than one way.

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maps and sets are meant to impose a strict weak ordering upon the data. Strick weak ordering maintains that no entries are equavalent (different to being equal).

You need to provide a functor that the map/set may use to perform a<b. With this functor the map/set sorts its items (in the STL from GCC it uses a red-black tree). It determines weather two items are equavalent if !a<b && !b<a -- the equavelence test.

The functor looks like follows:

template <class T>
struct less : binary_function<T,T,bool> {
  bool operator() (const T& a, const T& b) const {
    return a < b;

If you can provide a function that tells the STL how to order things then the map and set can do what you want. For example

template<typename T>
struct ItemHolder
    int insertCount;
    T item;

You can then easily write a functor to order by insertCount. If your implementation uses red-black trees your underlying data will remain balanced -- however you will get a lot of re-balancing since your data will be generated based on incremental ordering (vs. Random) -- and in this case a list with push_back would be better. However you cannot access data by key as fast as you would with a map/set.

If you want to sort by string -- provide the functor to search by string, using the insertCount you could potentiall work backwards. If you want to search by both you can have two maps.

map<insertcount, string> x; // auxhilary key
map<string, item> y; //primary key

I use this strategy often -- however I have never placed it in code that is run often. I'm considering boost::bimap.

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