Are c++ std::map<string,string> ordered? [duplicate]

Question

are STL maps ordered?

Specifically I need to know if std::map is ordered. So if I iterate over it, it will iterate with the first insert string first.

So will the below iterate A, C then B consistantly?

std::map<string,string> str_map;  str_map.insert(std::make_pair("A","Data")); str_map.insert(std::make_pair("C","Data")); str_map.insert(std::make_pair("B","Data")); 

Answer 1

are STL maps ordered?

Yes, a std::map<K,V> is ordered based on the key, K, using std::less<K> to compare objects, by default.

So if I iterate over it, it will iterate with the first insert string first?

No. It will iterate based on the sorted order, not the order that you inserted elements. In the case of std::string, it sorts in lexicographic order (alphabetic order).

If you want to iterate based on the insertion order, you're better off using a sequence container, such as a std::vector or a std::list.

Answer 2

std::maps are sorted using either the given type's operator< or using a custom comparison function/functor if one is supplied as an argument to the constructor.

So no, when you iterate over the map, the first item you get won't be the one you inserted first - it will be the one that comes first alphabetically.

Of course for your sample code that doesn't make a difference because "A" is the first key you inserted and also the first one alphabetically.