hash_map was a common extension provided by many library implementations. That is exactly why it was renamed to
unordered_map when it was added to the C++ standard as part of TR1. map is generally implemented with a balanced binary tree like a red-black tree (implementations vary of course).
hash_map and unordered_map are generally implemented with hash tables. Thus the order is not maintained.
unordered_map insert/delete/query will be O(1) (constant time) where map will be O(log n) where n is the number of items in the data structure. So
unordered_map is faster, and if you don't care about the order of the items should be preferred over
map. Sometimes you want to maintain order (ordered by the key) and for that
map would be the choice.
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