How to choose between map and unordered_map?

Question

Suppose I wanted to map data with a string as the key. What container should I have chosen, map or unordered_map? unordered_map takes up more memory so let's suppose memory isn't an issue, and the concern is speed.

unordered_map should generally give average complexity of O(1) with the worst case of O(n). In what cases would it get to O(n)? When does a map get more time efficient than unordered_map? Does it happen when n is small?

Assuming I would use STL unordered_map with the default haser Vs. map. string is the key.

If I'm going to iterate over the elements rather than access an individual element each time, should I prefer map?

Answer 1

                       | map              | unordered_map
---------------------------------------------------------
element ordering       | strict weak      | n/a 
                       |                  |
common implementation  | balanced tree    | hash table
                       | or red-black tree|  
                       |                  |
search time            | log(n)           | O(1) if there are no hash collisions
                       |                  | Up to O(n) if there are hash collisions 
                       |                  | O(n) when hash is the same for any key
                       |                  |     
Insertion time         | log(n)+rebalance | Same as search
                       |                  | 
Deletion time          | log(n)+rebalance | Same as search
                       |                  | 
needs comparators      | only operator <  | only operator ==
                       |                  |
needs hash function    | no               | yes
                       |                  |
common use case        | when good hash is| In most other cases. 
                       | not possible or  | 
                       | too slow. Or when|
                       | order is required|