Unordered map in C++ STL

By its name we can say that it comes under Associative containers with Unordered property. We know that any unordered container internally implemented with hash tables. So same has hashing concept in worst case any operation takes O(n) time and in average and best case it takes O(1) time. We can say it vary based on type of hash function we used. With comparison to map containers these work efficiently to find value by using key. And one main difference is that map elements are order but unordered map elements are not in order. Since it is based on key-value pair concept all keys must be unique.

This is a companion discussion topic for the original entry at