How is the unordered_map data structure implemented and how does it differ from regular maps?

unordered_map in C++ is implemented based on a hash table. It is a data structure that stores key-value pairs and provides fast access to elements by key on average in constant time O(1).

Unlike regular maps (std::map), which are implemented as balanced binary trees (usually red-black trees), unordered_map does not store elements in sorted order. This means that:

• Access and insertion in unordered_map are usually faster than in map, thanks to hashing.
• Elements in unordered_map are unordered, whereas in map they are sorted by key.

Example:

#include <iostream>
#include <unordered_map>

int main() {
    std::unordered_map<std::string, int> umap;
    umap["apple"] = 5;
    umap["banana"] = 3;

    std::cout << "apple: " << umap["apple"] << std::endl;
    return 0;
}

Thus, unordered_map is suitable when access speed is important and the order of elements does not matter.