What is the difference between std::map and std::unordered_map?

Point	std::map	std::unordered_map
Inside	Balanced tree	Hash table
Order	Sorted by key	No order
Find, insert	O(log n)	O(1) on average
Key needs	operator<	hash and ==
Iterators	Stay valid after insert	Can be invalidated by a rehash

#include <iostream>
#include <map>
#include <string>
#include <unordered_map>

int main() {
    std::map<std::string, int> sorted{{"pear", 3}, {"apple", 1}, {"mango", 2}};
    for (const auto& [k, v] : sorted) std::cout << k << ' ';        // always sorted
    std::cout << '\n';

    std::unordered_map<std::string, int> fast{{"pear", 3}, {"apple", 1}};
    std::cout << fast.at("pear") << ' ' << fast.contains("kiwi") << '\n';
}

Choose unordered_map when you only look up by key. Choose map when you need the order or range searches. See STL Containers.

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