Key: "user:101" ──► std::hash ──► Hash Code % BucketCount ──► Bucket Index [3]
│
┌─────────────┴─────────────┐
▼ ▼
[ "user:101", Data ] ──► [ Collided Node ]┌─────────────────────┬───────────────────────────┬───────────────────────────┐
│ Feature │ std::map / std::set │ std::unordered_map / set │
├─────────────────────┼───────────────────────────┼───────────────────────────┤
│ Data Structure │ Red-Black Tree (Balanced) │ Hash Table (Buckets) │
│ Element Ordering │ Strictly Sorted │ Unordered / Arbitrary │
│ Average Lookup Time │ O(log N) │ O(1) │
│ Worst-case Lookup │ O(log N) │ O(N) (Hash Collisions) │
│ Key Requirements │ operator< │ std::hash and operator== │
└─────────────────────┴───────────────────────────┴───────────────────────────┘#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include <string>
int main() {
// 1. std::unordered_map (O(1) Hash Map)
std::unordered_map<std::string, double> productPrices = {
{"Keyboard", 79.99},
{"Monitor", 249.50},
{"Mouse", 39.99}
};
// Fast O(1) average lookup
std::string query = "Monitor";
if (productPrices.find(query) != productPrices.end()) {
std::cout << "[Lookup] " << query << " costs $" << productPrices[query] << "\n";
}
// 2. std::unordered_set (O(1) Hash Set)
std::unordered_set<std::string> blacklistedIPs = {
"192.168.1.100", "10.0.0.50", "172.16.0.22"
};
std::string testIP = "10.0.0.50";
if (blacklistedIPs.count(testIP)) {
std::cout << "[Security Alert] IP " << testIP << " is BLACKLISTED!\n";
}
return 0;
}std::unordered_map when maximum lookup performance ($O(1)$) is needed and ordering does not matter..reserve(expectedCount) on unordered containers to pre-allocate hash buckets and avoid costly rehashing.std::map when you need sorted keys or range queries (lower_bound / upper_bound).