Containers in Practice

Unordered Set Lookup

An unordered set answers membership questions without requiring the values to stay sorted.

unordered set A `std::unordered_set` stores unique values and optimizes lookup instead of ordering.
contains Before C++20, `find` is the common membership check for unordered sets.

Unordered Set Lookup

role
unordered_set_lookup.cpp
Replay: real traced execution (multi-file project)
#include <iostream>
#include <string>
#include <unordered_set>

int main() {
    std::string role = "admin";

    std::unordered_set<std::string> allowed{"admin", "editor"};
    bool present = allowed.find(role) != allowed.end();

    std::cout << "role=" << role << std::endl;
    std::cout << "present=" << present << std::endl;
    return 0;
}

#include <iostream>
#include <string>
#include <unordered_set>

int main() {
    std::string role = "guest";

    std::unordered_set<std::string> allowed{"admin", "editor"};
    bool present = allowed.find(role) != allowed.end();

    std::cout << "role=" << role << std::endl;
    std::cout << "present=" << present << std::endl;
    return 0;
}

#include <iostream>
#include <string>
#include <unordered_set>

int main() {
    std::string role = "editor";

    std::unordered_set<std::string> allowed{"admin", "editor"};
    bool present = allowed.find(role) != allowed.end();

    std::cout << "role=" << role << std::endl;
    std::cout << "present=" << present << std::endl;
    return 0;
}

  1. role ← admin, allowed ← (empty), present ← 1

    5int main() {6    std::string role→ admin = "admin"; //@role="guest", "editor"78    std::unordered_set<std::string> allowed→ (empty){"admin", "editor"};9    bool present→ 1 = allowed(empty).find(roleadmin) != allowed.end();1011    std::cout << "role=" << roleadmin << std::endl;12    std::cout << "present=" << present1 << std::endl;13    return 0;14}
    outputrole=admin
present=1

  1. role ← guest, allowed ← (empty), present ← 0

    5int main() {6    std::string role→ guest = "guest";78    std::unordered_set<std::string> allowed→ (empty){"admin", "editor"};9    bool present→ 0 = allowed(empty).find(roleguest) != allowed.end();1011    std::cout << "role=" << roleguest << std::endl;12    std::cout << "present=" << present0 << std::endl;13    return 0;14}
    outputrole=guest
present=0

  1. role ← editor, allowed ← (empty), present ← 1

    5int main() {6    std::string role→ editor = "editor";78    std::unordered_set<std::string> allowed→ (empty){"admin", "editor"};9    bool present→ 1 = allowed(empty).find(roleeditor) != allowed.end();1011    std::cout << "role=" << roleeditor << std::endl;12    std::cout << "present=" << present1 << std::endl;13    return 0;14}
    outputrole=editor
present=1