std::unique_lock::lock(3)

std::unique_lock::lock(3) 💖

okay so welcome to the std::unique_lock::lock(3) page from the C++ Standard Libary manual, bestie — this one is short, iconic, and honestly giving “tiny function, big responsibilities” energy ✨

---

🌸 NAME 🌸

std::unique_lock::lock — std::unique_lock::lock (yes, it is literally its own name, we love a self-titled main character 👑)

---

✨ Synopsis ✨

void lock();

Since: C++11 🫶

---

🎀 Parameters 🎀

(none) — nothing to pass in, bestie. Zero. Nada. 💅

---

🦋 Return value 🦋

(none) — it returns nothing. It just does the thing. periodt.

---

🔥 Exceptions 🔥

• Any exceptions thrown by mutex()->lock(). 😭
• If there is no associated mutex, std::system_error with an error code of std::errc::operation_not_permitted.
• If the mutex is already locked by this unique_lock (in other words, owns_lock is true), std::system_error with an error code of std::errc::resource_deadlock_would_occur.

no cap, those last two are the “you tried to lock with no mutex” and “you already own this lock” situations — and C++ is absolutely going to tell you about it 💀

---

🌈 Example 🌈

The following example uses lock to re-acquire a mutex that was unlocked. Run this code 👇

#include <chrono>
#include <iostream>
#include <mutex>
#include <thread>
#include <vector>

int main()
{
    int counter = 0;
    std::mutex counter_mutex;
    std::vector<std::thread> threads;

    auto worker_task = [&](int id)
    {
        std::unique_lock<std::mutex> lock(counter_mutex);
        ++counter;
        std::cout << id << ", initial counter: " << counter << '\n';
        lock.unlock();

        // don't hold the lock while we simulate an expensive operation
        std::this_thread::sleep_for(std::chrono::seconds(1));

        lock.lock();
        ++counter;
        std::cout << id << ", final counter: " << counter << '\n';
    };

    for (int i = 0; i < 10; ++i)
        threads.emplace_back(worker_task, i);

    for (auto& thread : threads)
        thread.join();
}

---

🌟 Possible output 🌟

0, initial counter: 1
1, initial counter: 2
2, initial counter: 3
3, initial counter: 4
4, initial counter: 5
5, initial counter: 6
6, initial counter: 7
7, initial counter: 8
8, initial counter: 9
9, initial counter: 10
6, final counter: 11
3, final counter: 12
4, final counter: 13
2, final counter: 14
5, final counter: 15
0, final counter: 16
1, final counter: 17
7, final counter: 18
9, final counter: 19
8, final counter: 20

---

🧚 See also 🧚

• try_lock — tries to lock (i.e., takes ownership of) the associated mutex without blocking (public member function).
• unlock — unlocks (i.e., releases ownership of) the associated mutex (public member function).

---

source: http://cppreference.com ✨ version/date: 2024.06.10 🎀 page: std::unique_lock::lock(3) 💅