std::unique_lock<Mutex>::try_lock

bool try_lock();

The following examples try to acquire a mutex that was locked and unlocked.

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

using namespace std::chrono_literals;

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

    auto worker_task = [&](Id id, std::chrono::seconds wait, std::chrono::seconds acquire)
    {
        // wait for a few seconds before acquiring lock.
        std::this_thread::sleep_for(wait);

        std::unique_lock<std::mutex> lock(counter_mutex, std::defer_lock);
        if (lock.try_lock())
            std::cout << '#' << id << ", lock acquired.\n";
        else
        {
            std::cout << '#' << id << ", failed acquiring lock.\n";
            return;
        }

        // keep the lock for a while.
        std::this_thread::sleep_for(acquire);

        std::cout << '#' << id << ", releasing lock (via destructor).\n";
    };

    threads.emplace_back(worker_task, Id{0}, 0s, 2s);
    threads.emplace_back(worker_task, Id{1}, 1s, 0s);
    threads.emplace_back(worker_task, Id{2}, 3s, 0s);

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

Output:

#0, lock acquired.
#1, failed acquiring lock.
#0, releasing lock (via destructor).
#2, lock acquired.
#2, releasing lock (via destructor).