std::unique_lock::try_lock(3)

NAME
std::unique_lock::try_lock - std::unique_lock::try_lock

Synopsis
   bool try_lock(); (since C++11)
   Tries to lock (i.e., takes ownership of) the associated mutex without blocking.
   Effectively calls mutex()->try_lock().
   std::system_error is thrown if there is no associated mutex or if the mutex is
   already locked by this std::unique_lock.

Parameters
   (none)

Return value
   true if the ownership of the mutex has been acquired successfully, false otherwise.

Exceptions
     * Any exceptions thrown by mutex()->try_lock() (Mutex types do not throw in
       try_lock, but a custom Lockable might).
     * 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 std::unique_lock, std::system_error with
       an error code of std::errc::resource_deadlock_would_occur.

Example
   The following examples try to acquire a mutex that was locked and unlocked. // Run this code
 #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).

See also
   lock locks (i.e., takes ownership of) the associated mutex
                  (public member function)
                  attempts to lock (i.e., takes ownership of) the associated
   try_lock_for TimedLockable mutex, returns if the mutex has been unavailable for
                  the specified time duration
                  (public member function)
                  tries to lock (i.e., takes ownership of) the associated TimedLockable
   try_lock_until mutex, returns if the mutex has been unavailable until specified time
                  point has been reached
                  (public member function)
   unlock unlocks (i.e., releases ownership of) the associated mutex
                  (public member function)