std::unique_lock::unique_lock - cppreference.com

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# std::unique_lock::unique_lock

From cppreference.com
< cpp‎ | thread‎ | unique lock

unique_lock(); | (1) | (since C++11)
unique_lock( unique_lock&& other ); | (2) | (since C++11)
explicit unique_lock( mutex_type& m ); | (3) | (since C++11)
unique_lock( mutex_type& m, std::defer_lock_t t ); | (4) | (since C++11)
unique_lock( mutex_type& m, std::try_to_lock_t t ); | (5) | (since C++11)
unique_lock( mutex_type& m, std::adopt_lock_t t ); | (6) | (since C++11)
template< class Rep, class Period > unique_lock( mutex_type& m, const std::chrono::duration<Rep,Period>& timeout_duration ); | (7) | (since C++11)
template< class Clock, class Duration > unique_lock( mutex_type& m, const std::chrono::time_point<Clock,Duration>& timeout_time ); | (8) | (since C++11)

Constructs a `unique_lock`, optionally locking the supplied mutex.

1) Constructs a `unique_lock` with no associated mutex.
2) Move constructor. Initializes the `unique_lock` with the contents of `other`. Leaves `other` with no associated mutex.
3-8) Constructs a `unique_lock` with `m` as the associated mutex. Additionally:
3) Locks the associated mutex by calling m.lock(). The behavior is undefined if the current thread already owns the mutex except when the mutex is recursive.
4) Does not lock the associated mutex.
5) Tries to lock the associated mutex without blocking by calling m.try_lock(). The behavior is undefined if the current thread already owns the mutex except when the mutex is recursive.
6) Assumes the calling thread already owns `m`.
7) Tries to lock the associated mutex by calling m.try_lock_for(timeout_duration). Blocks until specified `timeout_duration` has elapsed or the lock is acquired, whichever comes first. May block for longer than `timeout_duration`.
8) Tries to lock the associated mutex by calling m.try_lock_until(timeout_time). Blocks until specified `timeout_time` has been reached or the lock is acquired, whichever comes first. May block for longer than until `timeout_time` has been reached.

### [edit] Parameters

other | - | another `unique_lock` to initialize the state with
m | - | mutex to associate with the lock and optionally acquire ownership of
t | - | tag parameter used to select constructors with different locking strategies
timeout_duration | - | maximum duration to block for
timeout_time | - | maximum time point to block until

### [edit] Exceptions

1, 2, 4)
`noexcept` specification:
noexcept

### [edit] Example

Run this code

    #include <cassert>
    #include <iostream> // std::cout
    #include <thread>
    #include <vector>
    #include <mutex>

    class Number;
    std::ostream& operator<<(std::ostream& stream, const Number& number);

    class Number {
     public:
      Number() : v_(1) {}

      // thread-safe update of 'a' and 'b'
      static void update(Number& a, Number& b, bool order) {
        // do not lock 'mutex_' of 'a' and 'b' sequentially,
        // two sequential lock may lead to deadlock,
        // that's why 'std::lock' exists (see below)
        GuardLock lock_a(a.mutex_, std::defer_lock);
        GuardLock lock_b(b.mutex_, std::defer_lock);

        // mutexes is not locked
        assert(!lock_a.owns_lock());
        assert(!lock_b.owns_lock());

        // unspecified series of calls...
        std::lock(lock_a, lock_b);

        // Result: 'a.mutex_' and 'b.mutex_' is in locked state
        // 'a' and 'b' can be modified safety
        assert(lock_a.owns_lock());
        assert(lock_b.owns_lock());

        if (order) {
          a.v_ += b.v_;
          b.v_ += a.v_;

          std::cout << a << b;
        }
        else {
          b.v_ += a.v_;
          a.v_ += b.v_;

          std::cout << b << a;
        }

        // 'lock_a' and 'lock_b' will be destroyed,
        // unlocking 'a.mutex_' and 'b.mutex_'
      }

      // not thread-safe; used before thread creation or in thread-safe 'update'
      std::ostream& print(std::ostream& stream) const {
        stream << v_ << " ";
        return stream;
      }

     private:
      using Mutex = std::mutex;
      using GuardLock = std::unique_lock<Mutex>;

      Mutex mutex_;
      int v_;
    };

    // not thread-safe; see 'Number::print'
    std::ostream& operator<<(std::ostream& stream, const Number& number) {
      return number.print(stream);
    }

    int main() {
      Number a, b;
      std::cout << a << b;

      std::vector<std::thread> threads;

      for (unsigned i = 0; i < 4; ++i) {
        // without 'std::lock' deadlock may occur in this situation:
        //   thread #1 lock 'a.mutex_'
        //   thread #2 lock 'b.mutex_'
        //   thread #1 try to lock 'b.mutex_' and blocked (it's locked by #2)
        //   thread #2 try to lock 'a.mutex_' and blocked (it's locked by #1)
        //   ... deadlock
        threads.emplace_back(Number::update, std::ref(a), std::ref(b), true); // #1
        threads.emplace_back(Number::update, std::ref(b), std::ref(a), false); // #2
      }

      for (auto& i: threads) {
        i.join();
      }

      std::cout << '\n';
    }

Output:

    1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584

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