NAME

std::unique_lock::unique_lock - std::unique_lock::unique_lock

Synopsis

unique_lock() noexcept; (1) (since C++11)
unique_lock( unique_lock&& other ) noexcept; (2) (since C++11)
explicit unique_lock( mutex_type& m ); (3) (since C++11)
unique_lock( mutex_type& m, std::defer_lock_t t ) noexcept; (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, (7) (since C++11)

const std::chrono::duration<Rep,Period>& timeout_duration );
template< class Clock, class Duration >

unique_lock( mutex_type& m, (8) (since C++11)

const std::chrono::time_point<Clock,Duration>& timeout_time );

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().
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 Mutex does not satisfy Lockable.
6) Assumes the calling thread already holds an non-shared lock (i.e., a lock
acquired by lock, try_lock, try_lock_for, or try_lock_until) on m. The behavior is
undefined if not so.
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. The behavior is
undefined if Mutex does not satisfy TimedLockable.
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. The behavior is undefined if Mutex does not satisfy TimedLockable.

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

Example

// Run this code

#include <iostream>
#include <thread>
#include <vector>
#include <mutex>
std::mutex m_a, m_b, m_c;
int a, b, c = 1;
void update()
{
{ // Note: std::lock_guard or atomic<int> can be used instead
std::unique_lock<std::mutex> lk(m_a);
a++;
}

{ // Note: see std::lock and std::scoped_lock for details and alternatives
std::unique_lock<std::mutex> lk_b(m_b, std::defer_lock);
std::unique_lock<std::mutex> lk_c(m_c, std::defer_lock);
std::lock(lk_b, lk_c);
b = std::exchange(c, b+c);
}
}

int main()
{
std::vector<std::thread> threads;
for (unsigned i = 0; i < 12; ++i)
threads.emplace_back(update);

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

std::cout << a << "'th and " << a+1 << "'th Fibonacci numbers: "
<< b << " and " << c << '\n';
}

Output:

12'th and 13'th Fibonacci numbers: 144 and 233