NAME¶

std::shared_lock::shared_lock - std::shared_lock::shared_lock

Synopsis¶

shared_lock() noexcept; (1) (since C++14)
shared_lock( shared_lock&& other ) noexcept; (2) (since C++14)
explicit shared_lock( mutex_type& m ); (3) (since C++14)
shared_lock( mutex_type& m, std::defer_lock_t t ) noexcept; (4) (since C++14)
shared_lock( mutex_type& m, std::try_to_lock_t t ); (5) (since C++14)
shared_lock( mutex_type& m, std::adopt_lock_t t ); (6) (since C++14)
template< class Rep, class Period >

shared_lock( mutex_type& m, (7) (since C++14)

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

shared_lock( mutex_type& m, (8) (since C++14)

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

Constructs a shared_lock, optionally locking the supplied mutex.

1) Constructs a shared_lock with no associated mutex.
2) Move constructor. Initializes the shared_lock with the contents of other. Leaves
other with no associated mutex.
3-8) Constructs a shared_lock with m as the associated mutex. Additionally:
3) Locks the associated mutex in shared mode by calling m.lock_shared().
4) Does not lock the associated mutex.
5) Tries to lock the associated mutex in shared mode without blocking by calling
m.try_lock_shared().
6) Assumes the calling thread already holds a shared lock (i.e., a lock acquired by
lock_shared, try_lock_shared, try_lock_shared_for, or try_lock_shared_until) on m.
The behavior is undefined if not so.
7) Tries to lock the associated mutex in shared mode by calling
m.try_lock_shared_for(timeout_duration), which 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
meet the SharedTimedLockable requirements.
8) Tries to lock the associated mutex in shared mode by calling
m.try_lock_shared_until(timeout_time), which 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 meet the SharedTimedLockable requirements.

Parameters¶

other - another shared_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 <chrono>
#include <iostream>
#include <shared_mutex>
#include <syncstream>
#include <thread>

std::shared_timed_mutex m;
int i = 10;

void read_shared_var(int id)
{
// both the threads get access to the integer i
std::shared_lock<std::shared_timed_mutex> slk(m);
const int ii = i; // reads global i

std::osyncstream(std::cout) << '#' << id << " read i as " << ii << "...\n";
std::this_thread::sleep_for(std::chrono::milliseconds(10));
std::osyncstream(std::cout) << '#' << id << " woke up..." << std::endl;
}

int main()
{
std::thread r1{read_shared_var, 1};
std::thread r2{read_shared_var, 2};

r1.join();
r2.join();
}

Possible output:¶

#2 read i as 10...
#1 read i as 10...
#2 woke up...
#1 woke up...