std::latch

 

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Threads thread (C++11) jthread (C++20) hardware_destructive_interference_sizehardware_constructive_interference_size (C++17)(C++17) this_thread namespace get_id (C++11) yield (C++11) sleep_for (C++11) sleep_until (C++11) Cooperative cancellation stop_token (C++20) inplace_stop_token (C++26) never_stop_token (C++26) stop_source (C++20) inplace_stop_source (C++26)   stop_callback (C++20) inplace_stop_callback (C++26) stop_callback_for_t (C++26) stoppable_token (C++26) unstoppable_token (C++26) stoppable-source (C++26) stoppable-callback-for (C++26)   Mutual exclusion mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17) timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14) Generic lock management lock (C++11) lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) once_flag (C++11) call_once (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lockdefer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)(C++11)(C++11)(C++11) Condition variables condition_variable (C++11) condition_variable_any (C++11) notify_all_at_thread_exit (C++11) cv_status (C++11) Semaphores counting_semaphorebinary_semaphore (C++20)(C++20) Latches and Barriers latch (C++20) barrier (C++20) Futures promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11) launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11) Safe Reclamation rcu_obj_base (C++26) rcu_domain (C++26) rcu_default_domain (C++26) rcu_synchronize (C++26) rcu_barrier (C++26) rcu_retire (C++26) Hazard Pointers hazard_pointer_obj_base (C++26) hazard_pointer (C++26) make_hazard_pointer (C++26)

Atomic types atomic (C++11) atomic_ref (C++20) atomic_flag (C++11) Initialization of atomic types atomic_init (C++11)(deprecated in C++20) ATOMIC_VAR_INIT (C++11)(deprecated in C++20) ATOMIC_FLAG_INIT (C++11) Memory ordering memory_order (C++11) kill_dependency (C++11) atomic_thread_fence (C++11) atomic_signal_fence (C++11) Free functions for atomic operations atomic_storeatomic_store_explicit (C++11)(C++11) atomic_loadatomic_load_explicit (C++11)(C++11) atomic_exchangeatomic_exchange_explicit (C++11)(C++11) atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11) atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11) atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11) atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11) atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11) atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11) atomic_fetch_maxatomic_fetch_max_explicit (C++26)(C++26) atomic_fetch_minatomic_fetch_min_explicit (C++26)(C++26) atomic_is_lock_free (C++11) atomic_waitatomic_wait_explicit (C++20)(C++20) atomic_notify_one (C++20) atomic_notify_all (C++20) Free functions for atomic flags atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11) atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11) atomic_flag_testatomic_flag_test_explicit (C++20)(C++20) atomic_flag_waitatomic_flag_wait_explicit (C++20)(C++20) atomic_flag_notify_one (C++20) atomic_flag_notify_all (C++20)

 

std::latch

Member functions

latch::latch

latch::~latch

latch::count_down

latch::try_wait

latch::wait

latch::arrive_and_wait

Constants

latch::max

 

Defined in header <latch>
class latch;		(since C++20)

The latch class is a downward counter of type std::ptrdiff_t which can be used to synchronize threads. The value of the counter is initialized on creation. Threads may block on the latch until the counter is decremented to zero. There is no possibility to increase or reset the counter, which makes the latch a single-use barrier.

Concurrent invocations of the member functions of std::latch, except for the destructor, do not introduce data races.

Data Members

Name	Definition
std::ptrdiff_t counter	the internal counter (exposition-only member object*)

Member functions

(constructor)	constructs a latch (public member function)
(destructor)	destroys the latch (public member function)
operator= [deleted]	latch is not assignable (public member function)
count_down	decrements the counter in a non-blocking manner (public member function)
try_wait	tests if the internal counter equals zero (public member function)
wait	blocks until the counter reaches zero (public member function)
arrive_and_wait	decrements the counter and blocks until it reaches zero (public member function)
Constants
max [static]	the maximum value of counter supported by the implementation (public static member function)

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_latch	201907L	(C++20)	std::latch

Example

Run this code

#include <functional>
#include <iostream>
#include <latch>
#include <string>
#include <thread>
 
struct Job
{
    const std::string name;
    std::string product{"not worked"};
    std::thread action{};
};
 
int main()
{
    Job jobs[]{{"Annika"}, {"Buru"}, {"Chuck"}};
 
    std::latch work_done{std::size(jobs)};
    std::latch start_clean_up{1};
 
    auto work = [&](Job& my_job)
    {
        my_job.product = my_job.name + " worked";
        work_done.count_down();
        start_clean_up.wait();
        my_job.product = my_job.name + " cleaned";
    };
 
    std::cout << "Work is starting... ";
    for (auto& job : jobs)
        job.action = std::thread{work, std::ref(job)};
 
    work_done.wait();
    std::cout << "done:\n";
    for (auto const& job : jobs)
        std::cout << "  " << job.product << '\n';
 
    std::cout << "Workers are cleaning up... ";
    start_clean_up.count_down();
    for (auto& job : jobs)
        job.action.join();
 
    std::cout << "done:\n";
    for (auto const& job : jobs)
        std::cout << "  " << job.product << '\n';
}

Output:

Work is starting... done:
  Annika worked
  Buru worked
  Chuck worked
Workers are cleaning up... done:
  Annika cleaned
  Buru cleaned
  Chuck cleaned

See also

barrier (C++20)

reusable thread barrier (class template)