std::latch

 

C++

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Freestanding and hosted

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Named requirements

Feature test macros (C++20)

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Concepts library (C++20)

Metaprogramming library (C++11)

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Technical specifications

Symbols index

External libraries

 

Concurrency support library

Threads thread (C++11) jthread (C++20) stop_token (C++20) stop_source (C++20) stop_callback (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) 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) 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_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) Memory ordering memory_order (C++11) kill_dependency (C++11) atomic_thread_fence (C++11) atomic_signal_fence (C++11) 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_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11) lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (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)

 

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.

Unlike std::barrier, std::latch can be decremented by a participating thread more than once.

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:

__cpp_lib_latch

Example

Run this code

#include <functional>
#include <iostream>
#include <latch>
#include <string>
#include <thread>
 
int main() {
  struct job {
    const std::string name;
    std::string product{"not worked"};
    std::thread action{};
  } 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 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 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 starting... done:
  annika worked
  buru worked
  chuck worked
Workers cleaning up... done:
  annika cleaned
  buru cleaned
  chuck cleaned

See also

barrier (C++20)

reusable thread barrier (class template)