NAME¶

std::latch - std::latch

Synopsis¶

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= latch is not assignable
[deleted] (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 the maximum value of counter supported by the implementation
[static] (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 reusable thread barrier
(C++20) (class template)