# std::latch

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< cpp‎ | thread

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)

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