<latch> provides std::latch, a simple synchronization primitive that counts down to zero. Threads call count_down() or count_down_and_wait() to decrement; other threads call wait() to block until the count reaches zero.

Unlike std::barrier, a latch is one-time use. It's useful for waiting for a fixed number of async tasks to complete.

This example uses a latch to wait for three worker threads to complete their tasks before the main thread continues.

// compile: g++ -std=c++20 -pthread -o latchexample latchexample.cpp
// run: ./latchexample
// description: wait for multiple threads using latch
 
#include <latch>
#include <iostream>
#include <thread>
 
int main() {
    std::latch done(3);
 
    auto worker = [&](int id) {
        std::cout << "thread " << id << " working\n";
        done.count_down();
    };
 
    std::thread t1(worker, 1);
    std::thread t2(worker, 2);
    std::thread t3(worker, 3);
 
    done.wait();
    std::cout << "all threads done\n";
 
    t1.join();
    t2.join();
    t3.join();
 
    return 0;
}