std::thread: How to wait (join) for any of the given threads to complete?

Question

For example, I have two threads, t1 and t2. I want to wait for t1 or t2 to finish. Is this possible?

If I have a series of threads, say, a std::vector<std::thread>, how can I do it?

Answer 1

There's always wait & notify using std::condition_variable, e.g.:

std::mutex m;
std::condition_variable cond;
std::atomic<std::thread::id> val;

auto task = [&] {
    std::this_thread::sleep_for(1s); // Some work

    val = std::this_thread::get_id();
    cond.notify_all();
};

std::thread{task}.detach();
std::thread{task}.detach();
std::thread{task}.detach();

std::unique_lock<std::mutex> lock{m};
cond.wait(lock, [&] { return val != std::thread::id{}; });

std::cout << "Thread " << val << " finished first" << std::endl;

_{Note: val doesn't necessarily represent the thread that finished first as all threads finish at about the same time and an overwrite might occur, but it is only for the purposes of this example.}