#include <cassert>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread>

int main() {
    std::mutex mutex;
    std::condition_variable changed;
    bool ready = false;
    int answer = 0;
    {
        std::jthread worker([&] {
            const int computed = 6 * 7;
            {
                std::lock_guard lock(mutex);
                answer = computed;
                ready = true;
            }
            changed.notify_one();
        });
        std::unique_lock lock(mutex);
        changed.wait(lock, [&] { return ready; });
        assert(answer == 42);
        std::cout << answer << "\n";
    }
}