Wait for shared state with a condition variable.

#include <iostream>
#include <string>
#include <string_view>
#include <vector>
#include <array>
#include <algorithm>
#include <numeric>
#include <map>
#include <unordered_map>
#include <set>
#include <queue>
#include <stack>
#include <deque>
#include <tuple>
#include <bitset>
#include <memory>
#include <optional>
#include <variant>
#include <functional>
#include <charconv>
#include <regex>
#include <fstream>
#include <sstream>
#include <filesystem>
#include <iomanip>
#include <cmath>
#include <cctype>
#include <limits>
#include <chrono>
#include <thread>
#include <future>
#include <mutex>
#include <atomic>
#include <condition_variable>

int main() {
    std::cout << std::boolalpha;
    std::mutex mutex;
    std::condition_variable changed;
    bool ready=false;
    int result=0;
    std::thread worker([&]{
        std::unique_lock<std::mutex> lock(mutex);
        changed.wait(lock,[&]{return ready;});
        result=42;
    });
    {
        std::lock_guard<std::mutex> lock(mutex);
        ready=true;
    }
    changed.notify_one();
    worker.join();
    std::cout << result << "\n";
}