c++ - 线程安全的 std::cout

Question

以下程序仍然将输出交织到 std::cout。我试过add a std::mutex to control access通过 std::lock_guard 到 std::cout，但它 still interleaves .

#include <iostream>
#include <chrono>
#include <thread>
#include <functional>
#include <mutex>
#include <condition_variable>

std::mutex global_mtx{};

class Timer {
public:
    Timer(size_t time, const std::function<void(void)>& f) : time{std::chrono::milliseconds{time}}, f{f} {}
    ~Timer() { wait_thread.join(); }

private:
    void wait_then_call()
    {
        std::unique_lock<std::mutex> lck{mtx};
        for(int i{10}; i > 0; --i) {
            {
                std::lock_guard<std::mutex>{global_mtx};
                std::cout << "Thread " << wait_thread.get_id() << " countdown at: " << '\t' << i << std::endl;

            }
            cv.wait_for(lck, time / 10);
        }
        f();
    }
    std::mutex mtx;
    std::condition_variable cv{};
    std::chrono::milliseconds time;
    std::function <void(void)> f;
    std::thread wait_thread{[this]() {wait_then_call(); }};
};

int main()
{
    auto f = []() {std::lock_guard<std::mutex>{global_mtx}; std::cout << "---------------- I waited to print! ----------------" << std::endl; };
    Timer t1{3'000,f};
    Timer t2{6'000,f};
    Timer t3{2'000,f};
    Timer t4{1'000,f};
}

我是否需要通过单独的类或专用线程来控制访问？

Answer 1

您的问题在这里:std::lock_guard<std::mutex>{global_mtx};创建一个锁守卫并立即释放它。您需要创建一个变量来持有锁，例如 std::lock_guard<std::mutex> lock{global_mtx}; .