第十节 future其他成员函数、shared_future、atomic

最新推荐文章于 2022-09-03 09:09:39 发布

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最新推荐文章于 2022-09-03 09:09:39 发布

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本文链接：https://blog.youkuaiyun.com/HITXJ/article/details/108607903

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本文深入探讨C++中的std::future及其成员函数，包括wait_for、get等，解析std::future_status枚举及std::shared_future的使用场景，通过实例说明不同状态的含义，帮助理解异步任务的管理和数据获取。

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一、std::future的其他成员函数

引言

#include <thread>
#include <future>

int mythread()
{
    cout << "mythread() start" << "threadid = " << std::this_thread::get_id() << endl;
    std::chrono::milliseconds dura(5000);
    std::this_thread::sleep_for(dura);
    cout << "mythread() end" << "threadid = " << std::this_thread::get_id() << endl;
    return 5;
}

int main()
{
    cout << "main threadid = " << std::this_thread::get_id() << endl;
    std::future<int> result = std::async(mythread); //这里线程就开始执行了
    cout << "continue......!" << endl;
    cout << result.get() << endl;
    cout << "I Love China!" << endl;
    return 0;
}

注意：async异步任务主线程会等待子线程执行完毕才会return返回

std::future::wait_for

主要返回一个枚举类型std::future_status

enum class future_status
{
  ready,
  timeout,
  deferred
};

timeout表示线程没执行完
ready表示线程已经执行完，已经成功返回了
deferred表示线程没开始执行

std::future_status::timeout

std::future_status::timeout表示线程超时

#include <thread>
#include <future>

int mythread()
{
    cout << "mythread() start" << "threadid = " << std::this_thread::get_id() << endl;
    std::chrono::milliseconds dura(5000);
    std::this_thread::sleep_for(dura);
    cout << "mythread() end" << "threadid = " << std::this_thread::get_id() << endl;
    return 5;
}

int main()
{
    cout << "main threadid = " << std::this_thread::get_id() << endl;
    std::future<int> result = std::async(mythread); //这里线程就开始执行了
    cout << "continue......!" << endl;

    std::future_status status = result.wait_for(std::chrono::seconds(1));
    if (status == std::future_status::timeout)  //超时：我想等待你(线程)1s，希望你返回，你没有返回，那么status = timeout
    {
        //超时：表示线程还没执行完
        cout << "超时，线程还没有执行完" << endl;
    }
    return 0;
}

std::future_status::ready

std::future_status::ready表示线程成功返回

#include <thread>
#include <future>

int mythread()
{
    cout << "mythread() start" << "threadid = " << std::this_thread::get_id() << endl;
    std::chrono::milliseconds dura(5000);
    std::this_thread::sleep_for(dura);
    cout << "mythread() end" << "threadid = " << std::this_thread::get_id() << endl;
    return 5;
}

int main()
{
    cout << "main threadid = " << std::this_thread::get_id() << endl;
    std::future<int> result = std::async(mythread); //这里线程就开始执行了
    cout << "continue......!" << endl;
    // cout << result.get() << endl;
    // cout << "I Love China!" << endl;

    std::future_status status = result.wait_for(std::chrono::seconds(6));
    if (status == std::future_status::timeout)  //超时：我想等待你(线程)1s，希望你返回，你没有返回，那么status = timeout
    {
        //超时：表示线程还没执行完
        cout << "超时，线程还没有执行完" << endl;
    }
    else if (status == std::future_status::ready)
    {
        cout << "线程成功执行完毕，返回" << endl;
        cout << result.get() << endl;
    }
    return 0;
}

现象：

main threadid = 139670424258368
continue......!
mythread() startthreadid = 139670406264576
mythread() endthreadid = 139670406264576
线程成功执行完毕，返回
5

std::future_status::deferred

表示延迟的意思
如果async的第一个参数被设置为std::launch::deferred，
即把std::future<int> result = std::async(mythread);换成std::future<int> result = std::async(std::launch::deferred,mythread);
则本条件成立.

补充：如果用std::launch::deferred参数，实际上没有创建真正的子线程，而是再主线程中执行，且在std::future::get()才能执行该线程

else if (status == std::future_status::deferred)
{
    cout << "线程被延迟执行" << endl;
    cout << result.get() << endl;
}

二、std::shared_future

引言

#include <thread>
#include <future>
int mythread(int mypar)
{
    cout << "mythread() start threadid = " << std::this_thread::get_id() << endl;
    std::chrono::milliseconds dura(5000);
    std::this_thread::sleep_for(dura);
    return 5;
}

void mythread2(std::future<int> &tmpf)
{
    cout << "mythread2() start" << " threadid = " << std::this_thread::get_id() << endl;
    auto result = tmpf.get();
    cout << "mythread2 result = " << result << endl;
    return;
}

int main()
{
    cout << "main threadid = " << std::this_thread::get_id() << endl;
    std::packaged_task<int(int)> mypt(mythread);
    std::thread t1(std::ref(mypt),1);
    t1.join();

    std::future<int> result = mypt.get_future();
    std::thread t2(mythread2,std::ref(result));
    t2.join();

    cout << "I Love China!" << endl;
    return 0;
}

问题：
为什么auto result = tmpf.get();第二次get会得到异常？
答：
主要是因为get的设计，是一个移动语义，get相当于把tmpf内容移动到result里面

如果有两个或者更多线程想要获得mythread结果

std::shared_future也是个类模板
std::future的get()函数转移数据
std::shared_future的get()函数复制数据

std::shared_future<int> result_s(std::move(result));

std::shared_future<int> result_s(result.share());

执行完毕之后result_s里有值，而result里空了
可以用std::future::share()检测是否为空

int main()
{
    cout << "main threadid = " << std::this_thread::get_id() << endl;
    std::packaged_task<int(int)> mypt(mythread);
    std::thread t1(std::ref(mypt),1);
    t1.join();

    std::future<int> result = mypt.get_future();
    // std::shared_future<int> result_s(std::move(result));
    std::shared_future<int> result_s(result.share());
    std::thread t2(mythread2,std::ref(result_s));
    t2.join();

    cout << "I Love China!" << endl;
    return 0;
}

void mythread2(std::shared_future<int> &tmpf)
{
    cout << "mythread2() start" << " threadid = " << std::this_thread::get_id() << endl;
    auto result = tmpf.get();
    cout << "mythread2 result = " << result << endl;
    return;
}