从 lambda 启动异步任务会阻塞当前线程 c++答案

【问题标题】：Starting an async task from a lambda blocks current thread c++从 lambda 启动异步任务会阻塞当前线程 c++
【发布时间】：2021-06-15 00:38:00
【问题描述】：

我正在从 lambda 函数中启动 std::async。即使使用std::launch::async 策略，任务也会在同一个线程上同步运行，因此会阻塞它。这是正常的还是我错过了什么？

int main()
{
    auto lambda = [&]
    {
        auto future = std::async(std::launch::async, [&]
        {
            using namespace std::chrono_literals;
            const auto delay = 5000ms;
            std::this_thread::sleep_for(delay);
            std::cout << "Done okay byeeeeeeee \n";
        });
    };
    
    lambda();
    for (long long i = 0 ; i < 10 ; ++i)
    {
        std::cout << "Doing stuff in main thread" << std::endl;
    }
}

【问题讨论】：

标签： c++ multithreading asynchronous

【解决方案1】：

我上次遇到了类似的问题，没有用 std::async 解决，而是用 std::thread 解决。

void get_sleep()
{
    std::this_thread::sleep_for(std::chrono::seconds(5));
    std::cout << "Done okay byeeeeeeee \n";
}

int main()
{
    std::thread t(get_sleep);
    t.detach();

    for (int i = 0 ; i < 10 ; ++i)
    {
        std::cout << "Doing stuff in main thread" << std::endl;
    }

    return 0;
}

【讨论】：

【解决方案2】：

问题在于future 变量，或者更确切地说是它的destruction。

析构函数可以阻塞直到异步完成。

【讨论】：

即使我删除了未来变量，它仍然会阻塞
@blondbeer 即使您没有将返回的对象分配给变量，std::async 仍然会创建并返回它。

【解决方案3】：

扩展至Some programmer dude's answer

考虑下面的类→我创建它是为了查看析构函数何时被调用。

struct MyStruct {
    ~MyStruct() {printf("DTOR\r\n");}
};

这是你的代码，我已经对代码进行了大量注释，所以让代码来说话

int main(){
//1 You are on main thread, create lambda on main thread
auto lambda = [&]{
    //3 lambda is being started to execute
    MyStruct myStruct{}; //4 This I added
    //4 std::async enqueues the inner lambda, which will run and return in the future
    auto future = std::async(std::launch::async, [&]
    {
        using namespace std::chrono_literals;
        const auto delay = 5000ms;
        std::this_thread::sleep_for(delay);
        std::cout << "Done okay byeeeeeeee \n";
    });
    //5 Going out of scope, destructors will be called therefore you will be blocked
};
//2 You are on main thread    
// You execute lambda on main thread, now you will enter into its contents    
lambda();
//6 After all destructors run you will continue here

【讨论】：