在构造函数中使用std :: async [英] Using std::async in constructor

问题描述

我对C ++ 11功能 std :: async 还是很陌生，我无法理解为什么下面的代码从不打印 bar .

I am quite new to the C++11 feature std::async and I fail to grasp why the code below never prints bar.

有人可以帮我一下吗?

class Thready {

  public:

    Thready() {
        std::async(std::launch::async, &Thready::foo, this);
    }

    void foo() {
        while (true) {
            std::cout << "foo" << std::endl;
        }
    }

    void bar() {
        while (true) {
            std::cout << "bar" << std::endl;
       }
    }
};

int main() {  
    Thready t;
    t.bar();
}

推荐答案

请参阅此页上的注释"部分: http://en.cppreference.com/w/cpp/thread/async

See "Notes" section on this page: http://en.cppreference.com/w/cpp/thread/async

该实现可能会扩展的第一个重载的行为std :: async通过启用附加的(实现定义的)位来实现默认启动策略.实施定义的启动示例策略是同步策略(在异步内立即执行调用)和任务策略(类似于异步，但线程本地不是)(清除)如果从std :: async获得的std :: future没有从或绑定到引用，则std :: future的析构函数将阻止在完整表达式的末尾直到异步操作完成后，基本上使诸如以下代码的代码同步:

The implementation may extend the behavior of the first overload of std::async by enabling additional (implementation-defined) bits in the default launch policy. Examples of implementation-defined launch policies are the sync policy (execute immediately, within the async call) and the task policy (similar to async, but thread-locals are not cleared) If the std::future obtained from std::async is not moved from or bound to a reference, the destructor of the std::future will block at the end of the full expression until the asynchronous operation completes, essentially making code such as the following synchronous:

std::async(std::launch::async, []{ f(); }); // temporary's dtor waits for f()
std::async(std::launch::async, []{ g(); }); // does not start until f() completes

(请注意，std :: futures的析构函数通过对std :: async的调用之外的其他方式获得，从不阻止)

(note that the destructors of std::futures obtained by means other than a call to std::async never block)

TL; DR:

尝试将std :: async调用的返回值保存到某个变量中:

try to save the returned value of std::async call into some variable:

auto handle = std::async(std::launch::async, &Thready::foo, this);

以下代码应该可以正常工作.

the following code should work as you expect.

#include <future>
#include <iostream>

class Thready {

  public:

    Thready() {
        handle = std::async(std::launch::async, &Thready::foo, this);
    }

    void foo() {
        while (true) {
            std::cout << "foo" << std::endl;
        }
    }

    void bar() {
        while (true) {
            std::cout << "bar" << std::endl;
       }
    }

    std::future<void> handle;
};

int main() {  
    Thready t;
    t.bar();
}

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