std :: async中的奇怪行为

Question

I want have one task that can be killable if it's running of time.

So i am tryng this:

#include <iostream>
#include <future>

using namespace std;

int main()
{
    auto handle = std::async(std::launch::deferred,[]
                        {
                            cout<<"Initializing thread..."<<endl; 
                            this_thread::sleep_for(std::chrono::seconds(5));
                        }
                );

    cout<<"Starting..."<<endl;
    handle.wait_for(std::chrono::seconds(2));

    cout<<"Finished correctly"<<endl;
    return 0;
}

The ouput:

Starting...

Finished correctly

Why don't print "Initializing thread..."? I tried swap both chronos and doesn't works anyway

Answer 1

You never demand the result of the task, so it isn't scheduled.

Replace deferred with async and you'll get what you expect.

Answer 2

deferred only evaluates on a non -timed wait function. If you change to:

handle.wait();

You'll see the thread start at that point.