prevent sleep_for from blocking background thread

Question

I'm writing in pure c++11 and want to do a simple 'wait x seconds and turn on a member variable' after turning it off. The member variable of the class in this example is a flag for 'animating'.

        cout << "stop animating!" << endl;
        this->animating = false;

        async(launch::async, [this] ()
        {
            this_thread::sleep_for(chrono::seconds{8});
            this->animating = true;
            std::cout << "start animating!" << std::endl;               
        });
        cout << "i'm here" << endl;

the this_thread::sleep_for blocks the entire program from continuing on (even though it is inside an async thread). because i dont see "I'm here" 8 seconds later. If the above code worked as intended, i would see "I'm here" immediately after "stop animating". This blocking is a problem for me because it locks up everything I care about like continuing to process 'input' like keyboard events, and the program also stops 'drawing' other objects on the screen.

Does anyone know how to achieve a simple delayed and async change of a member variable using standard c++11 (no frameworks like boost please)

in iOS it is very simple:

// Delay execution of my block for 10 seconds.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 10 * NSEC_PER_SEC), 
dispatch_get_main_queue(), ^
{
    //do whatever, 10 seconds later
});

Either use `std::thread([this](){...}).detach();` instead or try saving the result std::future and make sure it doesn't go out of scope. — , Mar 17 '18 at 18:32
yes. this appears to work, even better than keeping track of the auto future returned from async. i think this is the answer — user1709076, Mar 17 '18 at 18:56
Maybe have a look at this [answer](https://stackoverflow.com/a/32517201/3972710). As user2176127 mentioned it you may save the std::future otherwise its destructor waits for its result — NGI, Mar 17 '18 at 19:39

einpoklum · Accepted Answer · 2018-03-17T22:15:41.457

3

As per @user2176127 's comment - have you tried this? :

cout << "stop animating!" << endl;
this->animating = false;

std::thread delay_thread(
    [this]() {
        this_thread::sleep_for(chrono::seconds{8});
        this->animating = true;
        std::cout << "start animating!" << std::endl;               
    }
);
delay_thread.detach();
std::cout << "I'm here" << std::endl;

Also note you likely need to wrap the animating member in an std::atomic<>, i.e. if it was bool it now becomes an std::atomic<bool>, so as to ensure your main thread notices the change when it actually happens. (Using volatile won't help.)

edited Mar 17 '18 at 22:15

answered Mar 17 '18 at 18:43

einpoklum

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`volatile` isn't necessary. Unless you write low level code `volatile` is almost always useless. If you want to be pedantic about the `animating` variable use `std::atomic`. – Mar 17 '18 at 20:35
2

In C++ `volatile` has nothing to do with thread safety. While it will probably work in practice, you really need `atomic`, otherwise it's UB. – sbabbi Mar 17 '18 at 21:23
Keyword `volatile` is for use with memory-mapped hardware only. – Jive Dadson Mar 17 '18 at 21:39
@user2176127: Please note the recent (volatile -> atomic). – einpoklum Mar 17 '18 at 22:16
@JiveDadson: Signal handling. – einpoklum Mar 18 '18 at 00:04

prevent sleep_for from blocking background thread

1 Answers1