Is it ok to destroy a std::promise before future.get() is called?












4















I was wondering if it is ok to call promise.get_future(), move that future somewhere else (e.g. into a vector) and possibly let the promise die before even the future.get() is called. In the following example, the call gateway->refreshWithCallback executes the lambda in a thread so that the shared pointer may set the promise free even if in the second loop the future.get() was not called yet, which seems to work but I am angsty!



std::vector<std::future<bool>> futures;

for(GuiGateway *gateway : gateways){

    std::shared_ptr<std::promise<bool>> shared_promise_ptr(new std::promise<bool>());

    futures.push_back(shared_promise_ptr.get()->get_future());

    gateway->refreshWithCallback([shared_promise_ptr](bool success){

        shared_promise_ptr.get()->set_value(success);

    });

}



qDebug() << "waiting for futures";



for(std::future<bool> &future : futures){

    if(future.get() == false){

        qDebug() << "error retrieving all gateway data, aborting auto config";

        return;

    }

}





c++ concurrency promise future







share|improve this question


















  • 5





    Note that there exists std::make_shared which may be a bit more efficient at creating new objects that will be stored in a std::shared_ptr (allocates enough memory for the object and the control block in one shot) and covers an edge case regarding exception safety. It also saves on repetition, you can write auto shared_promise_ptr = std::make_shared<std::promise<bool>>();.

    – François Andrieux
    Jan 23 '18 at 18:18


















4















I was wondering if it is ok to call promise.get_future(), move that future somewhere else (e.g. into a vector) and possibly let the promise die before even the future.get() is called. In the following example, the call gateway->refreshWithCallback executes the lambda in a thread so that the shared pointer may set the promise free even if in the second loop the future.get() was not called yet, which seems to work but I am angsty!



std::vector<std::future<bool>> futures;

for(GuiGateway *gateway : gateways){

    std::shared_ptr<std::promise<bool>> shared_promise_ptr(new std::promise<bool>());

    futures.push_back(shared_promise_ptr.get()->get_future());

    gateway->refreshWithCallback([shared_promise_ptr](bool success){

        shared_promise_ptr.get()->set_value(success);

    });

}



qDebug() << "waiting for futures";



for(std::future<bool> &future : futures){

    if(future.get() == false){

        qDebug() << "error retrieving all gateway data, aborting auto config";

        return;

    }

}





c++ concurrency promise future







share|improve this question


















  • 5





    Note that there exists std::make_shared which may be a bit more efficient at creating new objects that will be stored in a std::shared_ptr (allocates enough memory for the object and the control block in one shot) and covers an edge case regarding exception safety. It also saves on repetition, you can write auto shared_promise_ptr = std::make_shared<std::promise<bool>>();.

    – François Andrieux
    Jan 23 '18 at 18:18
















4












4








4








I was wondering if it is ok to call promise.get_future(), move that future somewhere else (e.g. into a vector) and possibly let the promise die before even the future.get() is called. In the following example, the call gateway->refreshWithCallback executes the lambda in a thread so that the shared pointer may set the promise free even if in the second loop the future.get() was not called yet, which seems to work but I am angsty!



std::vector<std::future<bool>> futures;

for(GuiGateway *gateway : gateways){

    std::shared_ptr<std::promise<bool>> shared_promise_ptr(new std::promise<bool>());

    futures.push_back(shared_promise_ptr.get()->get_future());

    gateway->refreshWithCallback([shared_promise_ptr](bool success){

        shared_promise_ptr.get()->set_value(success);

    });

}



qDebug() << "waiting for futures";



for(std::future<bool> &future : futures){

    if(future.get() == false){

        qDebug() << "error retrieving all gateway data, aborting auto config";

        return;

    }

}





c++ concurrency promise future







share|improve this question














I was wondering if it is ok to call promise.get_future(), move that future somewhere else (e.g. into a vector) and possibly let the promise die before even the future.get() is called. In the following example, the call gateway->refreshWithCallback executes the lambda in a thread so that the shared pointer may set the promise free even if in the second loop the future.get() was not called yet, which seems to work but I am angsty!



std::vector<std::future<bool>> futures;

for(GuiGateway *gateway : gateways){

    std::shared_ptr<std::promise<bool>> shared_promise_ptr(new std::promise<bool>());

    futures.push_back(shared_promise_ptr.get()->get_future());

    gateway->refreshWithCallback([shared_promise_ptr](bool success){

        shared_promise_ptr.get()->set_value(success);

    });

}



qDebug() << "waiting for futures";



for(std::future<bool> &future : futures){

    if(future.get() == false){

        qDebug() << "error retrieving all gateway data, aborting auto config";

        return;

    }

}





c++ concurrency promise future




c++ concurrency promise future






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asked Jan 23 '18 at 18:16









Magnus LutzMagnus Lutz

1436




1436








  • 5





    Note that there exists std::make_shared which may be a bit more efficient at creating new objects that will be stored in a std::shared_ptr (allocates enough memory for the object and the control block in one shot) and covers an edge case regarding exception safety. It also saves on repetition, you can write auto shared_promise_ptr = std::make_shared<std::promise<bool>>();.

    – François Andrieux
    Jan 23 '18 at 18:18
















  • 5





    Note that there exists std::make_shared which may be a bit more efficient at creating new objects that will be stored in a std::shared_ptr (allocates enough memory for the object and the control block in one shot) and covers an edge case regarding exception safety. It also saves on repetition, you can write auto shared_promise_ptr = std::make_shared<std::promise<bool>>();.

    – François Andrieux
    Jan 23 '18 at 18:18










5




5





Note that there exists std::make_shared which may be a bit more efficient at creating new objects that will be stored in a std::shared_ptr (allocates enough memory for the object and the control block in one shot) and covers an edge case regarding exception safety. It also saves on repetition, you can write auto shared_promise_ptr = std::make_shared<std::promise<bool>>();.

– François Andrieux
Jan 23 '18 at 18:18







Note that there exists std::make_shared which may be a bit more efficient at creating new objects that will be stored in a std::shared_ptr (allocates enough memory for the object and the control block in one shot) and covers an edge case regarding exception safety. It also saves on repetition, you can write auto shared_promise_ptr = std::make_shared<std::promise<bool>>();.

– François Andrieux
Jan 23 '18 at 18:18














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If you provide a value to the std::promise before destroying it, the associated std::future will be able to retrieve it just fine. It does not depend on std::promise still existing. If you failed to provide a value to std::promise before destroying it, trying to get a result from the std::future will throw std::future_error but it is also well defined.






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    If you provide a value to the std::promise before destroying it, the associated std::future will be able to retrieve it just fine. It does not depend on std::promise still existing. If you failed to provide a value to std::promise before destroying it, trying to get a result from the std::future will throw std::future_error but it is also well defined.






    share|improve this answer




























      6














      If you provide a value to the std::promise before destroying it, the associated std::future will be able to retrieve it just fine. It does not depend on std::promise still existing. If you failed to provide a value to std::promise before destroying it, trying to get a result from the std::future will throw std::future_error but it is also well defined.






      share|improve this answer


























        6












        6








        6







        If you provide a value to the std::promise before destroying it, the associated std::future will be able to retrieve it just fine. It does not depend on std::promise still existing. If you failed to provide a value to std::promise before destroying it, trying to get a result from the std::future will throw std::future_error but it is also well defined.






        share|improve this answer













        If you provide a value to the std::promise before destroying it, the associated std::future will be able to retrieve it just fine. It does not depend on std::promise still existing. If you failed to provide a value to std::promise before destroying it, trying to get a result from the std::future will throw std::future_error but it is also well defined.







        share|improve this answer












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        answered Jan 23 '18 at 18:22









        François AndrieuxFrançois Andrieux

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