Using std::promises and futures is useful skill to create one-shot communication between a detecting task and reacting task.

Sometimes we want a task to detect some event and then inform a second asynchronously running task to proceed when some event has taken place (e.g., a data structure has been initialized, a stage of computation has been completed, a significant sensor value has been detected, etc.). In other words, a detecting task will detect a special event/condition, and a reacting task will wait until the detecting task notifies that the event occurs/condition changes.

If we only want to inform once, we can take use of the power of std::promises and futures (i.e., std::future and std::shared_future). Since both std::promise and futures are templates requiring a type parameter that indicates the type of data to be transmitted through the communications channel, we specify this type as void indicating that no data is to be conveyed:

• The detecting task will set its std::promise<void> when the event of interest occurs
• The reacting task will wait on its std::future<void> or std::shared_future<void>
• The communications channel wil permit the reacting task to know when the detecting task has written its void data by calling set_value on its std::promise

The essence of the technique looks like,

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std::promise<void> p;

void react();     // func for reacting task

void detect() {   // func for detecting task
    std::thread t([]{
        p.get_future().wait();  // suspend t until future is set
        react();
    });
    ...  // here t is suspend prior to call to react
    p.set_value();  // event detected, so t is unsuspended (and thus call react)
    ...   // do additional work, program is terminated if this part of code throws
    t.join();   // make t unjoinable
}

Taking use of std::future::share()¹, a general form is easy to implement where originally one reacting task extent to many:

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std::promise<void> p;  // as before

void detect()  // now for multiple reacting tasks
{
    auto sf = p.get_future().share();  // sf's type is std::shared_future<void>

    std::vector<std::thread> vt;  // container for reacting threads

    for (int i = 0; i < ThreadsToRun; ++i) {
        vt.emplace_back([sf]{ sf.wait();    // wait on local copy of sf
                              react(); });  // see item 42 for info on emplace_back
    }
    ...   // program is terminated if this part of code throws
    p.set_value();   // unsuspend all threads
    ...
    for (auto& t : vt) {  
        t.join();    // make all threads unjoinable
    }
}