How do I reverse set_value() and 'deactivate' a promise?

Question

I have a total n00b question here on synchronization. I have a 'writer' thread which assigns a different value 'p' to a promise at each iteration. I need 'reader' threads which wait for shared_futures of this value and then process them, and my question is how do I use future/promise to ensure that the reader threads wait for a new update of 'p' before performing their processing task at each iteration? Many thanks.

Answer 1

You can "reset" a promise by assigning it to a blank promise.

myPromise = promise< int >();

A more complete example:

promise< int > myPromise;

void writer()
{
    for( int i = 0; i < 10; ++i )
    {
        cout << "Setting promise.\n";
        myPromise.set_value( i );

        myPromise = promise< int >{};       // Reset the promise.

        cout << "Waiting to set again...\n";
        this_thread::sleep_for( chrono::seconds( 1 ));
    }
}

void reader()
{
    int result;
    do
    {
        auto myFuture = myPromise.get_future();
        cout << "Waiting to receive result...\n";
        result = myFuture.get();
        cout << "Received " << result << ".\n";
    } while( result < 9 );
}

int main()
{
    std::thread write( writer );
    std::thread read( reader );

    write.join();
    read.join();

    return 0;
}

A problem with this approach, however, is that synchronization between the two threads can cause the writer to call promise::set_value() more than once between the reader's calls to future::get(), or future::get() to be called while the promise is being reset. These problems can be avoided with care (e.g. with proper sleeping between calls), but this takes us into the realm of hacking and guesswork rather than logically correct concurrency.

So although it's possible to reset a promise by assigning it to a fresh promise, doing so tends to raise broader synchronization issues.

Answer 2

A promise/future pair is designed to carry only a single value (or exception.). To do what you're describing, you probably want to adopt a different tool.

If you wish to have multiple threads (your readers) all stop at a common point, you might consider a barrier.

Answer 3

The following code demonstrates how the producer/consumer pattern can be implemented with future and promise.

There are two promise variables, used by a producer and a consumer thread. Each thread resets one of the two promise variables and waits for the other one.

#include <iostream>
#include <future>
#include <thread>
using namespace std;

// produces integers from 0 to 99
void producer(promise<int>& dataready, promise<void>& consumed)
{
    for (int i = 0; i < 100; ++i) {
        // do some work here ...
        consumed = promise<void>{};      // reset
        dataready.set_value(i);          // make data available
        consumed.get_future().wait();    // wait for the data to be consumed
    
    }
    dataready.set_value(-1);                     // no more data
}

// consumes integers
void consumer(promise<int>& dataready, promise<void>& consumed)
{
    for (;;) {
        int n = dataready.get_future().get();    // wait for data ready
        if (n >= 0) {
            std::cout << n << ",";
            dataready = promise<int>{};  // reset
            consumed.set_value();        // mark data as consumed
            // do some work here ...
        }
        else
            break;
    }
}

int main(int argc, const char*argv[])
{
    promise<int> dataready{};
    promise<void> consumed{};

    thread th1([&] {producer(dataready, consumed); });
    thread th2([&] {consumer(dataready, consumed); });

    th1.join();
    th2.join();
    std::cout  << "\n";

    return 0;
}