CODEWITHSUNDEEP
multithreadingc++11future
James Franco
James Franco

Reputation: 4706

Shared future setting different values with get

I wanted to know if I could do something like this with shared_futures. Essentially I have two threads that receive a reference to a promise. Incase any of the thread returns an output by setting a value in the promise I would like to process that output and return back to listening for another assignment to a promise from the remaining thread. Can I do something like this. 

void tA(std::promise<string>& p )
{
  ....
  std::string r = "Hello from thread A";
  p.set_value(std::move(r));
}

void tB(std::promise<string>& p )
{
  ...
  std::string r = "Hello from thread A";
  p.set_value(std::move(r));
}

int main() {
std::promise<std::string> inputpromise;
std::shared_future<std::string> inputfuture(inputpromise.get_future());

//start the thread A
std::thread t(std::bind(&tA,std::ref(inputpromise));

//start the thread B
std::thread t(std::bind(&tA,std::ref(inputpromise));

std::future<std::string> f(p.get_future());

std::string response = f.get(); ------> Will this unblock when one thread sets a value to the promise and can i go back listening for more assignments on the promise ?

if(response=="b")

response = f.get(); -->listen for the assignment from the remaining thread
}

Upvotes: 1

Views: 206

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473447

You cannot call promise::set_value (or any equivalent function like set_exception) more than once. Promises are not intended to be used in this way, shared across threads. You have one thread which owns the promise, and one or more locations that can tell if the promise has been satisfied, and if so retrieve the value.

A promise is not the right tool for doing what you want. A future/promise is really a special case of a more general tool: a concurrent queue. In a true concurrent queue, generating threads push values into the queue. Receiving threads can extract values from the queue. A future/promise is essentially a single-element queue.

You need a general concurrent queue, not a single-element queue. Unfortunately, the standard library doesn't have one.

Upvotes: 2

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