CODEWITHSUNDEEP
c++c++11
notaorb
notaorb

Reputation: 2180

Using future<> and promise<> for message passing?

I'm trying write a simple program to pass message with future<> and promise<> so I understand how they work. The intent is to pass 10 objects from the produce to the consumer. Unfortunately, I'm running into compilation issues. Does anyone know where I have gone wrong? Thank you!

Update solution.

#include <iostream>
#include <thread>
#include <unistd.h>
#include <stdlib.h>
#include <future>
#include <functional>
#include <vector>
using namespace std;
using namespace std::chrono;

struct MSG {
  int val;
};

void consumer(vector<future<MSG>> *futv) {
  for (auto& p : *futv) {
    this_thread::sleep_for(milliseconds{rand() % 25});
    MSG m = p.get();
    cout << m.val << endl;
  }
}

void producer(vector<promise<MSG>> *promv) {
  MSG m = {0};

  for (auto& p : *promv) {
    this_thread::sleep_for(milliseconds{rand() % 25});
    p.set_value(m);
    m.val++;
  }
}


int main(int argc, char *argv[])
{
  vector<promise<MSG>> promv;
  vector<future<MSG>> futv;

  for (int i=0; i<10; i++) {
    promv.push_back(promise<MSG>());
    futv.push_back(promv[i].get_future());
  }

  thread tp {producer, &promv};
  thread tc {consumer, &futv};

  tp.join();
  tc.join();

  return 0;
}

Compilation...,

g++ -lpthread -pedantic -Wall test84.cc && ./a.out
0
1
2
3
4
5
6
7
8
9

Upvotes: 0

Views: 747

Answers (2)

rekkalmd
rekkalmd

Reputation: 181

You can use condition_variable, it is well placed than future/promise implementations in this case.

For example, taking two threads, product and consumer, when the product 'i' (product element) is finished, it will be passed on to a consumer 'i' who waits until the product is ready and is consumed afterwards.

future/promise is used, for instance, for multiple return value threads.

If you want your thread to return multiple values at different point of time then just pass multiple std::promise objects in thread and fetch multiple return values from thier associated multiple std::future objects.

Upvotes: 0

Alex Guteniev
Alex Guteniev

Reputation: 13634

The error is due to future being only movable but not copyable

for (auto p : futv) - this is an attempt to copy elements fro vector. Use the following loops to have reference to vector element instead:

for (auto& p : futv) or for (decltype(auto) p : futv)

Generally the idea may work as an exercise, but promises/futures are not really designed for that. The problem is that you cannot reuse elements of your array for another run. So for real uses, producer-consumer queues are implemented by using lower level primitives: mutexes, condition variables, atomics, etc.

Upvotes: 2

Related Questions