Packaged Task | Advanced C++ (Multithreading & Multiprocessing)

The  std::packaged_task class wraps any Callable objects (function, lambda expression, bind expression, or another function object) so that they can be invoked asynchronously. A packaged_task won’t start on its own, you have to invoke it, As its return value is stored in a shared state that can be called/accessed by  std::future objects.

Need of packaged_task

The main advantage of a packaged task is that it can link a callable object to a future and that is very important in a flooding environment. For example, if we have an existing function that fetches the data from Database (DB) and returns it. Now there is a need to execute this function in a separate thread. This can be done using:

std::packaged_task<>

 Otherwise, we’ll have to use:

std::promise<>

and have to change code but with the help of  std::packaged_task<> its simple and we don’t need to do that.

Member Functions

Some of the member functions in packaged_task are:

• Operator=- it moves packaged tasks and it’s a public member function.
• Swap- It just swaps to the packaged task or you can say exchange two packaged tasks with each other.
• get_future- It returns a std::future associated with the promised result.
• reset- This public member function just resets the task.
• (constructor)- As the name suggests this public member function constructs the packaged task.
• (destructor)- Similarly, (destructor) destructs the task object.

Non-Member Functions

One of the non-member functions is:

• swap(packaged_task)- It specializes the std::swap algorithm.

Below is the C++ program to implement the above functions-

C++

// C++ program to implement
// the functions
#include<bits/stdc++.h>

using namespace std;

// Factorial function
int factorial(int N)
{
    int res = 1;
    for (int i = N; i > 1; i--)
    {
        res *= i;
    }

    cout << "Result is = " <<
             res << "/n";
    return res;
}

// packaged task
std::deque<std::packaged_task<int(int)> > task_q;
std::mutex mu;
std::condition_variable cond;

void thread1()
{
  // packaged task
  std::packaged_task<int()> t;
  {
    std::unique_lock<std::mutex> locker(mu);
    cond.wait(locker, []()
              {
                return !task_q.empty();
              });
    t = std::move(task_q.front());
    task_q.pop_front();
  }
  t();
}

// Driver Code
int main()
{
      std::thread t1(thread1);

    // Create a packaged_task<> that
    // encapsulated the callback i.e. a function
    std::packaged_task<int()> t(bind(factorial,6));

    // Fetch the associated future<>
    // from packaged_task<>
      std::future<int> fu = t.get_future();
  {
    std::lock_guard<std::mutex> locker(mu);
    task_q.push_back(std::move(t));
  }

      cond.notify_one();

    // Fetch the result of packaged_task<>
    cout<<fu.get();

    // Join the thread. Its blocking and
    // returns when thread is finished.
      t1.join();

    return 0;
}

Output: