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

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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:


 Otherwise, we’ll have to use:


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++ program to implement
// the functions
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());
// 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);
    // Fetch the result of packaged_task<>
    // Join the thread. Its blocking and 
    // returns when thread is finished.
    return 0;



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Last Updated : 31 Jan, 2022
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