std::packaged_task (3) - Linux Manuals

std::packaged_task: std::packaged_task

NAME

std::packaged_task - std::packaged_task

Synopsis

Defined in header <future>
template< class > class packaged_task; //not defined (1) (since C++11)
template< class R, class ...Args > (2) (since C++11)
class packaged_task<R(Args...)>;

The class template std::packaged_task wraps any Callable target (function, lambda expression, bind expression, or another function object) so that it can be invoked asynchronously. Its return value or exception thrown is stored in a shared state which can be accessed through std::future objects.

Just like std::function, std::packaged_task is a polymorphic, allocator-aware container: the stored callable target may be allocated on heap or with a provided allocator. (until C++17)

Member functions

                          constructs the task object
constructor (public member function)
                          destructs the task object
destructor (public member function)
                          moves the task object
operator= (public member function)
                          checks if the task object has a valid function
valid (public member function)
                          swaps two task objects
swap (public member function)

Getting the result

                          returns a std::future associated with the promised result
get_future (public member function)

Execution

                          executes the function
operator() (public member function)
                          executes the function ensuring that the result is ready only once the current thread exits
make_ready_at_thread_exit (public member function)
                          resets the state abandoning any stored results of previous executions
reset (public member function)

Non-member functions

std::swap(std::packaged_task) specializes the std::swap algorithm
                              (function template)
(C++11)

Helper classes

std::uses_allocator<std::packaged_task> specializes the std::uses_allocator type trait
                                        (class template specialization)
(C++11)(until C++17)

Example

// Run this code

  #include <iostream>
  #include <cmath>
  #include <thread>
  #include <future>
  #include <functional>

  // unique function to avoid disambiguating the std::pow overload set
  int f(int x, int y) { return std::pow(x,y); }

  void task_lambda()
  {
      std::packaged_task<int(int,int)> task([](int a, int b) {
          return std::pow(a, b);
      });
      std::future<int> result = task.get_future();

      task(2, 9);

      std::cout << "task_lambda:\t" << result.get() << '\n';
  }

  void task_bind()
  {
      std::packaged_task<int()> task(std::bind(f, 2, 11));
      std::future<int> result = task.get_future();

      task();

      std::cout << "task_bind:\t" << result.get() << '\n';
  }

  void task_thread()
  {
      std::packaged_task<int(int,int)> task(f);
      std::future<int> result = task.get_future();

      std::thread task_td(std::move(task), 2, 10);
      task_td.join();

      std::cout << "task_thread:\t" << result.get() << '\n';
  }

  int main()
  {
      task_lambda();
      task_bind();
      task_thread();
  }

Output:

  task_lambda: 512
  task_bind: 2048
  task_thread: 1024