std::packaged_task(3)

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 ...ArgTypes > (2) (since C++11)
   class packaged_task<R(ArgTypes...)>;
   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 (until C++17)
   on heap or with a provided allocator.

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

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

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

Non-member functions
   std::swap(std::packaged_task) specializes the std::swap algorithm
   (C++11) (function template)

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

Example
// Run this code
 #include <cmath>
 #include <functional>
 #include <future>
 #include <iostream>
 #include <thread>
 // 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
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3117 C++17 deduction guides for packaged_task were missing added

See also
   future waits for a value that is set asynchronously
   (C++11) (class template)