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std::packaged_task(3)	       C++ Standard Libary	   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 (func-
     tion, 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_allo-
     cator 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 retroac-
     tively to
	previously published C++ standards.

	   DR	 Applied to		 Behavior as published		    Cor-
     rect 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)

http://cppreference.com 	   2024.06.10		   std::packaged_task(3)
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