std::packaged_task::packaged_task(3)

NAME
std::packaged_task::packaged_task - std::packaged_task::packaged_task

Synopsis
   packaged_task() noexcept; (1) (since C++11)
   template <class F> (2) (since C++11)
   explicit packaged_task( F&& f );
   template <class F, class Allocator> (since C++11)
   explicit packaged_task( std::allocator_arg_t, const Allocator& a, (3) (until C++17)
   F&& f );
   packaged_task( const packaged_task& ) = delete; (4) (since C++11)
   packaged_task( packaged_task&& rhs ) noexcept; (5) (since C++11)
   Constructs a new std::packaged_task object.
   1) Constructs a std::packaged_task object with no task and no shared state.
   2,3) Constructs a std::packaged_task object with a shared state and a copy of the
   task, initialized with std::forward<F>(f).
   The allocator a is used to allocate memory necessary to store the task.
   (until C++17)
     * These constructors do not
       (until C++17)
       This constructor does not
       (since C++17) participate in overload resolution if std::decay<F>::type is the
       same type as std::packaged_task<R(ArgTypes...)>.
     * The program is ill-formed if the INVOKE<R>(std::forward<F>(f),
       std::declval<Args>()...) expression (described in Callable) is ill-formed when
       treated as an unevaluated operand
       (i.e. std::is_invocable_r_v<R, F, Args...> is not true)
       (since C++17).
     * The behavior is undefined if the invocation on a copy of f behaves different
       from that on f.
   4) The copy constructor is deleted, std::packaged_task is move-only.
   5) Constructs a std::packaged_task with the shared state and task formerly owned by
   rhs, leaving rhs with no shared state and a moved-from task.

Parameters
   f - the callable target (function, member function, lambda-expression, functor) to
         execute
   a - the allocator to use when storing the task
   rhs - the std::packaged_task to move from

Exceptions
   2) Any exceptions thrown by copy/move constructor of f and possibly std::bad_alloc
   if the allocation fails.
   3) Any exceptions thrown by copy/move constructor of f and by the allocator's
   allocate function if memory allocation fails.

Example
// Run this code
 #include <future>
 #include <iostream>
 #include <thread>
 int fib(int n)
 {
     if (n < 3) return 1;
     else return fib(n-1) + fib(n-2);
 }
 int main()
 {
     std::packaged_task<int(int)> fib_task(&fib);
     std::cout << "starting task\n";
     auto result = fib_task.get_future();
     std::thread t(std::move(fib_task), 42);
     std::cout << "waiting for task to finish..." << std::endl;
     std::cout << result.get() << '\n';
     std::cout << "task complete\n";
     t.join();
 }

Output:
 starting task
 waiting for task to finish...
 267914296
 task complete
  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 2067 C++11 the deleted copy constructor took reference to made const
                       non-const