std::packaged_task<R(Args...)>::packaged_task

packaged_task() noexcept;	(1)	(since C++11)
template <class F> explicit packaged_task( F&& f );	(2)	(since C++11)
template <class F, class Allocator> explicit packaged_task( std::allocator_arg_t, const Allocator& a, F&& f );	(3)	(since C++11) (until C++17)
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) Constructs a std::packaged_task object with a shared state and a copy of the task, initialized with std::forward<F>(f). This constructor does not participate in overload resolution if std::decay<F>::type is the same type as std::packaged_task<R(ArgTypes...)>.

3) Constructs a std::packaged_task object with a shared state and a copy of the task, initialized with std::forward<F>(f). Uses the provided allocator to allocate memory necessary to store the task. This constructor does not participate in overload resolution if std::decay<F>::type is the same type as std::packaged_task<R(ArgTypes...)>.

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 possiblly 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.

4) (none)

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 non-const	made const

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), 40);
 
    std::cout << "waiting for task to finish...\n";
    std::cout << result.get() << '\n';
 
    std::cout << "task complete\n";
    t.join();
}

Output:

starting task
waiting for task to finish...
102334155
task complete

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Member functions
packaged_task::packaged_task
packaged_task::~packaged_task
packaged_task::operator=
packaged_task::valid
packaged_task::swap
Getting the result
packaged_task::get_future
Execution
packaged_task::operator()
packaged_task::make_ready_at_thread_exit
packaged_task::reset