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