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.
std::packaged_task
object with no task and no shared state.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) std::decay<F>::type
is the same type as std::packaged_task<R(ArgTypes...)>
. 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). f
behaves different from that on f
.std::packaged_task
is move-only.std::packaged_task
with the shared state and task formerly owned by rhs
, leaving rhs
with no shared state and a moved-from task.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 |
f
and possibly std::bad_alloc
if the allocation fails.f
and by the allocator's allocate
function if memory allocation fails.#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
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 |
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