std::packaged_task::packaged_task
From cppreference.com
< cpplrm; | threadlrm; | 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