std::packaged_task<R(Args...)>::packaged_task
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std::packaged_task
Member functions
packaged_task::packaged_task
Getting the result
Execution
Non-member functions
Helper classes
(until C++17)
Deduction guides (C++17)
packaged_task() noexcept;
(1)
(since C++11)
template< class F >
explicit packaged_task( F&& f );
(2)
(since C++11)
explicit packaged_task( F&& f );
template< class F, class Allocator >
explicit packaged_task( std::allocator_arg_t, const Allocator& a, F&& f );
(3)
(since C++11) explicit packaged_task( std::allocator_arg_t, const Allocator& a, F&& f );
(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.2,3) Constructs a
std::packaged_task object with a stored task of type std::decay <F>::type and a shared state. The stored task is initialized with std::forward <F>(f).
These overloads participate in overload resolution only if std::decay <F>::type is not the same type as std::packaged_task <R(Args...)>.
Let t1, t2, ..., tN be values of the corresponding types in Args, if INVOKE<R> (f, t1, t2, ..., tN) is not a valid expression, the program is ill-formed.
This overload participates in overload resolution only if std::remove_cvref_t <F> is not the same type as std::packaged_task <R(Args...)>.
If std::is_invocable_r_v <R, std::decay_t <F>&, Args...> is false, the program is ill-formed.
(since C++20)3) The allocator a is used to allocate memory necessary to store the task.
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.[edit] Parameters
f
-
the callable target to execute
a
-
the allocator to use when storing the task
rhs
-
the
std::packaged_task to move from
[edit] 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.[edit] 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
[edit] 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 parameter type of the copy constructor was packaged_task&
|
added const |
| LWG 2097 | C++11 | for overloads (2,3), F could be std::packaged_task<R(Args...)>
|
F is constrained
|
| LWG 4154 | C++11 | overloads (2,3) did not consider decaying | consider decaying |