std::packaged_task<R(Args...)>::packaged_task
From cppreference.com
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.2,3) 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 Let |
(until C++20) |
|
This overload participates in overload resolution only if If |
(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.Parameters
| f | - | the callable target 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 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 |