std::packaged_task<R(Args...)>::packaged_task
From cppreference.com
< cpp | thread | 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.2,3) 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 overloads participate in overload resolution only if std::decay<F>::type is not the same type as std::packaged_task<R(Args...)>.
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Let t1, t2, ..., tN be values of the corresponding types in |
(until C++17) |
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If std::is_invocable_r_v<R, F&, Args...> is false, the program is ill-formed. |
(since C++17) |
If invoking f itself and invoking a copy of f behave differently, the behavior is undefined.
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, function object) to execute |
| a | - | the allocator to use when storing the task |
| rhs | - | the std::packaged_task to move from
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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&
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added const |
| LWG 2097 | C++11 | for overloads (2,3), F could be std::packaged_task<R(Args...)>
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F is constrained
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