std::packaged_task<R(Args...)>::packaged_task - cppreference.com (original) (raw)

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 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(f, t1, t2, ..., tN) is not a valid expression, the program is ill-formed. (until C++20)
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)

  1. The allocator a is used to allocate memory necessary to store the task.

  2. The copy constructor is deleted, std::packaged_task is move-only.

  3. 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

  1. Any exceptions thrown by copy/move constructor of f and possibly std::bad_alloc if the allocation fails.

  2. Any exceptions thrown by copy/move constructor of f and by the allocator's allocate function if memory allocation fails.

[edit] Example

#include #include #include   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