# std::packaged_task<R(Args...)>::packaged_task

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< 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...)>.

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. | (until C++17)
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

### 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

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