std::packaged_task::packaged_task

From cppreference.com


packaged_task()	(1)

template <class F> explicit packaged_task(F&& f)	(2)

template <class F, class Allocator> explicit packaged_task(std::allocator_arg_t, const Allocator& a, F&& f)	(3)

packaged_task(const packaged_task&) = delete	(4)

packaged_task(packaged_task&& rhs)	(5)

1) Constructs a std::packaged_task object with no task and no shared state.

2) Constructs a std::packaged_task object with a shared state and a copy of the task, initialized with std::forward<F>(f).

3) Constructs a std::packaged_task object with a shared state and a copy of the task, initialized with std::forward<F>(f). Uses the provided allocator to allocate memory necessary to store the task.

4) The copy constructor is deleted, std::packaged_task is move-only. Note: C++11 does not specify const here, this is the defect 2067.

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 (function, member function, lambda-expression, functor) to execute
a	-	the allocator to use when storing the task
rhs	-	the `std::packaged_task` to move from

[edit] Exceptions

1)

noexcept specification:


noexcept

(since C++11)

2-3) Any exceptions thrown by copy/move constructor of f and possiblly std::bad_alloc if the allocation fails.

5)

noexcept specification:


noexcept

(since C++11)

[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), 40);
 
    std::cout << "waiting for task to finish...\n";
    std::cout << result.get() << '\n';
 
    std::cout << "task complete\n";                                                                                                                                                                           
    t.join();
}

Output:

starting task
waiting for task to finish...
102334155
task complete

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)