std::packaged_task - cppreference.com

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Defined in header `<future>`
template< class > class packaged_task;	(1)	(since C++11) (not defined)
template< class R, class ...Args > class packaged_task<R(Args...)>;	(2)	(since C++11)

The class template std::packaged_task wraps any Callable target (function, lambda expression, bind expression, or another function object) so that it can be invoked asynchronously. Its return value or exception thrown is stored in a shared state which can be accessed through std::future objects.

Just like std::function, std::packaged_task is a polymorphic, allocator-aware container: the stored callable target may be allocated on heap or with a provided allocator.

(until C++17)

Member functions

(constructor)	constructs the task object (public member function) [edit]
(destructor)	destructs the task object (public member function) [edit]
operator=	moves the task object (public member function) [edit]
valid	checks if the task object has a valid function (public member function) [edit]
swap	swaps two task objects (public member function) [edit]
Getting the result
get_future	returns a std::future associated with the promised result (public member function) [edit]
Execution
operator()	executes the function (public member function) [edit]
make_ready_at_thread_exit	executes the function ensuring that the result is ready only once the current thread exits (public member function) [edit]
reset	resets the state abandoning any stored results of previous executions (public member function) [edit]

Non-member functions

Helper classes

Deduction guides (since C++17)

Example

#include <cmath>
#include <functional>
#include <future>
#include <iostream>
#include <thread>

// unique function to avoid disambiguating the std::pow overload set
int f(int x, int y) { return std::pow(x, y); }

void task_lambda()
{
    std::packaged_task<int(int, int)> task([](int a, int b)
    {
        return std::pow(a, b); 
    });
    std::future<int> result = task.get_future();
	
    task(2, 9);

    std::cout << "task_lambda:\t" << result.get() << '\n';
}

void task_bind()
{
    std::packaged_task<int()> task(std::bind(f, 2, 11));
    std::future<int> result = task.get_future();
	
    task();

    std::cout << "task_bind:\t" << result.get() << '\n';
}

void task_thread()
{
    std::packaged_task<int(int, int)> task(f);
    std::future<int> result = task.get_future();
	
    std::thread task_td(std::move(task), 2, 10);
    task_td.join();

    std::cout << "task_thread:\t" << result.get() << '\n';
}

int main()
{
    task_lambda();
    task_bind();
    task_thread();
}

Output:

task_lambda: 512
task_bind:   2048
task_thread: 1024

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 3117	C++17	deduction guides for `packaged_task` were missing	added