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std::packaged_task

Defined in header `<future>`
template< class > class packaged_task; //not defined	(1)	(since C++11)
template< class R, class ...ArgTypes > class packaged_task<R(ArgTypes...)>;	(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)
(destructor)	destructs the task object (public member function)
operator=	moves the task object (public member function)
valid	checks if the task object has a valid function (public member function)
swap	swaps two task objects (public member function)
Getting the result
get_future	returns a `std::future` associated with the promised result (public member function)
Execution
operator()	executes the function (public member function)
make_ready_at_thread_exit	executes the function ensuring that the result is ready only once the current thread exits (public member function)
reset	resets the state abandoning any stored results of previous executions (public member function)

Non-member functions

std::swap(std::packaged_task)

(C++11)

specializes the std::swap algorithm
(function template)

Helper classes

std::uses_allocator<std::packaged_task>

(C++11) (until C++17)

specializes the std::uses_allocator type trait
(class template specialization)

Deduction guides(since C++17)

Example

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