NAME¶

std::packaged_task - std::packaged_task

Synopsis¶

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

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 (until C++17)
on heap or with a provided allocator.

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)
executes the function ensuring that the result is ready
make_ready_at_thread_exit only once the current thread exits
(public member function)
resets the state abandoning any stored results of previous
reset executions
(public member function)

Non-member functions¶

std::swap(std::packaged_task) specializes the std::swap algorithm
(C++11) (function template)

Helper classes¶

std::uses_allocator<std::packaged_task> specializes the std::uses_allocator type
(C++11)(until C++17) trait
(class template specialization)

Deduction guides(since C++17)

Example¶

// Run this code

#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

See also¶

future waits for a value that is set asynchronously
(C++11) (class template)