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Showing content from https://en.cppreference.com/w/cpp/language/../header/../thread/packaged_task.html below:

std::packaged_task - cppreference.com

template< class > class packaged_task;

(1) (since C++11)
(not defined)

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) [edit] Member functions constructs the task object
(public member function) [edit] destructs the task object
(public member function) [edit] moves the task object
(public member function) [edit] checks if the task object has a valid function
(public member function) [edit] swaps two task objects
(public member function) [edit] Getting the result returns a std::future associated with the promised result
(public member function) [edit] Execution executes the function
(public member function) [edit] executes the function ensuring that the result is ready only once the current thread exits
(public member function) [edit] resets the state abandoning any stored results of previous executions
(public member function) [edit] [edit] Non-member functions [edit] Helper classes [edit] Deduction guides (since C++17) [edit] 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

[edit] 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 [edit] See also waits for a value that is set asynchronously
(class template) [edit]

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