std::packaged_task

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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

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

Non-member functions

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

Helper classes

(C++11)(until C++17)
 specializes the std::uses_allocator type 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

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