Defined in header <future> | ||
---|---|---|
template< class > class packaged_task; //not defined | (1) | (since C++11) |
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.
Member functions
constructs the task object (public member function) | |
(destructor)
| destructs the task object (public member function) |
operator=
| 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 | |
operator()
| 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
specializes the std::uses_allocator type trait (class template specialization) |
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
See also
(C++11) | waits for a value that is set asynchronously (class template) |
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