std::packaged_task - cppreference.com (original) (raw)
| Defined in header | ||
|---|---|---|
| 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) |
|---|
Contents
- 1 Member functions
- 2 Non-member functions
- 3 Helper classes
- 4 Deduction guides (since C++17)
- 5 Example
- 6 Defect reports
- 7 See also
[edit] Member functions
| (constructor) | constructs the task object (public member function) [edit] |
|---|---|
| (destructor) | destructs the task object (public member function) [edit] |
| operator= | moves the task object (public member function) [edit] |
| valid | checks if the task object has a valid function (public member function) [edit] |
| swap | swaps two task objects (public member function) [edit] |
| Getting the result | |
| get_future | returns a std::future associated with the promised result (public member function) [edit] |
| Execution | |
| operator() | executes the function (public member function) [edit] |
| make_ready_at_thread_exit | executes the function ensuring that the result is ready only once the current thread exits (public member function) [edit] |
| reset | 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 #include #include #include #include // 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 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 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 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] | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |