std:: packaged_task
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| Member functions | ||||
| Getting the result | ||||
| Execution | ||||
| Non-member functions | ||||
| Helper classes | ||||
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(until C++17)
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| Deduction guides (C++17) |
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Defined in header
<future>
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template
<
class
>
class
packaged_task
;
//not defined
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(1) | (since C++11) |
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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.
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Just like
std::function
,
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(until C++17) |
Member functions
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constructs the task object
(public member function) |
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destructs the task object
(public member function) |
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moves the task object
(public member function) |
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checks if the task object has a valid function
(public member function) |
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swaps two task objects
(public member function) |
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Getting the result |
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returns a
std::future
associated with the promised result
(public member function) |
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Execution |
|
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executes the function
(public member function) |
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executes the function ensuring that the result is ready only once the current thread exits
(public member function) |
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resets the state abandoning any stored results of previous executions
(public member function) |
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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
#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
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
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(C++11)
|
waits for a value that is set asynchronously
(class template) |