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| Member functions | ||||
| Getting the result | ||||
| Execution | ||||
| Non-member functions | ||||
| Helper classes | ||||
(until C++17) | ||||
| Deduction guides(C++17) |
Defined in header <future> | ||
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 templatestd::packaged_task wraps anyCallable 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 throughstd::future objects.
Just likestd::function, | (until C++17) |
Contents |
| 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 astd::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] | |
| specializes thestd::swap algorithm (function template)[edit] |
(C++11)(until C++17) | specializes thestd::uses_allocator type trait (class template specialization)[edit] |
#include <cmath>#include <functional>#include <future>#include <iostream>#include <thread> // unique function to avoid disambiguating the std::pow overload setint f(int x,int y){returnstd::pow(x, y);} void task_lambda(){ std::packaged_task<int(int,int)> task([](int a,int b){returnstd::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: 512task_bind: 2048task_thread: 1024
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 forpackaged_task were missing | added |
(C++11) | waits for a value that is set asynchronously (class template)[edit] |
