std::shared_future
From cppreference.com
C++
Concurrency support library
|
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
std::shared_future
| Member functions | ||||
| Getting the result | ||||
| State | ||||
Defined in header <future>
|
||
|
template< class T > class shared_future;
|
(1) | (since C++11) |
|
template
<
class T >
class shared_future<T&
>
;
|
(2) | (since C++11) |
|
template
<>
class shared_future<
void
>
;
|
(3) | (since C++11) |
The class template std::shared_future provides a mechanism to access the result of asynchronous operations, similar to std::future, except that multiple threads are allowed to wait for the same shared state. Unlike std::future, which is only moveable (so only one instance can refer to any particular asynchronous result), std::shared_future
Access to the same shared state from multiple threads is safe if each thread does it through its own copy of a shared_future object.
Member functions
| constructs the future object (public member function) |
|
| destructs the future object (public member function) |
|
| assigns the contents (public member function) |
|
Getting the result | |
| returns the result (public member function) |
|
State | |
| checks if the future has a shared state (public member function) |
|
| waits for the result to become available (public member function) |
|
| waits for the result, returns if it is not available for the specified timeout duration (public member function) |
|
| waits for the result, returns if it is not available until specified time point has been reached (public member function) |
|
Example
A shared_future may be used to signal multiple threads simultaneously, similar to std::condition_variable::notify_all()
Run this code
#include <chrono> #include <future> #include <iostream> int main() { std::promise<void> ready_promise, t1_ready_promise, t2_ready_promise; std::shared_future<void> ready_future(ready_promise.get_future()); std::chrono::time_point<std::chrono::high_resolution_clock> start; auto fun1 = [&, ready_future]() -> std::chrono::duration<double, std::milli> { t1_ready_promise.set_value(); ready_future.wait(); // waits for the signal from main() return std::chrono::high_resolution_clock::now() - start; }; auto fun2 = [&, ready_future]() -> std::chrono::duration<double, std::milli> { t2_ready_promise.set_value(); ready_future.wait(); // waits for the signal from main() return std::chrono::high_resolution_clock::now() - start; }; auto fut1 = t1_ready_promise.get_future(); auto fut2 = t2_ready_promise.get_future(); auto result1 = std::async(std::launch::async, fun1); auto result2 = std::async(std::launch::async, fun2); // wait for the threads to become ready fut1.wait(); fut2.wait(); // the threads are ready, start the clock start = std::chrono::high_resolution_clock::now(); // signal the threads to go ready_promise.set_value(); std::cout << "Thread 1 received the signal " << result1.get().count() << " ms after start\n" << "Thread 2 received the signal " << result2.get().count() << " ms after start\n"; }
Possible output:
Thread 1 received the signal 0.072 ms after start Thread 2 received the signal 0.041 ms after start
See also
|
(C++11)
|
runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template) |
|
(C++11)
|
waits for a value that is set asynchronously (class template) |