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Difference between revisions of "cpp/thread/shared future"

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< cpp‎ | thread
(With std::shared_future, member functions on an individual object are still unsynchronized. The preferred way to use it would be to take a copy of the object instead and have each thread access its own copy.)
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The class template {{tt|std::shared_future}} provides a mechanism to access the result of asynchronous operations, similar to {{lc|std::future}}, except that multiple threads are allowed to wait for the same shared state. Unlike {{lc|std::future}}, which is only moveable (so only one instance can refer to any particular asynchronous result), {{tt|std::shared_future}} is copyable and multiple shared future objects may refer to the same shared state.
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Access to the same shared state from multiple threads is safe if each thread does it through its own copy of a {{tt|shared_future}} object.
  
 
===Member functions===
 
===Member functions===

Revision as of 04:05, 26 November 2014

 
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std::shared_future
 
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 is copyable and multiple shared future objects may refer to the same shared state.

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.

Contents

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) [edit]
State
checks if the future has a shared state
(public member function) [edit]
waits for the result to become available
(public member function) [edit]
waits for the result, returns if it is not available for the specified timeout duration
(public member function) [edit]
waits for the result, returns if it is not available until specified time point has been reached
(public member function) [edit]

Example

A shared_future may be used to signal multiple threads simultaneously, similar to std::condition_variable::notify_all()

Run this code
#include <iostream>
#include <future>
#include <chrono>
 
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 result1 = std::async(std::launch::async, fun1);
    auto result2 = std::async(std::launch::async, fun2);
 
    // wait for the threads to become ready
    t1_ready_promise.get_future().wait();
    t2_ready_promise.get_future().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
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