std::shared_future

 

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std::shared_future

Member functions

shared_future::shared_future

shared_future::~shared_future

shared_future::operator=

Getting the result

shared_future::get

State

shared_future::valid

shared_future::wait

shared_future::wait_for

shared_future::wait_until

 

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)

Member functions

(constructor)	constructs the future object (public member function)
(destructor)	destructs the future object (public member function)
operator=	assigns the contents (public member function)
Getting the result
get	returns the result (public member function)
State
valid	checks if the future has a shared state (public member function)
wait	waits for the result to become available (public member function)
wait_for	waits for the result, returns if it is not available for the specified timeout duration (public member function)
wait_until	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 <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 = [&]() -> 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 = [&]() -> 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