std::shared_future

C++

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Named requirements

Language support library

Concepts library (C++20)

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Regular expressions library (C++11)

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Technical Specifications

Thread support library

Threads
thread (C++11)
hardware_destructive_interference_sizehardware_constructive_interference_size (C++17)(C++17)
this_thread namespace
get_id (C++11) yield (C++11)	sleep_for (C++11) sleep_until (C++11)
Mutual exclusion
mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17)	timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14)
Generic lock management
lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)	lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11)
Condition variables
condition_variable (C++11)
condition_variable_any (C++11)
notify_all_at_thread_exit (C++11)
cv_status (C++11)
Futures
promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11)	launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

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)

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.

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 = [&, 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