std::shared_future

 

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Threads thread (C++11) jthread (C++20) 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) Cooperative cancellation stop_token (C++20) inplace_stop_token (C++26) never_stop_token (C++26) stop_source (C++20) inplace_stop_source (C++26)   stop_callback (C++20) inplace_stop_callback (C++26) stop_callback_for_t (C++26) stoppable_token (C++26) unstoppable_token (C++26) stoppable-source (C++26) stoppable-callback-for (C++26)   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 (C++11) lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) once_flag (C++11) call_once (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lockdefer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)(C++11)(C++11)(C++11) Condition variables condition_variable (C++11) condition_variable_any (C++11) notify_all_at_thread_exit (C++11) cv_status (C++11) Semaphores counting_semaphorebinary_semaphore (C++20)(C++20) Latches and Barriers latch (C++20) barrier (C++20) 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) Safe Reclamation rcu_obj_base (C++26) rcu_domain (C++26) rcu_default_domain (C++26) rcu_synchronize (C++26) rcu_barrier (C++26) rcu_retire (C++26) Hazard Pointers hazard_pointer_obj_base (C++26) hazard_pointer (C++26) make_hazard_pointer (C++26)

Atomic types atomic (C++11) atomic_ref (C++20) atomic_flag (C++11) Initialization of atomic types atomic_init (C++11)(deprecated in C++20) ATOMIC_VAR_INIT (C++11)(deprecated in C++20) ATOMIC_FLAG_INIT (C++11) Memory ordering memory_order (C++11) kill_dependency (C++11) atomic_thread_fence (C++11) atomic_signal_fence (C++11) Free functions for atomic operations atomic_storeatomic_store_explicit (C++11)(C++11) atomic_loadatomic_load_explicit (C++11)(C++11) atomic_exchangeatomic_exchange_explicit (C++11)(C++11) atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11) atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11) atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11) atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11) atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11) atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11) atomic_fetch_maxatomic_fetch_max_explicit (C++26)(C++26) atomic_fetch_minatomic_fetch_min_explicit (C++26)(C++26) atomic_is_lock_free (C++11) atomic_waitatomic_wait_explicit (C++20)(C++20) atomic_notify_one (C++20) atomic_notify_all (C++20) Free functions for atomic flags atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11) atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11) atomic_flag_testatomic_flag_test_explicit (C++20)(C++20) atomic_flag_waitatomic_flag_wait_explicit (C++20)(C++20) atomic_flag_notify_one (C++20) atomic_flag_notify_all (C++20)

 

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