NAME¶

std::shared_future - std::shared_future

Synopsis¶

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)
waits for the result, returns if it is not available for the specified
wait_for timeout duration
(public member function)
waits for the result, returns if it is not available until specified
wait_until 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

See also¶

async runs a function asynchronously (potentially in a new thread) and returns a
(C++11) std::future that will hold the result
(function template)
future waits for a value that is set asynchronously
(C++11) (class template)