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std::future

Defined in header `<future>`
template< class T > class future;	(1)	(since C++11)
template< class T > class future<T&>;	(2)	(since C++11)
template<> class future<void>;	(3)	(since C++11)

The class template std::future provides a mechanism to access the result of asynchronous operations:

An asynchronous operation (created via std::async, std::packaged_task, or std::promise) can provide a std::future object to the creator of that asynchronous operation.
The creator of the asynchronous operation can then use a variety of methods to query, wait for, or extract a value from the std::future. These methods may block if the asynchronous operation has not yet provided a value.
When the asynchronous operation is ready to send a result to the creator, it can do so by modifying shared state (e.g. std::promise::set_value) that is linked to the creator's std::future.

Note that std::future references shared state that is not shared with any other asynchronous return objects (as opposed to std::shared_future).

Member functions

(constructor)	constructs the future object (public member function)
(destructor)	destructs the future object (public member function)
operator=	moves the future object (public member function)
share	transfers the shared state from `*this` to a `shared_future` and returns it (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)

Examples

#include <iostream>
#include <future>
#include <thread>
 
int main()
{
    // future from a packaged_task
    std::packaged_task<int()> task([]{ return 7; }); // wrap the function
    std::future<int> f1 = task.get_future();  // get a future
    std::thread t(std::move(task)); // launch on a thread
 
    // future from an async()
    std::future<int> f2 = std::async(std::launch::async, []{ return 8; });
 
    // future from a promise
    std::promise<int> p;
    std::future<int> f3 = p.get_future();
    std::thread( [&p]{ p.set_value_at_thread_exit(9); }).detach();
 
    std::cout << "Waiting..." << std::flush;
    f1.wait();
    f2.wait();
    f3.wait();
    std::cout << "Done!\nResults are: "
              << f1.get() << ' ' << f2.get() << ' ' << f3.get() << '\n';
    t.join();
}

Output:

Waiting...Done!
Results are: 7 8 9

Example with exceptions

#include <thread>
#include <iostream>
#include <future>
 
int main()
{
    std::promise<int> p;
    std::future<int> f = p.get_future();
 
    std::thread t([&p]{
        try {
            // code that may throw
            throw std::runtime_error("Example");
        } catch(...) {
            try {
                // store anything thrown in the promise
                p.set_exception(std::current_exception());
            } catch(...) {} // set_exception() may throw too
        }
    });
 
    try {
        std::cout << f.get();
    } catch(const std::exception& e) {
        std::cout << "Exception from the thread: " << e.what() << '\n';
    }
    t.join();
}

Output:

Exception from the thread: Example

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

std::future

Member functions

Getting the result

State

Examples

Example with exceptions

See also