std::future<T>::get

Main template
T get();	(1)	(since C++11)
std::future<T&> specializations
T& get();	(2)	(since C++11)
std::future<void> specialization
void get();	(3)	(since C++11)

The get member function waits (by calling wait()) until the shared state is ready, then retrieves the value stored in the shared state (if any). Right after calling this function, valid() is false.

If valid() is false before the call to this function, the behavior is undefined.

Return value

1) The value v stored in the shared state, as std::move(v).

2) The reference stored as value in the shared state.

3) (none)

Exceptions

If an exception was stored in the shared state referenced by the future (e.g. via a call to std::promise::set_exception()) then that exception will be thrown.

Notes

The C++ standard recommends the implementations to detect the case when valid() is false before the call and throw a std::future_error with an error condition of std::future_errc::no_state.

Example

Run this code

#include <chrono>
#include <future>
#include <iostream>
#include <string>
#include <thread>
 
std::string time()
{
    static auto start = std::chrono::steady_clock::now();
    std::chrono::duration<double> d = std::chrono::steady_clock::now() - start;
    return "[" + std::to_string(d.count()) + "s]";
}
 
int main()
{
    using namespace std::chrono_literals;
 
    {
        std::cout << time() << " launching thread\n";
        std::future<int> f = std::async(std::launch::async, []
        {
            std::this_thread::sleep_for(1s);
            return 7;
        });
        std::cout << time() << " waiting for the future, f.valid() = "
                  << f.valid() << '\n';
        int n = f.get();
        std::cout << time() << " f.get() returned " << n << ", f.valid() = "
                  << f.valid() << '\n';
    }
 
    {
        std::cout << time() << " launching thread\n";
        std::future<int> f = std::async(std::launch::async, []
        {
            std::this_thread::sleep_for(1s);
            return true ? throw std::runtime_error("7") : 7;
        });
        std::cout << time() << " waiting for the future, f.valid() = "
                  << f.valid() << '\n';
 
        try
        {
            int n = f.get();
            std::cout << time() << " f.get() returned " << n
                      << ", f.valid() = " << f.valid() << '\n';
        }
        catch (const std::exception& e)
        {
            std::cout << time() << " caught exception " << e.what()
                      << ", f.valid() = " << f.valid() << '\n';
        }
    }
}

Possible output:

[0.000004s] launching thread
[0.000461s] waiting for the future, f.valid() = 1
[1.001156s] f.get() returned with 7, f.valid() = 0
[1.001192s] launching thread
[1.001275s] waiting for the future, f.valid() = 1
[2.002356s] caught exception 7, f.valid() = 0

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2096	C++11	overload (1) needed to check whether `T` is MoveAssignable	not required

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Member functions
future::future
future::~future
future::operator=
future::share
Getting the result
future::get
State
future::valid
future::wait
future::wait_for
future::wait_until