std::future<T>:: get

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

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

#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';
        }
    }
}

[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.

See also