std::future::get

T get();	(1)	(member only of generic `future` template) (since C++11)
T& get();	(2)	(member only of `future<T&>` template specialization) (since C++11)
void get();	(3)	(member only of `future<void>` template specialization) (since C++11)

The get method waits until the future has a valid result and (depending on which template is used) retrieves it. It effectively calls wait() in order to wait for the result.

The generic template and two template specializations each contain a single version of get. The three versions of get differ only in the return type.

The behavior is undefined if valid() is false before the call to this function.

Any shared state is released. valid() is false after a call to this method.

Parameters

(none)

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) Nothing.

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 implementations are encouraged 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 <thread>
#include <future>
#include <iostream>
#include <string>
#include <chrono>
 
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() << " future.get() returned with " << 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() << " future.get() returned with " << 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] future.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

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

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