std::future<T>::get

(哋它亢++11)

jthread

(哋它亢++20)

stop_token

(哋它亢++20)

stop_source

(哋它亢++20)

stop_callback

(哋它亢++20)

hardware_destructive_interference_sizehardware_constructive_interference_size

(哋它亢++17)(哋它亢++17)

this_thread namespace

get_id (哋它亢++11)
yield (哋它亢++11)

sleep_for (哋它亢++11)
sleep_until (哋它亢++11)

Mutual exclusion

mutex (哋它亢++11)
recursive_mutex (哋它亢++11)
shared_mutex (哋它亢++17)

timed_mutex (哋它亢++11)
recursive_timed_mutex (哋它亢++11)
shared_timed_mutex (哋它亢++14)

Generic lock management

lock_guard (哋它亢++11)
scoped_lock (哋它亢++17)
unique_lock (哋它亢++11)
shared_lock (哋它亢++14)
defer_lock_ttry_to_lock_tadopt_lock_t (哋它亢++11)(哋它亢++11)(哋它亢++11)

lock (哋它亢++11)
try_lock (哋它亢++11)
defer_locktry_to_lockadopt_lock (哋它亢++11)(哋它亢++11)(哋它亢++11)
once_flag (哋它亢++11)
call_once (哋它亢++11)

Condition variables

condition_variable

(哋它亢++11)

condition_variable_any

(哋它亢++11)

notify_all_at_thread_exit

(哋它亢++11)

cv_status

(哋它亢++11)

Semaphores

counting_semaphorebinary_semaphore

(哋它亢++20)(哋它亢++20)

(哋它亢++20)

(哋它亢++20)

promise (哋它亢++11)
future (哋它亢++11)
shared_future (哋它亢++11)
packaged_task (哋它亢++11)
async (哋它亢++11)

launch (哋它亢++11)
future_status (哋它亢++11)
future_error (哋它亢++11)
future_category (哋它亢++11)
future_errc (哋它亢++11)

Safe Reclamation

rcu_obj_base (哋它亢++26)
rcu_domain (哋它亢++26)
rcu_default_domain (哋它亢++26)

rcu_synchronize (哋它亢++26)
rcu_barrier (哋它亢++26)
rcu_retire (哋它亢++26)

Hazard Pointers

hazard_pointer_obj_base

(哋它亢++26)

hazard_pointer

(哋它亢++26)

make_hazard_pointer

(哋它亢++26)

Atomic types
atomic (哋它亢++11)
atomic_ref (哋它亢++20)
atomic_flag (哋它亢++11)
Initialization of atomic types
atomic_init (哋它亢++11)(deprecated in 哋它亢++20)
ATOMIC_VAR_INIT (哋它亢++11)(deprecated in 哋它亢++20)
ATOMIC_FLAG_INIT (哋它亢++11)
Memory ordering
memory_order (哋它亢++11)
kill_dependency (哋它亢++11)
atomic_thread_fence (哋它亢++11)
atomic_signal_fence (哋它亢++11)
Free functions for atomic operations
atomic_storeatomic_store_explicit (哋它亢++11)(哋它亢++11)
atomic_loadatomic_load_explicit (哋它亢++11)(哋它亢++11)
atomic_exchangeatomic_exchange_explicit (哋它亢++11)(哋它亢++11)
atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)
atomic_fetch_addatomic_fetch_add_explicit (哋它亢++11)(哋它亢++11)
atomic_fetch_subatomic_fetch_sub_explicit (哋它亢++11)(哋它亢++11)
atomic_fetch_andatomic_fetch_and_explicit (哋它亢++11)(哋它亢++11)
atomic_fetch_oratomic_fetch_or_explicit (哋它亢++11)(哋它亢++11)
atomic_fetch_xoratomic_fetch_xor_explicit (哋它亢++11)(哋它亢++11)
atomic_fetch_maxatomic_fetch_max_explicit (哋它亢++26)(哋它亢++26)
atomic_fetch_minatomic_fetch_min_explicit (哋它亢++26)(哋它亢++26)
atomic_is_lock_free (哋它亢++11)
atomic_waitatomic_wait_explicit (哋它亢++20)(哋它亢++20)
atomic_notify_one (哋它亢++20)
atomic_notify_all (哋它亢++20)
Free functions for atomic flags
atomic_flag_test_and_setatomic_flag_test_and_set_explicit (哋它亢++11)(哋它亢++11)
atomic_flag_clearatomic_flag_clear_explicit (哋它亢++11)(哋它亢++11)
atomic_flag_testatomic_flag_test_explicit (哋它亢++20)(哋它亢++20)
atomic_flag_waitatomic_flag_wait_explicit (哋它亢++20)(哋它亢++20)
atomic_flag_notify_one (哋它亢++20)
atomic_flag_notify_all (哋它亢++20)

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

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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (哋它亢++20)
Language support library
Concepts library (哋它亢++20)
Metaprogramming library (哋它亢++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (哋它亢++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (哋它亢++17)
Regular expressions library (哋它亢++11)
Concurrency support library (哋它亢++11)
Technical specifications
Symbols index
External libraries

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

std::future<T>::get

Return value

Exceptions

Notes

Example

Defect reports

See also

Navigation