std::promise::set_value

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators 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

Thread support library

Threads
thread (C++11)
hardware_destructive_interference_sizehardware_constructive_interference_size (C++17)(C++17)
this_thread namespace
get_id (C++11) yield (C++11)	sleep_for (C++11) sleep_until (C++11)
Mutual exclusion
mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17)	timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14)
Generic lock management
lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)	lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11)
Condition variables
condition_variable (C++11)
condition_variable_any (C++11)
notify_all_at_thread_exit (C++11)
cv_status (C++11)
Futures
promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11)	launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

std::promise

Member functions

promise::promise

promise::~promise

promise::operator=

promise::swap

Getting the result

promise::get_future

Setting the result

promise::set_value

promise::set_value_at_thread_exit

promise::set_exception

promise::set_exception_at_thread_exit

void set_value( const R& value );	(1)	(member only of generic promise template) (since C++11)
void set_value( R&& value );	(2)	(member only of generic promise template) (since C++11)
void set_value( R& value );	(3)	(member only of promise<R&> template specialization) (since C++11)
void set_value();	(4)	(member only of promise<void> template specialization) (since C++11)

Atomically stores the value into the shared state and makes the state ready.

The operation behaves as though set_value, set_exception, set_value_at_thread_exit, and set_exception_at_thread_exit acquire a single mutex associated with the promise object while updating the promise object.

An exception is thrown if there is no shared state or the shared state already stores a value or exception.

Parameters

value

-

value to store in the shared state

Return value

(none)

Exceptions

std::future_error on the following conditions:

• *this has no shared state. The error category is set to no_state.

• The shared state already stores a value or exception. The error category is set to promise_already_satisfied.

Additionally:

1, 3) Any exception thrown by the copy constructor of value

2) Any exception thrown by the move constructor of value

Example

This example shows how promise<void> can be used as signals between threads.

Run this code

#include <thread>
#include <future>
#include <cctype>
#include <vector>
#include <algorithm>
#include <iterator>
#include <iostream>
#include <sstream>

int main()
{
    std::istringstream iss_numbers{"3 4 1 42 23 -23 93 2 -289 93"};
    std::istringstream iss_letters{" a 23 b,e a2 k k?a;si,ksa c"};

    std::vector<int> numbers;
    std::vector<char> letters;
    std::promise<void> numbers_promise, letters_promise;

    auto numbers_ready = numbers_promise.get_future();
    auto letter_ready = letters_promise.get_future();

    std::thread value_reader([&]
    {
        // I/O operations.
        std::copy(std::istream_iterator<int>{iss_numbers},
                  std::istream_iterator<int>{},
                  std::back_inserter(numbers));

        //Notify for numbers.
        numbers_promise.set_value();

        std::copy_if(std::istreambuf_iterator<char>{iss_letters},
                     std::istreambuf_iterator<char>{},
                     std::back_inserter(letters),
                     ::isalpha);

        //Notify for letters.
        letters_promise.set_value();
    });


    numbers_ready.wait();

    std::sort(numbers.begin(), numbers.end());

    if (letter_ready.wait_for(std::chrono::seconds(1)) ==
            std::future_status::timeout)
    {
        //output the numbers while letters are being obtained.
        for (int num : numbers) std::cout << num << ' ';
        numbers.clear(); //Numbers were already printed.
    }

    letter_ready.wait();
    std::sort(letters.begin(), letters.end());

    //If numbers were already printed, it does nothing.
    for (int num : numbers) std::cout << num << ' ';
    std::cout << '\n';

    for (char let : letters) std::cout << let << ' ';
    std::cout << '\n';

    value_reader.join();
}

Output:

-289 -23 1 2 3 4 23 42 93 93 
a a a a b c e i k k k s s

See also

set_exception

sets the result to indicate an exception (public member function)