Std::promise::set_value (3) Linux Manual Page

std::promise<R>::set_value – std::promise<R>::set_value

Synopsis

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)

1-3) Atomically stores the value into the shared state and makes the state ready.
4) 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.
Calls to this function do not introduce data races with calls to get_future (but they need not synchronize with each other).

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

std::promise::set_value (3) Linux Manual Page