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std::promise<R>::set_value - cppreference.com

Main template

void set_value( const R& value );

void set_value( R&& value );

void set_value( R& value );

void set_value();

1-3) Atomically stores 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.

Calls to this function do not introduce data races with calls to get_future (therefore they need not synchronize with each other).

(none)

std::future_error on the following conditions:

• *this has no shared state. The error code is set to no_state.
• The shared state already stores a value or exception. The error code is set to promise_already_satisfied.

Additionally:

1) Any exception thrown by the constructor selected to copy an object of type R.

2) Any exception thrown by the constructor selected to move an object of type R.

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

#include <algorithm>
#include <cctype>
#include <chrono>
#include <future>
#include <iostream>
#include <iterator>
#include <sstream>
#include <thread>
#include <vector>
 
using namespace std::chrono_literals;
 
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(1s) == 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());
 
    // does nothing if numbers were already printed
    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

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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