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# std::promise::set_value

From cppreference.com
< cpp‎ | thread‎ | promise

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)

Stores the `value` into the shared state and makes the state ready.

The operation is atomic, i.e. it behaves as though they 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.

### [edit] Parameters

value | - | value to store in the shared state

### [edit] Return value

(none)

### [edit] 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:

1-2) Any exception thrown by the copy constructor of `value`
3) Any exception thrown by the move constructor of `value`

### [edit] 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

### [edit] See also

set_exception | sets the result to indicate an exception (public member function) [edit]

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