std::promise::set_value

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.

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:

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

3) Any exception thrown by the move constructor of value

Example

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

#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

Parameters

Return value

Exceptions

Example

See also