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std::promise<R>::set_value

From cppreference.com
< cpp‎ | thread‎ | promise
 
 
Concurrency support library
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Main template
void set_value( const R& value );
(1) (since C++11)
void set_value( R&& value );
(2) (since C++11)
std::promise<R&> specializations
void set_value( R& value );
(3) (since C++11)
std::promise<void> specialization
void set_value();
(4) (since C++11)
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).

Contents

[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 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.

[edit] Example

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

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 2098 C++11 overloads (1,2) could only throw the
exceptions thrown by the copy/move
constructor of R respectively
they can throw the exceptions thrown
by the actual constructor selected
to copy/move an object of type R

[edit] See also

sets the result to specific value while delivering the notification only at thread exit
(public member function) [edit]
sets the result to indicate an exception
(public member function) [edit]