std::promise::set_value - cppreference.com (original) (raw)

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.

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).

[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:

Any exception thrown by the constructor selected to copy an object of type R.
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 #include #include #include #include #include #include #include #include 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 numbers; std::vector letters; std::promise 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{iss_numbers}, std::istream_iterator{}, std::back_inserter(numbers)); // notify for numbers numbers_promise.set_value(); std::copy_if(std::istreambuf_iterator{iss_letters}, std::istreambuf_iterator{}, 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.