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std::promise<R>::set_value

From cppreference.com
< cpp‎ | thread‎ | promise
 
 
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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).

[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.

Run this code
#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]

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