Std::promise 是一個類
std::future也是一個類
std::future<bool>futObj 表示一個儲存bool值的future類物件。
std::promise<bool> proObj表示promise的一個類物件叫proObj，該物件儲存了一個型別為bool的值，該值可被future物件讀取（可能在另外一個執行緒中），因此 promise也提供了一種執行緒同步的手段。在 promise 物件構造時可以和一個共享狀態（通常是std::future）相關聯：futObj = proObj.get_futrue()。
std::promise::set_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.
    std::cout <<"num : ";
    for (int num : numbers) std::cout <<num << ' ';
    std::cout << '\n';

    std::cout <<"let: " ;
    for (char let : letters) std::cout <<let << ' ';
    std::cout << '\n';
 
    value_reader.join();
}
 

執行結果：