C++ Fundamentals for Professionals/

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Introduction to Promises and Futures

Introduction to Promises and Futures

This lesson gives an introduction to std::promise and std::future, which are used in C++ for multithreading.

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Promise and future make a mighty pair. A promise can put a value, an exception, or simply a notification into the shared data channel. One promise can serve many std::shared_future futures. With C++20, we may get extended futures that are composable.

Here is an introductory example of the usage of std::promise and std::future. Both communication endpoints can be moved to separate threads, so the communication takes place between threads.

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C++
// promiseFuture.cpp
#include <future>
#include <iostream>
#include <thread>
#include <utility>
void product(std::promise<int>&& intPromise, int a, int b){
  intPromise.set_value(a*b);
}
struct Div{
  void operator() (std::promise<int>&& intPromise, int a, int b) const {
    intPromise.set_value(a/b);
  }
};
int main(){
  int a = 20;
  int b = 10;
  std::cout << std::endl;
  // define the promises
  std::promise<int> prodPromise;
  std::promise<int> divPromise;
  // get the futures
  std::future<int> prodResult = prodPromise.get_future();
  std::future<int> divResult = divPromise.get_future();
  // calculate the result in a separate thread
  std::thread prodThread(product, std::move(prodPromise), a, b);
  Div div;
  std::thread divThread(div, std::move(divPromise), a, b);
  // get the result
  std::cout << "20*10 = " << prodResult.get() << std::endl;
  std::cout << "20/10 = " << divResult.get() << std::endl;
  prodThread.join();
  
  divThread.join();
  std::cout << std::endl;
  
}

Thread prodThread (line 36) gets the function product (lines 8 -10), the prodPromise (line 32) and the numbers a and b. To understand the arguments of prodThread, we have to look at the signature of the function. prodThread needs, as its first argument, a callable; this is the previously mentioned function product. The function product requires a promise of the kind ...

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