Python-like multiprocessing in C ++

Question

I'm new to C ++, and I'm starting from a long Python background.

I am looking for a way to run a function in parallel in C ++. I read a lot about std::async, but I'm still not very clear.

• The following code does a really interesting thing

  #include <future>
  #include <iostream>
  
  void called_from_async() {
    std::cout << "Async call" << std::endl;
  }
  
  int main() {
    //called_from_async launched in a separate thread if possible
    std::future<void> result( std::async(called_from_async));
  
    std::cout << "Message from main." << std::endl;
  
    //ensure that called_from_async is launched synchronously
    //if it wasn't already launched
    result.get();
  
    return 0;
  }
  

  If I run it several times, sometimes the output is what I expected:

  Message from main.
  Async call
  

  But sometimes I get something like this:

  MAessysnacg ec aflrlom main.
  

  Why isn’t happening cout? I clearly call the .get()AFTER method cout.

• About parallel launches. In case I have a code like this:

  #include <future>
  #include <iostream>
  #include <vector>
  
  int twice(int m) {
    return 2 * m;
  }
  
  int main() {
    std::vector<std::future<int>> futures;
  
    for(int i = 0; i < 10; ++i) {
      futures.push_back (std::async(twice, i));
    }
  
    //retrive and print the value stored in the future
    for(auto &e : futures) {
      std::cout << e.get() << std::endl;
    }
  
    return 0;
  }
  

  twiceWill all 10 function calls be executed simultaneously on separate kernels?

  In the case of no, is there something similar in C ++ like Python multiprocess lib?

  Basically what I'm looking for:

  n ? 1 n .

Answer 1

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