cpp

class Example {

  someAsyncronousMethod(void * completed ) {
    // This methods takes a long time, and runs on a separate thread so that it doesn't block execution.
    // When we are done ...
    if(completed != NULL)
      (*completed)();
  }

}

void foo() {
  std::cout << "Our long running method has completed."
}

int main(int argc, char ** argv) {
  
  Example ex;
  void * foo = &foo
  ex.someAsyncronousMethod(fooPtr);
  
  while(true) {
    //Event loop
  }
  return 0;
}

csharp

class MyWindow : Window
{
    Button _button;

    public MyWindow()
    {
        _button = new Button();
        // place the button in the window
        _button.Click += MyWindow.ButtonClicked;
    }

    static void ButtonClicked(object sender, RoutedEventArgs e)
    {
        MessageBox.Show("Button Clicked");
    }
}

java

// The callback interface
interface MyCallback {
    void callbackCall();
}

// The class that takes the callback
class Worker {
   MyCallback callback;

   void onEvent() {
      callback.callbackCall();
   }
}

// Option 1:

class Callback implements MyCallback {
   void callbackCall() {
      // callback code goes here
   }
}

worker.callback = new Callback();

// Option 2:

worker.callback = new MyCallback() {

   void callbackCall() {
      // callback code goes here
   }
};