Throw by Value, Catch by Reference with Beep

//throw by value, catch by reference, Item 73
#include <iostream>
#include <string>
using namespace std;

class Exception_just_a_class
{
};

class Exception_store_message
{
public:

  Exception_store_message(string message):what(message){}
  string get_what() { return what;}

  void set_what(const string new_what) { what = new_what;}

private:
  string what;
};

void madly_throw_exception()
{
 const char BELL = '\a';

  
  try{
    throw Exception_just_a_class();
  }
  catch (Exception_just_a_class){

    cout << "catch Exception_just_a_class through type\n";
    cout << BELL << endl;

    cout << "exception caught, hit enter to throw another exception\n";
    try{
      string line;
      getline(cin, line, '\n');

      if ( line.size() == 0)
 throw Exception_store_message("throw a second exception");
    }

    catch ( Exception_store_message &ex){
      cout << BELL << endl;

      cout << "(trow by value, catch by reference)\n";
      cout << ex.get_what() << endl;

      ex.set_what("new what");
      throw ; //rethrow modified Exception_store_message, that is, ex.
    }
  }
}

int main()
{
  try{
    madly_throw_exception();
  }
  catch (...) {

   cout << "I catch everythin before crash!\n" ;
  }
}