Question

I have a silly question. I read this article about std::exception http://www.cplusplus.com/doc/tutorial/exceptions/

On catch (exception& e), it says:

We have placed a handler that catches exception objects by reference (notice the ampersand & after the type), therefore this catches also classes derived from exception, like our myex object of class myexception.

Does this mean that by using “&” you can also catch exception of the parent class? I thought & is predefined in std::exception because it’s better to pass e (std::exception) as reference than object.

Answer 1

The reason for using & with exceptions is not so much polymorphism as avoiding slicing. If you were to not use &, C++ would attempt to copy the thrown exception into a newly created std::exception, potentially losing information in the process. Example:

#include <stdexcept>
#include <iostream>

class my_exception : public std::exception {
  virtual const char *what() const throw() {
    return "Hello, world!";
  }
};

int main() {
  try {
    throw my_exception();
  } catch (std::exception e) {
    std::cout << e.what() << std::endl;
  }
  return 0;
}

This will print the default message for std::exception (in my case, St9exception) rather than Hello, world!, because the original exception object was lost by slicing. If we change that to an &:

#include <stdexcept>
#include <iostream>

class my_exception : public std::exception {
  virtual const char *what() const throw() {
    return "Hello, world!";
  }
};

int main() {
  try {
    throw my_exception();
  } catch (std::exception &e) {
    std::cout << e.what() << std::endl;
  }
  return 0;
}

Now we do see Hello, world!.