Which is correct? catch (_com_error e) or catch (_com_error& e)?

Question

Which one should I use?

catch (_com_error e)  

or

catch (_com_error& e)

Answer 1

Definitely the second. If you had the following:

class my_exception : public exception
{
  int my_exception_data;
};

void foo()
{
  throw my_exception;
}

void bar()
{
  try
  {
    foo();
  }
  catch (exception e)
  {
    // e is "sliced off" - you lose the "my_exception-ness" of the exception object
  }
}