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

The second. Here is my attempt at quoting Sutter

"Throw by value, catch by reference"

Here's the full summary

---

The reason to avoid catching exceptions by value is that it implicitly makes a copy of the exception. If the exception is of a subclass, then information about it will be lost.

try { throw MyException ("error") } 
catch (Exception e) {
    /* Implies: Exception e (MyException ("error")) */
    /* e is an instance of Exception, but not MyException */
}

Catching by reference avoids this issue by not copying the exception.

try { throw MyException ("error") } 
catch (Exception& e) {
    /* Implies: Exception &e = MyException ("error"); */
    /* e is an instance of MyException */
}

Answer 2

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
  }
}

Answer 3

Also, note that, when using MFC, you may have to catch by pointer. Otherwise, @JaredPar's answer is the way you should normally go (and hopefully never have to deal with things that throw a pointer).

Answer 4

Personally, I would go for the third option:

catch (const _com_error& e)