exception handling - In C++, is there a difference between “throw” and “throw ex”?

Question

I'd like to ask this question (also here), but this time about C++.

What is the difference in C++ between

try { /*some code here*/}
catch(MyException& ex)
{ throw ex;} //not just throw

and

try {  /*some code here*/}
catch(MyException& ex)
{ throw;} //not throw ex

Is it just in the stack trace (which in C++ is in any case not a standard as in C# or Java)?

(If it makes any difference, I use MSVS 2008.)

Answer 1

throw; rethrows the same exception object it caught while throw ex; throws a new exception. It does not make a difference other than the performance reasons of creating a new exception object. If you have a exception hierarchy where there some other exception classes derived from MyException class and while throwing an exception you have done a throw DerivedClassException; it can be caught by the catch(MyException&). Now if you modify this caught exception object and rethrow it using throw; the type of exception object will still be DerivedClassException. If you do throw Ex; the object slicing occurs and the newly thrown exception will be of type MyException.