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;}

and

try {  /*some code here*/}
catch(MyException& ex)
{ throw;}

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 ex will make another copy and is not recommend use throw only to throw the current exception object.

Answer 2

You can use the throw; form with catch(...) (that is it is the only way to rethrow if you caught using catch(...) ).

Answer 3

throw can throw a nonstandard exception type that was caught by catch(...) (eg structured exception)

Answer 4

throw; rethrows the same exception object it caught while throw ex; throws a new exception. It does a make 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.

Answer 5

If you have an exception hierarchy, throw ex can slice your exception, while throw won't. For example:

#include <iostream> 
#include <string> 

using namespace std; 

struct base 
{ 
  virtual string who() {return "base";} 
}; 

struct derived : public base 
{ 
  string who() {return "derived";} 
}; 

int main() { 
  try { 
    try { 
      throw derived(); // throws a 'derived'
    }  
    catch (base& ex)  
    { 
      throw ex; // slices 'derived' object to be a 'base' object
    } 
  } 
  catch (base& ex) 
  { 
    cout<<ex.who()<<endl; // prints 'base'
  } 
}

Change throw ex to just throw, and you'll get an output of derived, which is what you probably expected to get.

Answer 6

Additionally, since it sometimes causes confusion, a bare throw; outside of an exception-handling context will abort the program.

Answer 7

There's a big difference. I wrote about it on my blog, at: http://cplusplus.co.il/2009/08/23/nuances-of-exception-rethrow/

You are more than welcome to have a look