[cxx-abi-dev] ABI modification for exception propagation

[David Vandevoorde]

On May 26, 2008, at 9:26 PM, Mark Mitchell wrote:
> Sebastian Redl wrote:
>> Mark Mitchell wrote:
>>> Ideally, it should be possible to freely mix old and new object  
>>> files in a single application.  It should be possible to throw an  
>>> exception in an old file and catch it in a new one, and vice versa.
>> Hmm, I think that's not possible. Definitely not in the separation  
>> model.
>
> If true, that's very unfortunate.  C++ has gotten a somewhat well- 
> deserved black eye by not maintaining binary compatibility over  
> time.  If you've needed about long-term binary compatibility, C has  
> been a better choice for a long time.  The C++ ABI was designed in  
> part to put an end to that problem.  I would hope that we could  
> extend the ABI in a way that would permit old code to continue to  
> work, without
>
> Is the issue that current_exception may need additional information  
> in order to locate the copy constructor for the thrown object.  Is  
> that the issue?


Note that it doesn't have to copy the thrown object: It can  
essentially return a smartptr<__cxa_exception> (which is what  
std::exception_ptr would be) for the top exception.  The constructor  
for this smart pointer must behave a bit like __cxa_rethrow() in that  
it must avoid having __cxa_end_catch destroy the exception.  Instead,  
that destruction must now be handled by the destructor of  
smartptr<__cxa_exception> (when the reference count goes to zero).

	Daveed