Re: [boost] Exception Visitor

From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2006-07-21 07:39:29

Oleg Abrosimov wrote:

>static void execute(Iterator*, LastIterator*, F f)
>{
>     assert(std::uncaught_exception());
>     typedef typename boost::mpl::deref<Iterator>::type item;
>
>     try
>     {
>         throw;
>     }
>     //...
>}
>  
>
uncaught_exception() always returns false in VC++6, so if you want the 
thing to work there you need to put this assert under conditionals.

But actually, it will never work and always assert(). The standard says:

*Returns:*
    true after completing evaluation of a /throw-expression/ until  either
    completing   initialization  of  the  /exception-declaration/  in  the
    matching handler or after  entering  terminate()
    for  eny  reason other than an explicit call to terminate().  [/Note:/
    This includes stack unwinding --/end note/]

At the point you enter execute(), the matching handler (the catch(...) 
outside) has completed initialization of its exception declaration.

The use case for uncaught_exception() is quite simple: it allows 
destructors to take different actions depending on whether they're 
destructed due to stack unwind or a different reason.
I've used it in a thin wrapper around jpeglib: there you have to wrap 
the encoding/decoding process in matching function calls, with different 
end calls depending on whether an error occurred. I wrote a sentry class 
that calls the start function on construction and uses 
uncaught_exception() to decide which of the end functions it should call 
on destruction. If the guarded block exits with an exception, the error 
abort function will be called, otherwise the normal decoding finished 
function will be called.
Another option is for destructors that perform operations that could 
fail. They could throw an exception in case of that failure, unless 
they've determined, via uncaught_exception(), that an exception is 
already pending. Then they don't throw, to avoid terminate() being called.

As a simple way of remembering when uncaught_exception() returns true, 
simply remember that this snippet will always call terminate() instead 
of actually throwing.

if(uncaught_exception()) throw 1;

Sebastian Redl