Re: [boost] Exception Visitor

From: David Abrahams (dave_at_[hidden])
Date: 2006-07-19 07:52:02

"AlisdairM" <alisdair.meredith_at_[hidden]> writes:

> Jeremy Day wrote:
>
>> > Personally I would prefer it if handlers automatically got the same
>> > order as would be provided by overload resolution.  It's almost
>> > always a mistake to write
>> > 
>> >   catch(Base&) { ... }
>> >   catch(Derived&) { ... }
>> 
>> 
>> Surely it would be possible to reorder the type list that catcher uses
>> (which it gets from the exception handling class, such as
>> my_exception_handler) so that the order is always from most derived
>> (such as Derived&) to least derived (such as Base&).  Is there an MPL
>> algorithm somewhere that will do that?  I feel certain that I saw
>> some code by Andrei Alexandrescu that did something along those lines.
>
> Note the word 'almost' in Dave's statement ;?)

Don't read too much into that.

> No, I don't know any GOOD reason to write catch clauses in Base/Derived
> order, but I can write code that will be caught by the derived handler:
>
> struct my_exception : std::runtime_error, std::logic_error {
> ...
> };
>
> void test() {
>   try {
>     throw my_exception();
>   }
>   catch( const std::exception & ) {}  
>   catch( const std::runtime_error & ) {}
> }
>
>
> Now the question is - can I find a good example with an exception
> hierarchy designed in this way, where catching on the derived type
> first would break code?  In priciple I can fake up the example, but how
> likely is it to be a problem in real-world use?

Not. :)

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com