The Right Approach to Exceptions

[Andrei Alexandrescu]

On 2/20/12 12:02 PM, H. S. Teoh wrote:
> On Mon, Feb 20, 2012 at 06:55:13PM +0100, deadalnix wrote:
>> Le 20/02/2012 18:11, Andrei Alexandrescu a écrit :
> [...]
>>> That extra processing must format the message given the information
>>> passed by the exception. _Definitely_ it doesn't make sense to put
>>> the formatting processing in the exception. That's why shipping the
>>> information outside the exception in a generic format is necessary,
>>> hence the Variant[string].
>>>
>>>
>>> Andrei
>>
>> And so variant is the way to go ?
>>
>> Clearly, this is a very strong arguement in favor of typed Exception,
>> that provide usefull information about what went wrong.  This is a
>> safe approach.
>>
>> Because this Variant stuff is going to require massive ducktyping of
>> Exceptions, with all possible errors involved. The keys in the
>> Variant[string] will depend on the Exception the dev is facing. This
>> should be avoided and should warn us about the requirement of typed
>> Exceptions.
>
> Exactly! Just because you use a Variant doesn't magically free you from
> needing to know what exactly is that exception that you caught in the
> first place.

I think this is a confusion. You seem to advocate that exceptions are 
somehow exempt from class hierarchy design.

> To make any sense of what's stored in the Variant, you
> still have to know what is the exception type, and based on the type
> interpret the Variant. So you end up with:
>
> 	catch(Exception e) {
> 		switch(e.type) {
> 			case BlahException:
> 				handleBlahException(e.data.blah_info);
> 				break;
> 			case BlehException:
> 				handleBlehException(e.data.bleh_info);
> 				break;
> 			case BluhException:
> 				handleBluhException(e.data.bluh_info);
> 				break;
> 			...
> 		}
> 	}
>
> which is exactly the kind of ugliness that class hierarchies were
> invented to solve.

No, you don't end up with that at all. In fact it's your code that 
"knows exactly" the concrete type that suffers from duplication. I'm 
sorry, I think this is a complete confusion. Hopefully my post of a 
minute ago will help it.


Andrei