The Right Approach to Exceptions

[Steven Schveighoffer]

On Sat, 18 Feb 2012 13:52:05 -0500, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> There's a discussion that started in a pull request:
>
> https://github.com/alexrp/phobos/commit/4b87dcf39efeb4ddafe8fe99a0ef9a529c0dcaca
>
> Let's come up with a good doctrine for exception defining and handling  
> in Phobos. From experience I humbly submit that catching by type is most  
> of the time useless.

OK, so after reading about 100 or so of these messages, I stopped.  Sorry  
if this has been said before, but here is my take:

many many times when dealing with exceptions I hate to do this:

class ExceptionTypeA : Exception {...}
class ExceptionTypeB : Exception {...}


try
{
}
catch(ExceptionTypeA ex)
{
    // code
}
catch(ExceptionTypeB ex)
{
    // same f'ing code
}

I know, I could do this:

catch(Exception e)
{
    if(cast(ExceptionTypeA)e || cast(ExceptionTypeB)e)
    {
       // code
    }
    else
      throw e;
}

But that sucks, and as others have pointed out, it kills the stack trace.

So I love the idea that others have specified to have a 'template  
constraint' type piece for catch.  IMO, it should not be a template, but a  
runtime check.  i.e., I don't think we need to templatize the catch, we  
just need to add extra code to the 'should we catch' check that currently  
consists of 'does the type match'.  This would be a huge improvement over  
existing exception catching techniques.

So the above would become:

catch(Exception e) if(e is ExceptionTypeA or ExceptionTypeB) // not sure  
of the exact syntax, maybe the if statement I used above?

I agree the constraints should be pure and nothrow.


On to my second point.  One of the issues I have with Java is that  
exceptions are *overused*.  For example, EOF should not be an exception,  
most files have ends, it's not a very exceptional situation.  If there is  
an intuitive way to use an existing return value to convey an error rather  
than an exception, I'd prefer the return value.  There is a runtime cost  
just for setting up a try/catch block, even if no exceptions are thrown.

-Steve