The Right Approach to Exceptions

[Andrei Alexandrescu]

On 2/18/12 6:03 PM, Jonathan M Davis wrote:
> On Saturday, February 18, 2012 17:30:10 Andrei Alexandrescu wrote:
>> The alternative is with virtuals. Do you see a lot of virtuals in base
>> exceptions? Do you see dramatically different interface for different
>> exception types?
>
> You mean virtual functions? The problem is that each exception type could have
> information specific to _it_ which makes no sense in a base class. For
> instance, Exception does to have errno in it. FileException does.
>
> If we have GetOptException, it should have a variable for which flag failed.
> Exception doesn't have that. And what if the flag failed because it was given a
> bad argument? Then the exception needs a field for that argument. Then you can
> get something like
>
> try
>      getopt(args, ...)
> catch(MissingArgumentException mae)
> {
>      stderr.writefln("%s is missing an argument", mae.flag);
>      return -1;
> }
> catch(InvalidArgumentException iae)
> {
>      stderr.writelfln("%s is not a valid argument for %s. You must give it a
> %s.", mae.arg, mae.flag, mae.expectedType);
>      return -1;
> }
> catch(UnknownFlagException ufe)
> {
>      stderr.writefln("%s is not a known flag.", ufe.ufe);
>      return -1;
> }
> catch(GetOptException goe)
> {
>      stderr.writefln("There was an error with %s",  goe.flag);
>      return -1;
> }
> //A delegate that you passed to getopt threw an exception.
> catch(YourException ye)
> {
>      //...
> }
> catch(Exception e)
> {
>      stderr.writeln("An unexpected error occured.");
>      return -1;
> }
>
> You can't do _anything_ like that right now.

Of course I can. They call it toString(). The code above pretty much 
proves my point with so much wonderful irony.

Andrei