Question w.r.t. `'class Foo' has virtual functions but non-virtualdestructor` warning.

[Chris Lattner]

On Fri, 4 Mar 2005, Mark Mitchell wrote:
>> I've run into this warning with C++ code as well, and it is quite annoying. 
>> There are lots of possible reasons to want to do this sort of thing, and 
>> adding a virtual dtor increases the size of the vtable for the class.
>
> Yeah, there goes one whole pointer per class in your program image. :-) 
> Seriously, you'd have to have a lot of classes and not very many objects 
> before that got to be *too* big a deal.  But, we do have customers who count 
> every byte, so I'm not totally discounting this objection.

For me, it's much more of an irritation factor than anything else.  I 
agree that saving a couple pointers per class isn't a big deal.

>> It seems that the warning could be improved to be emitted when the *delete* 
>> is seen of a class without a virtual dtor (but that does have virtual 
>> methods).  If you never actually do the questionable behavior, you'd never 
>> get the warning.  It seems like a bug to emit it for the class definition.
>
> Age-old debate: better to warn early about possibly broken interfaces, or 
> late about definitely broken usage?  I think that warning early, together 
> with what DJ is calling fine-grained warning control is the best solution.

I don't agree at all on this.  It's not a matter of warning vs not 
warning: it's a matter of emitting bogus warnings *sometimes* when you can 
emit the proper warning *all of the time*.  Given a choice of an accurate 
warning vs an warning that fires on a superset of the cases, I'd prefer 
the former.  Warning "late" may just be a synonym for warning only where 
there is a problem, as opposed to in every translation unit that includes 
the header. :)

-Chris

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