Re: [boost] Re: Yet another Variant class

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-06-30 12:36:38

On Saturday 29 June 2002 03:26 pm, Itay Maman wrote:
> >I'm happy with that. Personally, I would strongly
> >discourage the use of variant_cast anyway because it
> >is the only mechanism to access a variant that is
> >not guaranteed to succeed at run-time. Visitors are
> >typesafe as would be a switch-on-type construct.
>
> (Following this line) Then, what do you think about
> getting rid of variant_cast? This, obviously,
> contradicts the "variant with an any-like interface"
> feature, but it might have one major positive effect:
> It requires any-based code to be rewritten in a safer
> manner.

If boost::any didn't exist I might agree. But boost::any has been around for 
quite a while and there has been no alternative variant type in Boost. I'm 
afraid that there are many users using boost::any as the variant type now in 
progress. Would the omission of variant_cast make the conversion from 
boost::any to variant too involved, such that users won't bother? 

The other question is whether there are cases where variant_cast really is 
useful. For instance, the context may dictate that only one of the types in 
the variant is legal. I have come across this case a few times recently in my 
own projects. 

So I guess I'm backing away from my current statement. I think that 
variant_cast is a lot like std::auto_ptr::release: it's powerful, maybe a 
little dangerous, but necessary for some cases. We should strongly encourage 
the use of the safer access methods (visitor & typeswitch), but supply 
variant_cast for those corner cases that need it.

> >1) Great Stack_Holder::swap :)
> >
> >>I am wondering if there's a more efficient way for
> >>implementing an exception-safe swap() ?
> >
> >For the general case, I don't think there is. There
> >are tons of optimization opportunities for special
> >cases:
> >
> >  1) Same type: cast and swap
> >  2) Both incomplete types: swap pointers
> >  3) Both POD types: swap the bytes
>
> Il'l add this to my "to do" list

Maybe at the bottom :). You can be assured that variant would never be held 
back in a formal review for lack of optimizations, and these optimizations 
could be added later without any effect on the interfaces.

        Doug