Re: [boost] [variant] wish list

From: Yuval Ronen (ronen_yuval_at_[hidden])
Date: 2006-06-07 07:15:12

Eric Friedman wrote:
> As for operator!= (and operator<=, etc.), while I do think it may be 
> useful, it raises the question of what requirements variant imposes on 
> its bounded types. Specifically, adding support for these operators is 
> not so straightforward as saying operator!= is simply !(*this == rhs). 
> The idea of operator== and operator< is that if the current bounded type 
> is the same in both variant instances, then operator overload _for that 
> type_ will be invoked. To support this same behavior for the other 
> operators would require adding to the bounded type requirements. See 
> <http://boost.org/doc/html/variant/reference.html#variant.concepts> for 
> a refresher on variant's approach to concepts. Maybe this is an 
> overly-pedantic response on my part, though.

I think it *is* as straightforward as saying operator!= is simply 
!(*this == rhs). There is no other way that makes sense.

Operators >, <=, >=, on the other hand, are completely different. I 
think there is no natural ordering of variant objects, so not supporting 
them is logical. Operator < is an exception because it's used for 
ordered conrainers. IOW, I completely agree with your decision to supply 
operator < and not supplying operators >, <=, >=.

> The reason (or at least one of them) that apply_visitor accepts visitors 
> by reference and not by value is that I had intended to introduce a 
> dynamic_visitor counterpart to static_visitor in a more general 
> Boost.Visitor library. Passing by value would interact poorly with 
> polymorphic dynamic visitors due to slicing. If you are interested, I 
> believe Boost.Visitor is probably still sitting around in the Boost 
> Sandbox CVS on Sourceforge.

Then maybe different names should be used for dynamic_visitors, for 
example apply_dyn_visitor(). We need to consider whether the need for 
dynamic_visitors is large enough in order to justify the negative impact 
on the static_visitor's interface. Have you found there is great need 
for dynamic visitors? I have to admit that I've never needed them... Not 
to mention that the whole standard library is based on static functors 
passed by value...

> Variant already supports reference types, at least on modern compilers. 
> However, references are not assignable, and so as noted in the concepts 
> documentation for variant, this implies that the resultant variant is 
> not assignable. To address your comment about boost::optional, then, 
> variant does not allow rebinding of references.

No problems here.
I just want to express my opinion that the decision regarding reference 
support should apply both to Optional and to Variant.

> I'll have to more carefully look at your proposals for additional get 
> functions to see whether I am convinced of their utility.

I'd appreciate that very much.

Thanks,
Yuval