Re: [boost] [any] interest in boost::variant style visitors for boost::any?

From: Korcan Hussein (korcan_h_at_[hidden])
Date: 2006-12-14 13:10:05

"Steven Watanabe " wrote:
>Do you intend to make boost::any a typedef then?

That would do it unless there is some non-obvious ambiguity that this may 
cause.

>Meaning boost::apply_visitor(printer, a) I assume?

Yep but an apply_visitor function isn't reallly necessary.

>Do you mean
>
>struct deref_print : 
>boost::dynamic_visitor<std::vector<int>::const_iterator> {    void 
>operator()(std::vector<int>::const_iterator iter) const {       std::cout 
><< *iter;    } };
>
>std::vector<int> v(1); boost::any a = v.begin(); 
>boost::apply_visitor(deref_print(), a);

Basically what i meant was you have 4 cases of:

     cosntant visitor visits constant and  non-constant visitee
     non-constant visitor visits constant and  non-constant visitee

     constant visitee accepts cosntant & non-constant visitor
     non-constant visitee accepts cosntant & non-constant visitor

>For this last I don't think there is a good non-intrusive solution.

Are you talking about the complete solution or catch-all part? i don't see 
exactly how this is an intrusive solution so please elaborate/clarify.

>If the default is to throw an exception then it effectively limits the 
>contained type to those enumerated. In this case it is probably better to 
>use variant in the first place.

I don't see what the problem is since a user can override the default or 
later extend there original visitor to support new (set) type(s), as well as 
override the definition of the old supported types if he/she wishes. This 
solution seems to be much better than type casting (for every type wanted) & 
type switching code that you have to do with boost::any::.

If no one likes the idea of a default catch-all/generic visit and have a 
better alternative then i'm open to it since this isn't set in stone and 
isn't diffcult to change since the implementation is so trivial :)

Actually when i think about it the "catch-all" isn't even really 
need/necessary, the original reason why i put it in in the first place was 
as a constrast to generic visits of with boost::variant visitors.

Anyways it looks as if no one is interested, even just to make a comment of 
any form :(

Thanks
Korcan Hussien.

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