Why does std.variant not have a tag?

[Robert Jacques]

On Monday, 5 November 2012 at 14:13:41 UTC, evansl wrote:
> On 11/05/12 00:33, Robert Jacques wrote:
>> On Sunday, 4 November 2012 at 22:33:46 UTC, Alex Rønne 
>> Petersen
>> wrote:
>>> On 05-11-2012 00:31, evansl wrote:

[snip]

> If std.Algebraic is like Boost.Variant, then duplicate
> bounded types are not allowed and leads to the problem mentioned
> in the post on the spirit mailing list which I linked to in my
> OP.  OOPS,  now I see why reading that post was not clear 
> enough.
> Maybe this earlier post in same spirit thread would be clearer.
>
> http://article.gmane.org/gmane.comp.parsers.spirit.general/17113
>
> In particular, note the phrase:
>
>   neither can you use variant<string,string,int>
>   because variant can't take duplicate types.
>
> This can lead to problems in the spirit parser because
> the attributes of parsing:
>
>     a | b
>
> where:
>   phrase a has an attribute of type A
>   phrase b has an attribute of type B
> is:
>
>     variant<A,B>
>
> as noted near the bottom of:
>
>
> http://www.boost.org/doc/libs/1_51_0/libs/spirit/doc/html/spirit/abstracts/attributes/compound_attributes.html
>
> and if A and B are the same, then there's a problem because
> Boost.variant can't handle duplicates.
>
> Hope that's clearer.
>
> -regards,
> Larry

Thank you for the clarification. Implementing an id seems 
reasonable feature request for algebraic. I've added a bugzilla 
request for it: 
http://d.puremagic.com/issues/show_bug.cgi?id=8962 Please have a 
look in case I missed anything.

> BTW, recently there was a review of
> another Boost library that has some similarity to
> Boost.any.  It's called type_erasure:
>
>
> http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure/doc/html/index.html
>
> Since std.Variant is similar to Boost.any (as noted above),
> and since Boost.any is, in some ways, like Boost.type_erasure,
> and since you're working on a revised std.Variant,
> you might be interested in looking at type_erasure.

Thanks, I'll take a look at it.