From: Eric Friedman (ebf_at_[hidden])
Date: 2003-11-04 22:07:23
Persson Jonas wrote:
>>-----Original Message-----
>>From: Eric Friedman [mailto:ebf_at_[hidden]]
>>Sent: den 4 november 2003 00:46
>>To: Persson Jonas
>>Subject: Re: Variant size bug?
>
[snip]
>>Well, yes: variant<> *is* variant<boost::empty>.
>>As it stands currently,
>>there is no such thing as a variant with no content; the closest
>>approximation is a variant with boost::empty content.
>
> Yes I understand that, but *why*? What are the conceptual reasons
> for not allowing truly empty variants?
There are no conceptual obstacles per se; rather, the design was crafted
so that visitation would always be a successful operation (i.e.,
visitation itself would never fail). With truly empty content, I believe
you can see this is not possible.
Anyhow, as I just posted on the list, I've changed variant so that, at
least in the 1.31 release, variant<> is not supported at all. Similarly,
variant on an empty type sequence will lead to a compile-time error.
While this change perhaps does not help you with your metaprogramming
'termination' condition, it does open up variant for future changes
without negative effects on backwards compatibility.
Eric