[boost] Re: Variant size bug?

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