[boost] Re: Variant size bug?

From: Eric Friedman (ebf_at_[hidden])
Date: 2003-11-03 18:45:34

Persson Jonas wrote:

>>-----Original Message-----
>>From: Eric Friedman [mailto:ebf_at_[hidden]]
>>Sent: den 31 oktober 2003 22:31
>>To: boost_at_[hidden]
>>Subject: [boost] Re: Variant size bug?
>>
>>Well, technically it's not a bug: variant<> *is* 
>>variant<boost::empty>.
>>
>>I agree this may be unexpected, and so should probably be noted more 
>>explicitly in the docs.
>>
>>Also, we _can_ argue whether this unexpectedness constitutes a bug in 
>>variant's design. (It's certainly not an *implementation* bug though.)
> 
> 
> Well, I would argue that it is a (design) bug.  It breaks the
> typelist <-> variant symmetry, and causes some of the mpl stuff to 
> give the wrong result.

Before I comment on this, do you mean the (implementation) bug you 
identified elsewhere in this thread?

If so, as I replied to your message, that bug has been fixed.

If not, then I think you'll have to be clearer.

> Is there any reason why the user would want the interface to work 
> this way?

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.

Thus, visitors to variant<> must handle boost::empty. That 
variant<>::types contains boost::empty seems only logical.

Is there any reason you want the interface to work *differently*?

> I use mpl to unroll the types in the variant, so I must have a 
> termination condition. As it stands now I cannot use mpl::size or 
> mpl::empty for that. How do I check at compile time if the variant 
> contains any types or not?

Again, you'll have to be clearer.

Thanks,
Eric