Re: [boost] [variant] Warning: variadic templates in boost::variant

Subject: Re: [boost] [variant] Warning: variadic templates in boost::variant
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2013-12-09 09:52:12

AMDG

On 12/09/2013 04:16 AM, Andrey Semashev wrote:
> On Monday 09 December 2013 14:02:12 Peter Dimov wrote:
>> Antony Polukhin вроте:
>>> However this:
>>> * breaks ABI (now variant is declared as `template <typename T0,
>>> typename... TN> class variant`)
>>
>> Why not just template<class... TN> class variant? It's true that the
>> degenerate case of variant<> isn't particularly useful, but then again,
>> neither is the singular case of variant<X>. Is there a good reason to
>> prohibit variant<>?
> 
> What should be the storage for such a variant then? Also, most of the 
> interface of variant<> becomes invalid (e.g. type(), which(), assignment from 
> elements, apply_visitor()). With such drastic interface differences, there 
> should be a compelling reason to have such a specialization, and I don't see 
> one.
> 

Boost.Variant explicitly guarantees that it always
holds one of the variant types.  variant<> doesn't
really make sense, because it's impossible to satisfy
this invariant.  variant<X> isn't very useful, but it
does have a sensible meaning.

In Christ,
Steven Watanabe