Re: [boost] Another variant type (was: [peer review queue tardiness] [was Cleaning out the Boost review queue] Review Queue mem

Subject: Re: [boost] Another variant type (was: [peer review queue tardiness] [was Cleaning out the Boost review queue] Review Queue mem
From: Agustín K-ballo Bergé (kaballo86_at_[hidden])
Date: 2015-04-05 16:30:59

On 4/4/2015 9:31 PM, Larry Evans wrote:
> On 04/04/2015 05:49 PM, Steven Watanabe wrote:
>> AMDG
>>
>> On 04/04/2015 04:33 PM, Larry Evans wrote:
>>> On 04/04/2015 05:08 PM, Matt Calabrese wrote:
>>>> On Sat, Apr 4, 2015 at 3:06 PM, Matt Calabrese <rivorus_at_[hidden]> wrote:
>>>>
>>>>> You need to use a recursive union if you are to get some constexpr support.
>>>>>
>>>>
>>>> Well, let me rephrase, you can expand it out by preprocessor to a certain
>>>> limit and get that too, but you can't just use aligned storage. At some
>>>> point you need to hit a recursive case.
>>>>
>>> Could you please elaborate on why recursion cannot be avoided?
>>>
>>
>> Variadic parameter packs can't be expanded on
>> class (or in this case union) members.
>>
>> template<class... T>
>> union variant_storage {
>>      T t;... // illegal
>> };
>
> Why not:
>
>    template<class... T>
>    struct variant_storage
>    : std::aligned_union
>      < 0 //Let aligned_union decide the size needed.
>      , char //handle case where size_of...(T) == 0.
>      , T...
>      >
>    {
>    };
>
> This provides the storage needed. No constexpr functions
> yet.  The egg code mentioned by gonzalobg88_at_[hidden]:
>
> https://github.com/eggs-cpp/variant/blob/master/include/eggs/variant/detail/storage.hpp#L88
>
> had several constexpr( or at least EGGS_CXX11_CONSTEXPR) functions.
> Are one of those requiring a recursive union for implementation?

Yes, pretty much all of them require a recursive union for constexpr 
support. The details can be found here 
http://talesofcpp.fusionfenix.com/post-20/eggs.variant---part-ii-the-constexpr-experience 
and here https://akrzemi1.wordpress.com/2012/12/13/constexpr-unions/

Regards,
-- 
Agustín K-ballo Bergé.-
http://talesofcpp.fusionfenix.com