Re: [boost] [variant] awkward recursion

Subject: Re: [boost] [variant] awkward recursion
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-10-30 10:16:12

on Mon Oct 29 2012, Mathias Gaunard <mathias.gaunard-AT-ens-lyon.org> wrote:

> On 29/10/12 22:33, Dave Abrahams wrote:
>>
>> on Mon Oct 29 2012, Mathias Gaunard <mathias.gaunard-AT-ens-lyon.org> wrote:
>>
>>> On 29/10/12 20:29, Dave Abrahams wrote:
>>>>
>
>>>> Using recursive_variant_ works out quite nicely, except that
>>>> it doesn't allow me to separately declare "object" and "array" and
>>>> re-use them in the definition of "value":
>>>>
>>>>     typedef boost::make_recursive_variant<
>>>>         null
>>>>       , std::string
>>>>       , double
>>>>       , std::map<std::string, boost::recursive_variant_>
>>>>       , std::vector<boost::recursive_variant_>
>>>>       , bool
>>>>     >::type value;
>>>
>>> I'd use this method, it's clearly the cleanest.
>>> I don't understand what's your problem with it.
>>
>> As I had feared, and as
>> http://news.gmane.org/find-root.php?message_id=%3cm2objlggaw.fsf%40pluto.luannocracy.com%3e
>> demonstrates, extra dynamic allocations will be used to store maps and
>> vectors (or anything at all?) in the variant.

I should also point out that it might be putting the indirection in the
"wrong" place.  having the recursive_wrapper around the whole map or
vector is clearly more efficient than having it around (part of) their
elements.

> Why not fix the implementation then instead of writing hacks?

I realized it's not completely fixable as Steven points out.  I actually
did think of trying to do something about other issues,
https://svn.boost.org/trac/boost/ticket/7601, but...

> Of course, the code of variant is horrible enough to make it very
> difficult to do anything with it.

...exactly.

-- 
Dave Abrahams
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