Re: [boost] optional<optional<T>>

Subject: Re: [boost] optional<optional<T>>
From: Joel de Guzman (joel_at_[hidden])
Date: 2009-09-03 00:02:38

Andrey Semashev wrote:
> OvermindDL1 wrote:
>> On Wed, Sep 2, 2009 at 8:43 PM, Andrei
>> Alexandrescu<andrei_at_[hidden]> wrote:
>>> Hello,
>>>
>>>
>>> I'm defining an "optional" type for D's standard library modeled 
>>> similarly
>>> to Boost.optional. An interesting question came up - should
>>> optional<optional<T>> fold itself into optional<T>, or is "double 
>>> optional"
>>> an interesting concept of its own?
>>>
>>> I thought I'd ask here because by now there's a significant body of
>>> experience with optional<T>. I perused the online documentation and the
>>> forum and couldn't find information about that specific detail.
>>
>> Personally I would find it interesting as it can hold extended data,
>> but perhaps it would be nice to fold someway so it takes less data,
>> perhaps one bit per sub-optional used up to the overall max of a char
>> or int or something (would anyone really have 8 optionals embedded?)
> 
> I think, if folding should be supported then it should be done through 
> an external metafunction, something like:
> 
>   typedef typename fold_optionals< T, 7 >::type single_optional;
> 
> I think, the default behavior of optional should stay as is.

I agree. It is easy enough to detect and collapse in an application
specific way. Your recipe is similar to what we do.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net