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

Subject: Re: [boost] optional<optional<T>>
From: Joel de Guzman (joel_at_[hidden])
Date: 2009-09-03 00:01:29

Andrey Semashev wrote:
> Joel de Guzman wrote:
>> Andrei Alexandrescu 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.
>>
>> We use boost.optional extensively in Boost.Spirit. IIRC, we collapse
>> such things. Usually they occur in generic code. I see no real need
>> for optional<optional<T>>. If there's a use for it, I too would like
>> to hear it.
> 
> I use double optionals. Indeed, this appears in a generic code, but it's 
> intended.
> 
> The point is that I use the outer optional to in-place construct 
> objects, and it sometimes happens that the objects themselves are 
> optionals. Making them not optional would not be correct from the 
> application logic point of view.

Use case, please?

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