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

Subject: Re: [boost] optional<optional<T>>
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2009-09-02 23:29:03

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.