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

Subject: Re: [boost] optional<optional<T>>
From: Daniel Frey (d.frey_at_[hidden])
Date: 2009-09-03 03:09:07

On 03.09.2009, at 04:43, Andrei Alexandrescu wrote:

> 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 think optional<optional<T>> should not be folded into optional<T>.

One use-case occurs with databases. Consider "SELECT myint FROM  
mytable WHERE mywhatever='foo'". When returning the result of such a  
query, the C++ function that executed it could return an empty outer  
optional if the database returned no row (WHERE-clause didn't match  
anything), an empty inner optional when myint is SQL's NULL and the  
integer value of myint in the then non-empty inner optional<int>  
otherwise.

Regards, Daniel