Re: [boost] compact_optional -- prompting interest

Subject: Re: [boost] compact_optional -- prompting interest
From: Matt Calabrese (rivorus_at_[hidden])
Date: 2015-09-27 07:00:41

On Sat, Sep 26, 2015 at 1:12 PM, Nevin Liber <nevin_at_[hidden]> wrote:
>
> I'm strongly against the word "optional" appearing the name, for the
> following reasons:
>
>
>    - optional<int> allows me to use every single value that can be stored
>    in an int.  This doesn't.
>    - optional<string> allows one to shorten the lifetime of the string it
>    holds.  This doesn't.
>    - optional<T> has a nothrow default constructor.  This doesn't.
>
> At best, it resembles optional only superficially.  Please give it a
> different name.

This. However, I do think that there is a place for a separate
intrusive_optional as well, though the empty value should not be a valid
value of the type, but rather, just be able to take advantage of its
storage, and the type should not be constructed while the in the empty
state. I.E. an optional reference only takes up the size of a pointer
because it uses the "0" value for empty. Similarly, any type that contains
a reference (or a pointer that cannot be null) also can take advantage of
this by way of an intrusive optional. The user would have to provide a
predicate that operates on the raw storage of the value, and access it in a
way that is well-formed whether the object is constructed there or not, so
it is a little bit trickier to specify properly.

-- 
-Matt Calabrese