Re: [boost] [optional] Specializing optional to save space

Subject: Re: [boost] [optional] Specializing optional to save space
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2015-09-27 17:19:53

On 27.09.2015 23:38, Nevin Liber wrote:
> On 27 September 2015 at 15:03, David Stone <david_at_[hidden]> wrote:
>
>> Andrzej posted another thread about creating a new compact_optional class.
>> The goal of this class is to make it easy to create a new optional type
>> with a special sentinel value. This allows easy customization per instance
>> of an optional value.
>>
>> This post is about the opposite approach: making it easier to specialize
>> optional to provide special behavior for all instances of a given optional
>> type.
>>
>> When I bring this up, people often compare it to the vector<bool>
>> specialization, but this is a completely different issue.
>
>
> No, it's the same issue.  It'll have subtly different behavior.  Generic
> programming counts on the behavior being the same.
>
> The only reason vector<bool> is bad is because of its spelling.  Had it
> been a different name, most people would be happy.
>
> optional<T> controls the lifetime of T.  If you have a sentinel instead,
> then the lifetime of the engaged T is exactly the same as the lifetime of
> optional<T>.

I got the impression that the OP intended that the specialized 
optional<> still controlled the lifetime of the adopted object the way 
it does now. The proposed change basically offloads engagement checking 
to a user-specializable trait but really nothing more than that.