Re: [boost] variant - off topic

Subject: Re: [boost] variant - off topic
From: Robert Ramey (ramey_at_[hidden])
Date: 2019-04-08 14:58:46

On 4/8/19 12:29 AM, Gavin Lambert via Boost wrote:
> On 8/04/2019 18:29, Robert Ramey wrote:
>> LOL - Looks good to me!  So now we have std::variant that is 
>> guaranteed to have a valid state with no double buffer BS and no extra 
>> overhead.
> 
> It's restricted to only trivial types, however, which sharply limits its 
> usability.  (Mostly just to primitives and PODs, though that's an 
> oversimplification.)
> 
> Or did you mean to check is_nothrow_constructible rather than 
> is_trivially_constructible?

I looked at both nothrow and trivial and couldn't figure out the 
difference between them with spending more time than I wanted to at the 
time.  After, I was just throwing out an idea for discussion.

Note that this approach is not really a rival to making a new variant. 
It's to explicit limited the usage a variant to those situations where 
there are none of the problems and tradeoffs that implementations of 
variants have to address.

Basicall, I'm giving up on trying to understand all this stuff.  I just 
need something that "just works" in an efficient simple way or "just 
fails(tm)" if that's not possible.  It it just fails, I'll fall back to 
the "fancy variant" (and spend a lot of time investigating the tradeoffs 
and error/exception handling requirements) or alter my types so the 
safe_variant just works.

> (Also, for robustness you should check assignment as well as construction.)

Hmmm - interesting point.  My goal was to make it impossible for 
safe_variant to end up in the the dreaded stateless state.  I was 
gathering from the discussions that assignment to variant entailed 
construction of a new instance of one of the constituent types was key 
place where the stateless problem can occur.  So you're absolutely right 
in that the checks have to be implemented for all operations which might 
be used.

Robert Ramey

> 
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