Re: Mother of all Variants

From: Gavin Lambert (boost_at_[hidden])
Date: 2019-06-28 04:40:42

On 28/06/2019 16:12, David Sankel wrote:
> Many would prefer their PIMPL classes to not have an artificial
> empty/partially formed state because of the increased semantic complexity
> that implies.

Maybe, but that seems a bit daft to me.

The whole point of move construction and assignment is to steal things 
from an object that's about to be thrown away -- spending time and 
resources to allocate more memory for it just to preserve a "never 
empty" state is counterproductive in that scenario.

Granted that in the case where someone misbehaves and does try to use an 
invalid operation on an empty object (ie. anything other than 
assignment, destruction, or empty()/explicit bool/whatever), people like 
neither the wide interface of empty checks in each public method nor the 
implementation-defined-behaviour of accessing a null pointer -- but at 
least on most platforms the latter is reasonably guaranteed to crash 
immediately rather than having any other adverse effect, so it seems 
like the lesser of all evils to me.

Just accept that it can be empty, and that your program will crash if 
you do something inappropriate with an empty object.  No reason to 
introduce a "never empty" invariant where it will harm performance.