Re: [boost] [outcome] outcome without empty state?

Subject: Re: [boost] [outcome] outcome without empty state?
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-05-24 23:45:52

On 25/05/2017 07:54, Niall Douglas wrote:
>> 3. A default-constructed result<T> is initialized as if
>> `result<T>{error_code_extended{}}`.
> 
> I feel lots of gut feelings against that default. It seems a bit of a hack.
> 
> Also, I remember Expected used to do this as a default, but it was
> undone. I would assume someone from WG21 gave a very good reason why.
> 
> I will say that I cannot think of a good technical reason not to default
> construct to a null error_code. But just because you can doesn't mean
> you should.

Visions of "An error occurred: The operation succeeded" dialog boxes 
scroll through my brain.

I like Peter Dimov's suggestion of defaulting to a non-null error code, 
especially where T is not void.  This requires explicitly acknowledging 
success.  (I'm not sure about the case where T is void.  I'm initially 
inclined to want the same for consistency, but I suspect that would be 
too painful to actually use in practice.)

Perhaps a suitable default would be std::errc::state_not_recoverable? 
That seems appropriate while being fairly rare as a regular error code. 
(Or of course since we're talking about std::error_code, a custom error 
category that isn't convertible to errc could be used for this instead.)