Re: [boost] Outcome v2 is feature complete

Subject: Re: [boost] Outcome v2 is feature complete
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-07-11 14:59:17

On 11/07/2017 15:41, Peter Dimov via Boost wrote:
> Andrzej Krzemienski wrote:
> 
>> maybe result<int, int> indeed looks suspicious, but using
>> result<error_code, error_code> might come as quite natural:

An excellent example of what we want to strongly discourage.

I really strongly believe that for result and outcome, you must avoid
choosing types for success and failure which are not very obviously
distinct.

> Re the multi-arg constructor, one obvious use case is this:
> 
>    return { errno, std::generic_category() };
> 
> or
> 
>    return { GetLastError(), std::system_category() };
> 
> although the main purpose is to forward arguments to custom ECs.
> 
> I'm working on a prototype that showcases these but it's not ready yet.

That you showed above was my biggest motivator in favour of multi-arg
converting constructors.

But countering it, the syntax is unfortunately similar to aggregate
initialisation which is probably why std::variant doesn't allow it. I'd
be interested if someone from WG21 could tell us actually.

And besides, just prefixing the braces with 'std::error_code' means your
code compiles as intended on Outcome v2. I'm all for eliminating
boilerplate, but it does depend at what cost.

Niall

-- 
ned Productions Limited Consulting
http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/