Re: [boost] [config] Positive feature macros, r2

Subject: Re: [boost] [config] Positive feature macros, r2
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2018-01-25 17:08:03

On 01/25/18 19:51, Peter Dimov via Boost wrote:
> I'd like to revisit the topic of Boost.Config feature macros, 
> specifically that in my opinion, positive feature macros 
> (BOOST_CXX17_FEATURE) make more sense than negative macros 
> (BOOST_NO_CXX17_FEATURE) in the current environment.
> 
> Negative macros made sense in the past when they signified bugs and 
> deficiencies in the compiler as measured against a static standard. 
> Nowadays, the standard, such as it is, is in constant motion and macros 
> indicate merely a current state of development for both the compiler and 
> the standard.
> 
> The arguments for positive macros are the same as last time:
> 
> - adding a new negative macro requires changes to all legacy compilers 
> that do not and will never implement the feature; a positive macro only 
> requires changes to active compilers.
> 
> - positive macros are a better match for the standard feature macros, 
> which are positive. When we see a standard feature macro being set, we 
> typically would set ours as well. This means that the amount of changes 
> required to add a new macro is further reduced; we only need to touch 
> compiler-specific config files if the compiler either does not implement 
> feature macros, or sets them incorrectly (in our considered opinion.)
> 
> TL;DR we should switch to positive feature macros in Boost.Config, and 
> delaying this switch just accumulates more unnecessary code in 
> Boost.Config.

My opinion hasn't changed since the last discussion - if we move to 
positive macros then please convert all existing macros as well. I 
wouldn't want to remember which one is positive or negative.