Re: [Boost-build] feature, properties, variants, and all the rest

Subject: Re: [Boost-build] feature, properties, variants, and all the rest
From: Chambers, Matthew (matt.chambers42_at_[hidden])
Date: 2017-08-07 15:05:53

On 8/7/2017 9:42 AM, Stefan Seefeld via Boost-build wrote:
> On 08/07/2017 10:27 AM, Chambers, Matthew via Boost-build wrote:
>> On 8/5/2017 9:56 PM, Stefan Seefeld via Boost-build wrote:
>>>>    The need for propagated features stems from the fact that
>>>> Boost.Build allows multiple build configurations to be
>>>> handled simultaneously.  They're basically used for anything
>>>> that would be global in other build systems, such as the toolset,
>>>> debug vs. release, static vs. shared, etc.
>>> Ah, right. Well, right now I don't support multiple build
>>> configurations. (You can clone artefacts and apply different sets of
>>> features to them all you want, such as a static and a shared version of
>>> a library. But still, both are represented as two distinct objects, and
>>> I haven't encountered the need to propagate features across the entire
>>> graph.)
>> lib a : a.cpp ;
>>
>> exe b : b.cpp : <library>a.cpp/<link>shared ;
>> exe c : c.cpp : <library>a.cpp/<link>static ;
>>
>> This very basic example illustrates the need for propagated features.
>> I don't want to define (clone?) a twice.
> faber supports that, too, but doesn't require the "propagate" feature
> attribute for that:
> https://github.com/stefanseefeld/faber/blob/develop/examples/implicit_rules/fabscript#L21-L23
> (And I'm not sure that this is in fact an example of "propagate" at all,
> not even in b2.)

Doh, you're right. I'd need an extra level of indirection for propagate to matter.

lib z : z.cpp ;
lib a : a.cpp z ;

exe b : b.cpp : <library>a.cpp/<link>shared ;
exe c : c.cpp : <library>a.cpp/<link>static ;

These are the propagated features from builtin.jam:

feature.feature toolset            :                 : implicit propagated symmetric ;
feature.feature stdlib             : native          : propagated composite ;
feature.feature link               : shared static   : propagated ;
feature.feature runtime-link       : shared static   : propagated ;
feature.feature runtime-debugging  : on off          : propagated ;
feature.feature optimization       : off speed space : propagated ;
feature.feature profiling          : off on          : propagated ;
feature.feature inlining           : off on full     : propagated ;
feature.feature threading          : single multi    : propagated ;
feature.feature rtti               : on off          : propagated ;
feature.feature exception-handling : on off          : propagated ;

Link/ABI compatibility is another big use case for propagated when using things like runtime-debugging and optimization (at least for MSVC).

-Matt