Re: [Boost-build] Conditional targets not resolving properly

Subject: Re: [Boost-build] Conditional targets not resolving properly
From: Johan Larsson (hnefi.mail_at_[hidden])
Date: 2009-09-16 04:23:18

On Mon, Sep 14, 2009 at 11:01 AM, Vladimir Prus <ghost_at_[hidden]> wrote:

> On Thursday 10 September 2009 Johan Larsson wrote:
>
> > Hello.
> >
> > As part of a project, I have a library with an associated Jamfile I need
> to
> > build. I want to build this particular library as a static lib, so I've
> > added <link>static to the project requirements in the Jamfile for the
> > library. So far so good.
> >
> > However, I also want to exclude a particular source file from the library
> if
> > I use a particular toolset (gcc in this case). So I've created two
> targets;
> > one with all the sources and no particular requirements, and one without
> the
> > source file in question with <toolset>gcc added as a requirement, like
> so:
> >
> >
> >
> > project A
> >   : requirements
> >     <link>static
> >     <toolset>gcc:<cxxflags>-DNO_SMART
> >   : usage-requirements
> >     <linkflags>-lA
> >     <include>.
> >   ;
> >
> > alias commonsources
> >   :  (snip)
> >   ;
> >
> > lib libA
> >   : commonsources
> >     SOURCE.cpp
> >   ;
> >
> > lib libA
> >   : commonsources
> >   : <toolset>gcc
> >   ;
> >
> > The problem is that when building with a toolset that is not gcc, bjam
> fails
> > to find an appropriate target. The output it gives me is as follows:
> > error: No best alternative for ./libA
> >     next alternative: required properties: <link>static
> >         not matched
> >     next alternative: required properties: <link>static <toolset>gcc
> >         not matched
>
> Johan,
>
> you are running into a known issue:
>
>        https://zigzag.lvk.cs.msu.su:7813/boost.build/ticket/16
>
> Can you let me know if the workarounds suggested by Matthew and Phillip
> work for you?
>
> Thanks,
> Volodya
>

They both work - and thank you to both Matthew and Phillip. However, both
workarounds have issues:

Matthew's solution produces the desired behaviour to the letter, but I've
found no way to specify which sources to add or exclude in the conditional
rule except by hardcoding them into the rule itself. This means that if a
user wants to produce this kind of behaviour, they have to write a specific
rule for each Jamfile where this behaviour is needed. Not very practical.

Phillip's solution is more elegant, but does not produce strictly the
behaviour I need. Specifically, I need to exclude a source file if a
specific toolset is used. This means I need to be able to either specify a
negative condition (such as <toolset> ! gcc or similar) or specify a remove
source rule (<removesource>SOURCE.cpp or similar), neither of which I've
been able to do. So instead, I'll have to specify each positive case
explicitly (<toolset>gcc:... <toolset>co:... <toolset>intel:...) which is
obviously not very maintainable.

So in the end, they both can be used, but not without issues. The solution
proposed in the ticket you linked would be a very, very welcome addition,
should it ever get fixed.