Re: [Boost-build] [BBv2] PCH problems (was: removing properties from a property set)

From: Vladimir Prus (ghost_at_[hidden])
Date: 2005-11-23 10:36:00

On Wednesday 23 November 2005 11:57, Reece Dunn wrote:
> Reece Dunn wrote:
> > Q: Given a property set $(ps), how do I remove a property if I don't
> > know it's value.
> >
> > e.g.
> >
> >     local newps = property-set.remove "<pch-header>" : $(ps) ;
>
> I have worked out howe to do this:

The standard way is this:

        local newps = [ property-set.create
                                [ property.change [ $(ps).raw ] : <pch-header> ] ] ;

The "change" rule is used to change the value of feature to some other value,
but with empty value it just removes everything.

> and am using it here...
>
>          # Run the PCH generator, passing in the source file for the PCH.
>
>          local r =
>            [ generator.run $(project) $(name) :
>              [
>                property-set.create
>                  <pch-source>$(sources[2]) # mypch.cpp
>                  $(newps)
>              ] : $(sources)
>            ] ;
>
> But although the code works, it is in the wrong place! This is because
> with:
>
>     lib a : pch_a ... ;
>     lib b : pch_b a ... ;
>
> we have:
>
>     PCH(A) --> LIB(A) -\
>            PCH(B) --> LIB(B)
>
> where LIB(B) inherits both PCH(A) and PCH(B) properties. :(

Yes, usage requirements are propagated infinitely. Is the above real-world 
case? Can you explain this to me? Should pch-related usage requirements 
*always* apply only to direct depenents? Are there any other corner cases?

I think it's possible to make pch-related feature "propagated to direct 
dependents only".

> NOTE: I have attached the --debug-generators for the above scenario if
> someone wants to help find a solution.
>
> The alternative that Alexey suggested of having:
>
>     lib a : ... : <pch>pch_a ;
>     lib b : a ... : <pch>pch_b ;
>
> does not (I think) allow you to add a generator for the PCH file to
> generate the *.pch file.

What do you mean by "add a generator for the PCH file"? I think the biggest 
problem with this approach (assuming PCH rule is modified to add no usage 
requirements) is extracting header name from the target. I honestly don't 
know about right solution.

- Volodya