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From: Vladimir Prus (ghost_at_[hidden])
Date: 2005-11-23 11:20:54

On Wednesday 23 November 2005 19:02, Reece Dunn wrote:

> > > 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?
> This would be a real-world case. Especially for complicated projects that
> have many libraries all dependant on each other, all (or most) using PCH
> support.

Why can't single PCH header be created, then?

> >I think it's possible to make pch-related feature "propagated to direct
> >dependents only".
> That would solve this problem. Do you know how to do this?

Sure :-) Go to build/targets.jam, class basic-target, method
compute-usage-requirements. Find line:

 result = [ $(result).add
          [ $(subvariant).sources-usage-requirements ] ] ;

Remove properties you don't like for sources usage requirements before adding

Sorry for not being very helpful with coding this, but I promise to commit the
patch even if <pch*> features are hardcoded here ;-) I'll add new feature
attribute if and when we'll have another case where propagation to direct
dependents only is desired.

To clarify -- yes, I think it's fine to hardcode features here, provided you
add a comment explaining why they are special. I don't want to introduce
generic mechanism for the sake of one use-case.


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