Re: [Boost-build] <asynch-exceptions>on cannot be specified in usage-requirements

From: Vladimir Prus (ghost_at_[hidden])
Date: 2006-12-26 17:51:10

On Tuesday 19 December 2006 00:28, David Abrahams wrote:

> > Because if you allow non-free usage requirements, then in:
> > 	
> > 	exe a : a.cpp lib1 lib2 ;
> >
> > usage requirements brought by lib1 might well affect some
> > conditional requirements in lib2, and in order to figure out the
> > final properties for all targets you potentially need to take
> > multiple passed though all targets, re-computing usage requirements
> > and propagating all the way up and down. I never had time or
> > motivation to spell down such an algorithm.
> 
> As a first cut, you could just turn it into an error if you find
> conditional requirements that depend on usage requirements.  That
> would allow the user to explicitly request the build properties that
> come from the usage requirement from the command-line, thereby driving
> the conditions from the top down.

It would still be somewhat awkward algorithm. For

        exe a : a.cpp lib1 lib2 ;

you need to build both lib1 and lib2, get their usage requirements, and if
any includes non-feature features, build one of the targets, or sometimes both,
again. So, it's twice the work and I'm not sure how it will scale for deep
target dependencies. 

I'd rather if Boost.Test said "I want the dependents to request <async-exceptions>on"
and fail if it's not the case. Then, the user can request that property from the command
line, or project requirements.

- Volodya