Re: Conditional requirements

From: David Abrahams (gclbb-jamboost_at_[hidden])
Date: 2003-07-18 18:07:48

Vladimir Prus <ghost_at_[hidden]> writes:

> Hello,
> I'd like to tighten up the rules for conditional requirements. 
>
> Consider:
>
> exe a : a.cpp : 
> <variant>debug:<optimization>speed 
> <optimization>off:<define>USE_SLOW_BUT_TESTED_ALGORITHM
> ;
>
> Now, someone just calls "bjam" and "a" will be built with default properties:
> "<variant>debug <optimization>off". Requirements are evaluated and the result 
> is that <optimization>speed is used and <define>USE_SLOW_BUT_TESTED_ALGORITHM
> is added.
>
> And that's confusing! I'd expect the target to be build either with 
> <optimization>speed or <optimization>off and <define> to appear only in 
> second case. But the target is build with <optimization>off and <define>.
> IOW, one conditional requirements changes property used as condition for 
> another. 

This is just one example of why Ali and I wanted to define
conditional properties to be evaluated in order. It's really easy to
imagine other cases. For example, 

on platforms X,Y, and Z use toolset A
and on toolset A define FOOBAR

> I think the simplest solution is to declare that in conditional
> property, part after ":" can be only free or incidental

Why would you want to impose that limitation?

> and that free/incidental features cannot be present before
> ":". Opinions? Anybody using non-free conditional properties?

I don't know; this sounds terribly limiting to me.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com