Re: [Boost-build] Sharing variables across multiple files

From: Jurko Gospodnetić (jurko.gospodnetic_at_[hidden])
Date: 2008-02-10 13:51:47

   Hi Fredrik.

> I've been trying to figure out how to share variables across multiple  
> Jamfiles, and so far I haven't found any info on how to do so. I however  
> found the following statement on the boost build wiki:
> 
[...many lines snipped...]
> I read that each Jamfile has it's own namespace, and that would explain  
> why above usage doesn't work.
> But is it possible with some tricks (i.e. exporting it somehow)? If so,  
> what am I missing? And if it's not possible, then what would you recommend  
> me to do?

   I believe what you're looking for are the path-constant & constant 
rules defined by Boost Build.

Example:
   path-constant SOURCE_BASE  : "./../../../../Source" ;
   constant      BOOSTVERSION : "boost_1_34_1"         ;

   It is true that all Jamfiles use a separate namespace (module in 
Boost Build lingo) and that is why variables from one Jamfile are not 
directly accessible from other Jamfiles.

   There is also another way to achieve this which would also allow you 
to make non-constant shareable variables but it takes a bit more work:
   1. Place your values in a separate module
   2. Read them from other modules using the modules.peek rule.
   3. Write to them from other modules using the modules.poke rule.

   Example:

module AAA
{
   xxx = "Here I am!" ;
}

# Reading...
yyy = [ modules.peek AAA : xxx ] ;

# Writing...
modules.poke AAA : xxx : new-value ;

   constant rules work around the problem of you not knowing what a 
Jamfile module is called and so not being able to access it directly as 
shown above, at least not without using some internal implementation 
details. What they do is ensure that a copy of their constant values is 
made in each new Jamfile module. That should also explain why those 
variables are 'constants' - there is actually no single variable but one 
copy per Jamfile.

   path-constant rule helps with automatically rooting specified 
relative paths based to your project's folder.

   Hope this helps.

   Best regards,
     Jurko Gospodnetić