Re: [Boost-build] Boost-build Digest, Vol 69, Issue 8

Subject: Re: [Boost-build] Boost-build Digest, Vol 69, Issue 8
From: Gevorg Voskanyan (v_gevorg_at_[hidden])
Date: 2011-05-13 10:45:52

Michael Krajnak wrote:

> Great!  That works, what I have is:
> 
>     import toolset  ;
>     import os ;
>     local stlDir = [ os.environ  "STLPORT" ] ;
>     ...
>     using stlport : 5.2.1  :
>     $(stlDir)/stlport :
>     $(stlDir)/lib/evc9-x86  ;
> 
> I do have a question about the "?=" syntax in your  example.
> 
>     programFiles ?= [ os.envron "ProgramFiles" ]  ;
> 
> I assume this is "set value if not already set", something like "if  
>!programFiles then programFiles = ..."

Yes.

> I wanted to a hard code a default  value so I put something like:
> 
>     stlDir ?= [  "default/path/to/stlport"] ;
> 
> But I get the following  message:
> 
>     user-config.jam:43: in modules.load rule  D:/dev/sapphire/stlport unknown 
>in module user-config.

Yes, as [ ... ] is the rule (function) invocation syntax. E.g. [ os.environ 
"ProgramFiles" ] means call the os.environ rule with "ProgramFiles" as argument; 
in C++ the equivalent would be os.environ("ProgramFiles") .

> I can live without  this, but at this point I would like to understand what's 
>happening  here.

I hope the above explains it. So to fix the error you need to write:  stlDir ?= 
"default/path/to/stlport" ;

> Thanks!
> 
> Mike

HTH,
Gevorg