Re: [Boost-build] How can I pass vars to actions

Subject: Re: [Boost-build] How can I pass vars to actions
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2014-03-20 14:57:16

AMDG

On 03/20/2014 11:10 AM, Jose . wrote:
> I read the BBv2 documentation and samples. I don't want to create my own
> generator, just to see "Hola" "Adios" as the output of this program:
> 
> rule myecho
> {
>     _MSG on $(<) = $(MSG) ;
> }
> actions myecho bind _MSG
> {
>     echo "MSG = $(_MSG)" ;
> }
> 

Note: bind _MSG means that $(_MSG) is a target
and you want the name of the file, rather than
the internal name.  It doesn't really make sense
in this example.

> constant MSG : "hola" ;
> make Hola : : @myecho ;
> 
> constant MSG : "adios" ;
> make Adios : : @myecho ;
> 
> Why I only see "adios" "adios" ? I guess it's because the variable inside
> the action is not evaluated until the action executes. But the rule myecho
> should be executed in the first pass, hence _MSG should contain "Hola" for
> target Hola, and "Adios" for target Adios.
> 
> Am I missing anything?

Yes.  Boost.Build doesn't make two passes.  It makes three.
1) Parse Jamfiles
2) Generate actual targets
3) Run updating actions

(This should make sense if you keep in mind that a
target in the Jamfile can end up creating several
actual targets for debug and release builds, static
vs. shared libraries, different compilers, etc.)

the rules are run in step 2 after MSG has already been
set to "adios".

> How can I pass a different message to a Make rule
> depending on the target? Can I avoid creating a generator?
> 

This is what features (e.g. cxxflags) are for.

import feature : feature ;
feature message : : free ;

rule myecho ( targets * : sources * : properties * )
{
  _MSG on $(targets) =
    [ feature.get-values <message> : $(properties) ] ;
}

make adios : : @myecho : <message>adios ;

In Christ,
Steven Watanabe