Re: [Boost-build] Problem with ambiguity error

From: Daniel Einspanjer (deinspanjer_at_[hidden])
Date: 2006-03-13 10:21:17

"Vladimir Prus" <ghost_at_[hidden]> wrote in message 
news:200603131220.05610.ghost_at_cs.msu.su...
> Boost.Build will find all generator invocation sequences from INI to CPP,
> subject to restriction that no generator can be invoked twice. This gives:
>   (1)  INI -> CPP
>   (2)  INI -> CPP -> CPP
> and it's an ambiguity. The possible solution is to make CPP -> CPP 
> generator
> run only when explicitly needed. For example:
>
> 2. You can make customerinfo-generator demand a very specific source type, 
> and
> use cast. Basically, that's what you tried to did, but you should not 
> derive
> CUSTOMERINFO from CPP. If you do, then sequence (2) above will still be
> found, because CUSTOMERINFO types inherits all generators for from type 
> CPP.

This was exactly the problem.  Making CUSTOMERINFO not inherit from CPP 
makes the generators work regardless of import order.  I'm sorry you had to 
struggle with me so long to get a clear explanation from me.

> 1. If the generator should be invoked only when some property, say 
> <customer>
> is in build properties, use this:

In our case, <customer> is always in the build properties.  It is a global 
feature that defaults to our "internal" customer name if none were 
specified.

>  generators.register [ new customerinfo-generator customerinfo.transform :
>       CUSTOMERINFO : CUSTOMERINFO : <allow>customerinfo.transform ] ;
>
> and add <allow>customerinfo.transform to requirements if you want this
> generator to run. The <allow> property is the standard one. You can use a
> helper rule to automatically add this property when declaring targets:
>
>   rule customerinfo-transformed ( name : sources * : requirements * .. )
>   {
>        customerinfo $(name)
>           : $(sources)
>           : $(requirements) <allow>customerinfo.transform
>           .........
>   }

This example of using <allow> is in my notebook as being potentially useful. 
:)  I'm not sure how the helper rule helps though.  Where would you invoke 
the rule? In the requirements section of a target that consumes the 
customerinfo.cpp file?

> I don't know which method is best for you, but all should work.

To be completely frank, I acknowlege these generators are a trifle 
over-engineered.  I could have easily done everything in them with a simple 
make target.
I created them to both better my understanding of the BB system and to 
demonstrate the power and flexibility of the system to our dev team.

> Speaking about imports order -- that's still a bug in Boost.Build, I'll 
> see
> into it.

Hopefully it will be a bug fix that enhances the reliability of BB rather 
than just fixing this one nit. :)

Thanks again for your persistent help with this issue.
Daniel Einspanjer