Re: [jamboost] Re: boost.build-v2: usage-requirements for header-only libs?

From: Michael Stevens (Michael.Stevens_at_[hidden])
Date: 2003-08-29 04:18:28

Vladimir Prus wrote:

> David Abrahams wrote:
>
> In fact, immediately after I went online after replying Peter, I 
> realized we
> can support this in a simple way in V2.
>
> alias aHeaderOnlyLib : : : : <include>include ;
>
> Now, alias does not support usage requirements, but it's trivial to 
> add. Any
> objections to this approach?
>
> W.r.t. projects: the 'usage-requirements' attribute on projects have 
> effect
> only on targets declarated in that project, so <use>/boost does not work.

I was thinking about the issue of library header files a bit further. In 
particular with composite builds with include headers from disparate 
projects.
Let me give a concreate example. I have a library and example source 
that I distribute (actually called) Bayes++. This library reliese on 
Boost for several header only libraries. When a users wants to build my 
examples they must follow a few instructions so the build succeds in 
finding the Boost headers.

This last step is fraught with danger! A simple typo or renamed 
directory will result in failure. The problem for the poor user is that 
it only shows up at a very late stage. When the compiler #include the 
first header from Boost. The usually result in the normal indecipherable 
stream of errors.

My ideal solution is that the error comes much earlier (and more 
meaningfully) from the build system. It this case the build system would 
need to know about the set of header files that can be included from the 
library.
Obviously the build system can do more then just issue (hopefull 
meaningful) error messages. It can also determine the necessary build 
dependencies to these headers.

At the moment I don't have a clear notion of how best to implement this.
It would seem logicaly that the library itself sepecify these header 
dependancies. That is the present BOOST_ROOT/Jamfile would specify the 
full set of headers in BOOST_ROOT/boost.
Alternetively a system based on the existance of header directories may 
be simpler to maintain.

I think I should experiment!

Michael Stevens

My build instruct