[Boost-build] Question about try-one-generator-really

From: David Abrahams (dave_at_[hidden])
Date: 2007-03-10 01:10:32

  # Returns usage requirements + list of created targets
  local rule try-one-generator-really ( project name ? : generator : 
      target-type : property-set : sources * )
  {
      local targets =
        [ $(generator).run $(project) $(name)
                         : $(property-set)
                         : $(sources)
        ] ;

      local usage-requirements ;
      local success ;

      generators.dout [ indent ] returned $(targets) ;

      if $(targets)
      {
          success = true ;

          if  [ class.is-a $(targets[1]) : property-set ]
          {
              usage-requirements = $(targets[1]) ;
              targets = $(targets[2-]) ;
          }
          else
          {
              usage-requirements = [ property-set.empty ] ;
          }
      }

Apparently this means it's optional for the first element returned
from running a generator property-set of usage requirements.  Why is
that a good idea?  Wouldn't it be better to use a single protocol (all
generators return either nothing, or a sequence starting with a
property-set of usage requirements and followed by generated targets)?

Now as I modify gcc-linking-generator, I have to worry that
unix-linking-generator might not give me a property set of usage
requirements.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com