[Boost-build] PCH and property propagation

From: Vladimir Prus (ghost_at_[hidden])
Date: 2006-10-23 14:46:58

I think I have the final question about PCH interface. Now if we write:

   cpp-pch pch : pch.hpp : ......... ;
   exe hello : hello.cpp pch : ....... ;

the user is required to make sure that properties used to compile pch and the 
exe are "compatible". I wonder if we can improve that. One approach is to 
propagate all features, including free one, from exe to pch. So that if you 
write:

   cpp-pch pch : pch.hpp ;
   exe hello : hello.cpp pch : <define>FOO ;

the <define>FOO is propagated to 'pch'. That's "implementable", but then what 
about:

   cpp-pch pch : pch.hpp ;
   exe hello : hello.cpp pch : <define>FOO ;
   exe hello2 : hello.cpp pch : <define>BAR ;

In this case, Boost.Build won't know with what properties to build 'pch' and 
will error out. And FOO/BAR really might affect just the executables, and not 
the PCH. So, I think that requiring user to set right properties on PCH might 
be a good idea, after all.

Comments?

- Volodya