Re: [boost] Hybrid compilation model?

From: Jeff Garland (jeff_at_[hidden])
Date: 2007-04-06 12:42:00

Beman Dawes wrote:

> I'll like to challenge Boosters to think about this problem a bit more, 
> and I'd love to see someone who understands the challenges take, say, 
> Boost.System and demonstrate how it could be packaged for either 
> header-only or compiled-library use. The important goal would be to 
> abstract what to done into a general set of guidelines (and any 
> configuration support needed). In other words, we don't so much need a 
> solution for Boost.System as for any Boost library that would benefit 
> from a hybrid computation model.
>

I'd do it, but I just dont' have time right now.  Here's the basic details. 
BTW, this is detailed in Chapter 10 of Efficient C++ (Bulka/Mayhew) circa 2000 
-- and I'm guessing they didn't invent it -- so, this isn't exactly a new 
idea.  In their case the rational was performance tuning.

Here's the basic idea:
   1) in the header, conditionally include the implementation file (eg: .ipp)
   2) in the implementation file, conditionally inline methods
   3) in the .cpp file include .ipp and turn off the conditional inlining

For system it appears that it's essentially error_code.hpp/cpp we need to 
convert.  So here's what needs to be done.  In error_code.hpp we add something 
like this at the near the end of the file:

//make system all inline
#ifdef BOOST_USE_OS_NATIVE_HEADERS_INLINE
#include <error_code.ipp>
#endif

Now, in the IPP file you'll need a macro to optionally define inline:

//code to define inline
#ifdef BOOST_USE_OS_NATIVE_HEADERS_INLINE
#define BOOST_INLINE inline
#endif

and the methods need to be declared like this:

   BOOST_INLINE
   int
   errno_ed( const error_code & ec )

To be safe at the end of the .ipp you'll probably want this:

#ifdef BOOST_USE_OS_NATIVE_HEADERS_INLINE
#undef BOOST_INLINE
#endif

Note that in system.cpp code looks like there's some namespace scope const 
arrays and things that probably need to be turned into enums or in general 
cleaned up for header only use.

So the result is that when someone does

   #def BOOST_USE_OS_NATIVE_HEADERS_INLINE 1
   #include <boost/system.hpp>

the effect is all inlined code -- they don't need to link the library.

Now the error_code.cpp effectively just includes the .ipp with the macro 
undefined.  So it builds the functions into the library because BOOST_INLINE 
resolves to an empty string.

So, for those that just use the usual:

   #include <boost/system.hpp>

they will need to link the library.

That's the essence of the technique...I'm sure someone with an hour or 2 to 
goof around with it could implement it on system.

Jeff