[boost] [config] Boost PP and variadic macros

Subject: [boost] [config] Boost PP and variadic macros
From: Edward Diener (eldiener_at_[hidden])
Date: 2011-04-03 11:58:58

This is largely directed to John Maddock, the maintainer of config, but 
of course anyone else can respond who is interested.

I have been working to integrate my VMD library, and essentially 
variadic macro support, into Boost PP, with  help and discussion offline 
of Paul Mensonides. I am now at the testing phase and all is now going 
well. This is currently being done in a separate library but when Paul 
and I are satisfied we intend to update the trunk with our changes. The 
library is at https://svn.boost.org/svn/boost/branches/pplib/variadics.

One problem that I have can be explained by the fact that Boost PP 
currently does not depend on any other Boost library, not even Boost 
config. The reason for this is that Boost PP has been designed to work 
with C compilers ( or C++ compilers in C mode ) as well as C++ 
compilers. This means that the code to detect variadic macro support in 
Boost is duplicated in two places, both in config, which I basically 
started with the blessing of John Maddock with the 
BOOST_NO_VARIADIC_MACROS macro, and in Boost PP in a configuration file 
which I have coded and essentially duplicates the code in config for 
variadic macro support.

I do not like this. Duplicating functionality in two different places, 
even as simple as determining variadic macro support for compilers, is 
error prone and a maintenance problem. Changes in one place have to be 
done in the other, and this would need to constantly be tracked.

My initial thought was to see if config could work with C compilers, but 
all investigation there has led me to believe that it would be too many 
unnecessary changes to config, would complicate config greatly, would 
need extensive testing against all compilers, and that since config is 
central to all of Boost such a potential massive change to it it is a 
bad idea.

So my current thought is to remove the code in config which tests for 
variadic macro support for a given compiler, to determine if 
BOOST_NO_VARIADIC_MACROS should be set, into a separate header file for 
each compiler which might set BOOST_NO_VARIADIC_MACROS, and then 
#include that header file in the compiler's header file, at the place 
where currently the setting of BOOST_NO_VARIADIC_MACROS exists ( or 
possible at the end ). The separate header file itself for testing 
variadic macro support is easily C compatible and I could use it in my 
Boost PP configuration file. This would eliminate duplication of the 
code to test for variadic macro support in config and Boost PP while 
keeping the goal of allowing Boost PP to still work with C compilers.

Is this possible/desirable as far as config goes ? It would only affect 
the current code for setting BOOST_NO_VARIADIC_MACROS, hardly a Boost 
world-shaking event, and can easily be done by me and tested in 
regression tests.

If it is, one further consideration is that the code in the Boost PP 
configuration file still would have to internally duplicate the checking 
and order of each compiler in the 'select_compiler_config.h' in config, 
but this is at least much better, and much less of a maintenance problem 
for Boost PP, than having to duplicate the code for checking for 
variadic macro support itself.

Even this could be worked around, with a probably undesirable change to 
'select_compiler_config.h', which I will proceed to suggest nonetheless. 
If we could add to that file another set of #defines for the 
corresponding variadic macro checking header file for each compiler that 
needs it. Something like:

#define BOOST_COMPILER_CONFIG_NVM 
"boost/config/compiler/somecompiler_nvm.hpp"

then in my Boost PP config header file I could go:

#include <boost/config/select_compiler_config.hpp>
#ifdef BOOST_COMPILER_CONFIG_NVM
#include BOOST_COMPILER_CONFIG_NVM
#endif

and I could then check BOOST_NO_VARIADIC_MACROS and this will work for C 
compilers ( 'select_compiler_config.hpp' itself has nothing related to 
C++ and should work with any C compiler AFAICS ).

But I would understand that littering 'select_compiler_config.hpp' with 
another set of #defines just to satisfy my need in Boost PP may not be 
acceptable to the clean design of that file, even if it would eliminate 
my need to reduplicate its logic in the Boost PP configuration file for 
variadic macro support in Boost PP which I have created.