Build your own boost::preprocessor library

Recently I realized that using boost::preprocessor could improve a design in our project. We have a framework which allows consumers to listen to events emit by modules. But we need to write a wrapper for each event. for an event like OnLogin, we write the wrapper this way:

void Fire_OnLogin(bool bReconnect)
{
	BEGIN_FIRE_EVENT(IMyEvent)
		pEvent->OnLogin(bReconnect);
	END_FIRE_EVENT
}

By using boost::pp, we could write something like the following to accomplish the same goal:

MY_EVENT(IMyEvent, OnLogin, 1, (bool))

Here is how MY_EVENT is implemented:

#include <boost/preprocessor/repetition.hpp>
#include <boost/preprocessor/array.hpp>
#define __MY_PARAMS(z, n, args) BOOST_PP_ARRAY_ELEM(n, args) p##n
#define MY_EVENT(cls, evt, argCount, argList)	\
	void Fire_##evt(\
		 BOOST_PP_ENUM(argCount, __MY_PARAMS, (argCount, argList)))\
	{\
		BEGIN_FIRE_EVENT(cls);\
			pEvent->evt(BOOST_PP_ENUM_PARAMS(argCount, p));\
		END_FIRE_EVENT;\
	}

I’m not going to explain the code line by line. If you want to know more about BOOST_PP_ENUM, BOOST_PP_ARRAY_EVEM, .., please refer to the boost documentation Everything looks great, except that we don’t have boost available in my project. And as a facility in a framework, I don’t like the idea asking the customer to install boost before using it. As preprocessor is a header only library, maybe we could extract it using bcp and then ship it along with the framework. This sounds good, but let’s see if we have better option. What if we build a preprocessor library? Note my wording, we’re gonna build instead of writing one. Thanks to g++ that it has a parameter -M for generating the include-dependency file list:

nickx:/tmp $ cat boostpp.cpp
#include <boost/preprocessor/config/config.hpp>
#include <boost/preprocessor/repetition.hpp>
#include <boost/preprocessor/array.hpp>
nickx:/tmp $ g++ -M boostpp.cpp
boostpp.o: boostpp.cpp \
  /usr/local/include/boost/preprocessor/config/config.hpp \
  /usr/local/include/boost/preprocessor/repetition.hpp \
  /usr/local/include/boost/preprocessor/repetition/deduce_r.hpp \
  /usr/local/include/boost/preprocessor/detail/auto_rec.hpp \
  ...</pre>

cat these files to produce a single file: (Some bash tricks are applied to remove a few unwanted characters)

nickx:/tmp $ cat $(g++ -M boostpp.cpp |grep boost/preprocessor/|sed -e 's/\\//g')
# /* **************************************************************************
#  *                                                                          *
#  *     (C) Copyright Paul Mensonides 2002.
#  *     Distributed under the Boost Software License, Version 1.0. (See
#  *     accompanying file LICENSE_1_0.txt or copy at
#  *     http://www.boost.org/LICENSE_1_0.txt)
#  *                                                                          *
#  ************************************************************************** */
#
# /* See http://www.boost.org for most recent version. */
#
# ifndef BOOST_PREPROCESSOR_CONFIG_CONFIG_HPP
# define BOOST_PREPROCESSOR_CONFIG_CONFIG_HPP
#
# /* BOOST_PP_CONFIG_FLAGS */
#
# define BOOST_PP_CONFIG_STRICT() 0x0001
# define BOOST_PP_CONFIG_IDEAL() 0x0002
#
# define BOOST_PP_CONFIG_MSVC() 0x0004
# define BOOST_PP_CONFIG_MWCC() 0x0008
# define BOOST_PP_CONFIG_BCC() 0x0010
# define BOOST_PP_CONFIG_EDG() 0x0020
...

for boost 1.48, the generated file has 8062 lines. don’t forget removing the include directives.

nickx:/tmp $ cat $(g++ -M boostpp.cpp |grep boost/preprocessor/|\
sed -e 's/\\//g')|grep '# *include' -v|wc
    7801   60707  584467

Almost done. As we’re still using the BOOST_PP_* macros, it would have name conflicts if a customer already has boost preprocessor included in her project. We could fix this simply by replacing ‘BOOST’.

nickx:/tmp $ cat $(g++ -M boostpp.cpp |grep boost/preprocessor/|\
sed -e 's/\\//g')|grep '# *include' -v|sed -e 's/BOOST/MY/g'>mypreprocessor.h

Here is a bash script to wrap all this up:

#!/bin/bash 

src=/tmp/boostpp.cpp

cat>$src<<EOF
#include <boost/preprocessor/config/config.hpp>
#include <boost/preprocessor/repetition.hpp>
#include <boost/preprocessor/array.hpp>
EOF

cat $(g++ -M $src |grep boost/preprocessor/|sed -e 's/\\//g')|grep include -v|sed -e 's/BOOST/MY/g'>mypreprocessor.h

Rewrite MY_EVENT as:

#include "mypreprocessor.h"
#define __MY_PARAMS(z, n, args) MY_PP_ARRAY_ELEM(n, args) p##n
#define MY_EVENT(cls, evt, argCount, argList)	\
	void Fire_##evt(\
		 MY_PP_ENUM(argCount, __MY_PARAMS, (argCount, argList)))\
	{\
		BEGIN_FIRE_EVENT(cls);\
			pEvent->evt(MY_PP_ENUM_PARAMS(argCount, p));\
		END_FIRE_EVENT;\
	}

That’s all. hope you enjoy it.