Boost ptr_map iterator incompatibility

August 11, 2009

Boost library provides a very powerful set of data structures and algorithms which are intended to be cross platform and rather concise. Unfortunately it happens, that the API of the library changes from version to version. The problem which I faced recently was related to ptr_map container iterator and different methods allowing for accessing keys and values of the container. In boost <=1.33 the mentioned fields could be accessed via iterator->key() and (*iterator) methods respectively. Starting from version 1.34 the authors of the library introduced approach which is compatible with map container from STL and uses iterator->first and iterator->second fields.

Unfortunately I was forced to use different platforms for development (Ubuntu 9.04 – Boost 1.39) and deploying (CentOS 5.3 with Boost 1.33), that’s why some problems appeared.

In order to solve them I used some conditional compilation tricks and some basic AutoConf routines.

First of all in configure.ac file I added the following condition, which tries to compile a given instruction using ptr_map iterator from Boost > 1.33.

If it succeeds the HAVE_PTR_MAP_ITR_SECOND value is defined in config.h file and we are sure that the platform supports STL compatible method of accessing map entries.

AC_TRY_COMPILE(
[#include ],
[boost::ptr_map::iterator itt;int *i=itt->second;],
AC_DEFINE(HAVE_PTR_MAP_ITR_SECOND,1,
[ptr_map::iterator defines second field]))

Later I created a short macro which will be replaced with appropriate code depending on the HAVE_PTR_MAP_ITR_SECOND condition:

#ifdef HAVE_PTR_MAP_ITR_SECOND
#define ptr_map_itr_second(s) ((s)->second)
#else
#define ptr_map_itr_second(s) (&(*s))
#endif

The exemplary code showing application of the created macro can be found below:

...
ptr_map::iterator it;
it = ports_map_.find("key");

if (it != ports_map_.end())
{
  OffPortRange *port_range =  ptr_map_itr_second(it);
  ....
}