Conflict between Boost, OpenCV and Eigen libraries?

Question

my question is somewhat related to Static linking of Boost and OpenCV libs with Eclipse CDR. errors, whereas I'm trying to do a bit more than described here: How to create a program that can read all the images in folder using Boost and OpenCV?, namely traverse a directory using Boost's filesystem library and do some processing on the image files with OpenCV.

I compiled filesystem and other libraries with MinGW, and try to run Boost 1.45, OpenCV 2.2 and Eigen2 with Eclipse CDT on a Windows 7 64-bit system. The filesystem library compiles and runs without problems if used in a project by itself, but in combination with the other two libraries from above, I get the following errors:

In file included from C:\boost_1_45_0/boost/filesystem/v3/path_traits.hpp:22:0, 
                 from C:\boost_1_45_0/boost/filesystem/v3/path.hpp:25, 
                 from C:\boost_1_45_0/boost/filesystem.hpp:32, 
                 from ..\src\ComputeNatScaleFunction.cpp:18: 
C:\boost_1_45_0/boost/type_traits/decay.hpp: In instantiation of 'boost::decay<cv::<anonymous enum> >': 
C:\cmake_binaries\include/opencv2/core/operations.hpp:766:23:   instantiated from here 
C:\boost_1_45_0/boost/type_traits/decay.hpp:28:66: error: 'cv::' is/uses anonymous type 
C:\boost_1_45_0/boost/type_traits/decay.hpp:28:66: error:   trying to instantiate 'template struct boost::remove_reference' 
C:\boost_1_45_0/boost/type_traits/decay.hpp:38:17: error: 'cv::' is/uses anonymous type 
C:\boost_1_45_0/boost/type_traits/decay.hpp:38:17: error:   trying to instantiate 'template struct boost::remove_reference' 
C:\boost_1_45_0/boost/type_traits/decay.hpp: In instantiation of 'boost::decay<cv::<anonymous enum> >': 
C:\cmake_binaries\include/opencv2/core/operations.hpp:917:21:   instantiated from here 
C:\boost_1_45_0/boost/type_traits/decay.hpp:28:66: error: 'cv::' is/uses anonymous type 
C:\boost_1_45_0/boost/type_traits/decay.hpp:28:66: error:   trying to instantiate 'template struct boost::remove_reference' 
C:\boost_1_45_0/boost/type_traits/decay.hpp:38:17: error: 'cv::' is/uses anonymous type 
C:\boost_1_45_0/boost/type_traits/decay.hpp:38:17: error:   trying to instantiate 'template struct boost::remove_reference' 
C:\boost_1_45_0/boost/type_traits/decay.hpp: In instantiation of 'boost::decay<Eigen::<anonymous enum> >': 
C:\Eigen2/Eigen/src/Core/GenericPacketMath.h:116:18:   instantiated from here 
C:\boost_1_45_0/boost/type_traits/decay.hpp:28:66: error: 'Eigen::' is/uses anonymous type 
C:\boost_1_45_0/boost/type_traits/decay.hpp:28:66: error:   trying to instantiate 'template struct boost::remove_reference' 
C:\boost_1_45_0/boost/type_traits/decay.hpp:38:17: error: 'Eigen::' is/uses anonymous type 
C:\boost_1_45_0/boost/type_traits/decay.hpp:38:17: error:   trying to instantiate 'template struct boost::remove_reference' 

etc.

Any hints as to why these libraries might conflict with each other? The compiler is not getting past the include of filesystem (i.e. line 18).

Answer 1

Using the boost::filesystem namespace before including Eigen causes the compiler to fail:

#include <boost/filesystem.hpp>
using namespace boost::filesystem;
#include <Eigen/Core>

fails, but

#include <boost/filesystem.hpp>
#include <Eigen/Core>
using namespace boost::filesystem;

works.

The reason is that if boost::filesystem is added to the global namespace, it pollutes it, and causes some code (here: eigen) that depends on unpolluted namespace to cause errors during compilation. There's nothing strange about it. Normally you should never put "using" lines before your includes are finished.