Thank you Steven for your reply. It works great. Here is what I finally did: // struct histogram_functor typename boost::enable_if< boost::mpl::contains< boost::mpl::transform<gray_image_types,add_view_type<boost::mpl::_1>
::type, ViewType>, result_type>::type operator()(const ViewType& v) const { // ... }
template<class ViewType> typename boost::enable_if< boost::mpl::or_< boost::mpl::contains< boost::mpl::transform< rgb_image_types, add_view_type<boost::mpl::_1 > >::type, ViewType>, boost::mpl::contains< boost::mpl::transform< rgba_image_types, add_view_type<boost::mpl::_1 > >::type, ViewType> >, result_type>::type operator()(const ViewType& v) const { // ... } // ... template <typename ImageType> struct add_view_type { typedef typename ImageType::view_t type; }; However, I have one other need: type reduction. For instance, all rgb image types have the same overload operator(). I do not know if the solution I came up instantiate the operator for all types, but if it is the case, it is a valid but not a good solution. What I need is to have the same operator() for all rgb types. I have read in a paper from L .Bourdev ( www.lubomir.org/academic/Minimizing*CodeBloat*.pdf) that I could use type reduction, but I need some help. Can you help ? Here is the definition of rgb_image_types: #define RGB_IMAGE_TYPES (boost::gil::rgb8_image_t)(boost::gil::rgb16_image_t)(boost::gil::rgb32_image_t) typedef boost::mpl::vector< BOOST_PP_SEQ_ENUM( RGB_IMAGE_TYPES ) > rgb_image_types; Best regards, Olivier