I pass a mpl::vector with duplicates to mpl::unique and the result still has duplicates. What am I doing wrong? http://coliru.stacked-crooked.com/a/fe85b5739eb4c15f #include <boost/mpl/vector.hpp> #include <boost/mpl/unique.hpp> #include <boost/mpl/equal_to.hpp> #include <boost/mpl/placeholders.hpp> #include <boost/type_traits/is_same.hpp> #include <boost/mpl/print.hpp> template< typename... Args > void test( const Args&... args ) { using namespace boost::mpl; using namespace boost::mpl::placeholders; typedef vector<Args...> args_types; typedef typename unique<args_types, boost::is_same<_1, _2> >::type unique_types; print< unique_types > x; } int main() { int a = 0; char b = 0; test( a, b, a ); } + g++-4.8 -std=c++11 -O2 -Wall -pedantic -pthread main.cpp main.cpp: In instantiation of ‘void test(const Args& ...) [with Args = {int, char, int}]’: main.cpp:22:19: required from here main.cpp:15:27: warning: unused variable ‘x’ [-Wunused-variable] print< unique_types > x; ^ + ./a.out