I see examples of enable_if used to specify a return type to a function: template <class T> typename enable_if_c<boost::is_arithmetic<T>::value, T>::type foo(T t) { return t; } Is it possible to use enable_if with a constructor? For example, I'd like to define a constructor that allows only certain values of an enum: #include <boost/utility/enable_if.hpp> class foo { public: enum state { state1, state2, state3 }; // This should compile only when passed (s == state2 or s == state3) template<int ID> foo(boost::enable_if_c<some_boolean_condition, state>::type s) : status_(s) {} private: state status_; }; Even if this turns out to be impossible, I'd like to go on and try to define some_boolean_condition if I may. I see that Aleksey plans a contains_c metafunction for 1.33, http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?MPL_TODO_List Equivalent URL: http://tinyurl.com/4toly so my limited understanding leads me to some_boolean_condition is equivalent to: mpl::contains_c< mpl::vector_c< int, state2, state3 >, ID >::value Does that look correct? In the absence of contains_c, I've been trying to use mpl::find_if but my predicate compiles for both "valid" and "invalid" values, so I must be doing something wrong. I'd be grateful if someone could put me back on track. Regards, Angus #include <boost/type_traits/is_same.hpp> #include <boost/mpl/assert.hpp> #include <boost/mpl/end.hpp> #include <boost/mpl/equal_to.hpp> #include <boost/mpl/find_if.hpp> #include <boost/mpl/lambda.hpp> #include <boost/mpl/integral_c.hpp> #include <boost/mpl/not.hpp> #include <boost/mpl/vector_c.hpp> class foo { public: enum state { state1, state2, state3 }; }; int main() { // Should lead to a compile-time failure. #define TESTED_VALUE foo::state1 // Should compile. // #define TESTED_VALUE foo::state2 namespace mpl = boost::mpl; typedef mpl::vector_c< int, foo::state2, foo::state3 > allowable_values; typedef mpl::find_if< allowable_values, mpl::equal_to< mpl::_1, mpl::integral_c<allowable_values::value_type, TESTED_VALUE> > >::type iter; BOOST_MPL_ASSERT(( mpl::not_< boost::is_same< iter::type, mpl::end<allowable_values>::type > > )); return 0; }