Hi, I want to warn user about a possible miss use of a function. Now I'm only able to stop the compilation with the following code template <typename ValueType> class simVec3 { ......... /// Unary multiply by scalar. /// @param rsh The scalar to multiply the vector with template<class T> simVec3<ValueType>& operator *= (const T& rhs) { // The function should not be use like this // simVec<int> vec(1, 1, 1); // a *= 0.3; // since what a will contain is [0, 0, 0] BOOST_MPL_ASSERT_NOT(( boost::mpl::and_< \ boost::is_floating_point<T>, \ boost::is_integral<ValueType> > )); ValueType s=static_cast<ValueType>(rhs); vec[0]*=s; vec[1]*=s; vec[2]*=s; return *this; } What I would want is something like #if A_meta_function_returning_1_if_int_double #pragma message("simVec::Possible type truncation") #endif instead of the BOOST_MPL_ASSERT_NOT I also tried something like: template <bool x> struct AA; template <> struct AA<true> { enum {value = 1 }; }; template <> struct AA<false> { enum {value = 0}; }; template <typename U, typename V> double myfct(U u, V v) { #if ( AA< boost::mpl::and_< boost::is_integral<U>, \ boost::is_floating_point<V> > > ::value ) #pragma message("****** Hellop *********") #endif return u + v; } But the compilation is complaining about an unmatched parenthesis Remi