Hi! I may have posted the first half of this question once, sorry for that! : ) The question in short: which boost library enables me to get the "wider" datatype of two, such that get_wider::type will be float, and get_wider::type will also be float. Hopefully get_wider::type will generate a compile-time error, since the two are not convertible to each other. More verbosely: Let's take a look at this simple functor class: template struct mixed_plus : public binary_function // result_type: bad solution! { result_type operator() (first_argument_type left, second_argument_type right) const { return left + right; } }; All is fine if I write this: cout << mixed_plus()(5.5, 5) << endl; OUTPUT: 11.5 (as expected) But when I write this: cout << mixed_plus()(5, 5.5) << endl; OUTPUT: 10 (as expected but not as intended) I vaguely remember that there is a boost library that is able to generate the "wider" data type of two data type, with the help of which I would be able to have the result_type typedef always the appropriate one.