(In the previous mail, "function call operator" should be "dereference operator". Sorry for this, I fix it and resend a mail.) A long-standing request that transform_iterator should use boost::result_of will be resolved in Boost 1.47. This is really nice news, but I have a concern related to this. In the new code, transform_iterator uses  result_of<UnaryFunc(typename std::iterator_traits<Iterator>::reference)> to determine the return type of a functor. However, since transform_iterator's dereference operator is const,  result_of<const UnaryFunc(typename std::iterator_traits<Iterator>::reference)> should be used instead. There is a patch for this fix in #5127: https://svn.boost.org/trac/boost/ticket/5127 Alternatively, to remove this inconsistency, we can strip the constness of transform_iterator's dereference operator by either A. Make UnaryFunc member data mutable as suggested in #4969: https://svn.boost.org/trac/boost/ticket/4969 or B. Add non-const overloads to  - transform_iterator's dereference() function  - iterator_core_access' dereference() function  - iterator_facade's operator*() Patches for A (make_mutable.patch) and B (add_nonconst_overloads.patch) are attached in this mail. Comments? Regards, Michel