I see from the source code that the transform range is calling make_transform_iterator which takes two arguments, and that

"If Reference is use_default then the reference member of transform_iterator is result_of::type."

The documentation for result_of explains the need, and shows how to use a result template instead of a result_type simple type in this case. My code follow from the example:

struct ASCII_lower { // tell details to result_of template, since VS8 doesn't support decltype keyword. template <typename> struct result; template struct result { typedef T type; };

template< typename CharT > CharT operator() (const CharT& ch) const { if (ch >= 'A' && ch <= 'Z') return ch+0x20; return ch; // unchanged } };

But, I still get an error that result_type is not found!

B:\boost/iterator/transform_iterator.hpp(42) : error C2039: 'result_type' : is not a member of 'vst::internal::ASCII_lower' B:\boost/mpl/eval_if.hpp(41) : see reference to class template instantiation 'boost::detail::function_object_result<UnaryFunc>' being compiled with [ UnaryFunc=vst::internal::ASCII_lower ] B:\boost/iterator/iterator_adaptor.hpp(172) : see reference to class template instantiation 'boost::mpl::eval_if' being compiled with [ C=boost::is_sameboost::use_default,boost::use_default,

F1=boost::detail::function_object_resultvst::internal::ASCII_lower, F2=boost::mpl::identityboost::use_default ]

It appears to be using function_object_result, not result_of as documented. And that is: template <class UnaryFunc> struct function_object_result { typedef typename UnaryFunc::result_type type; }; which simply turns result_type into type to make eval_if happy, and doesn't do anything fancier.

So, it doesn't work as documented, and neither provides another way of passing the parameter in (exposing more arguments from the underlying transform_iterator). Is that a bug, or what?

What version of boost are you using? The following code compiles fine for me with 1.47. I know some of the result_of support has been added recently, so it's possible you're using an older version that does not contain these fixes. #include #include struct ASCII_lower { // tell details to result_of template, since VS8 doesn't support decltype keyword. template <typename > struct result; template struct result { typedef T type; }; template <typename CharT> CharT operator()(const CharT& ch) const { if (ch >= 'A' && ch <= 'Z') return ch + 0x20; return ch; // unchanged } }; int main() { const char s[] = "HELLO"; assert('h' == *boost::begin(s | boost::adaptors::transformed(ASCII_lower()))); return 0; } Regards, Nate.