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?
—John
