diff --git a/sprout/algorithm/adjacent_find.hpp b/sprout/algorithm/adjacent_find.hpp
index 7f0c1821..9a41503c 100644
--- a/sprout/algorithm/adjacent_find.hpp
+++ b/sprout/algorithm/adjacent_find.hpp
@@ -109,7 +109,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR ForwardIterator
 		adjacent_find(
 			ForwardIterator first, ForwardIterator last, BinaryPredicate pred,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			typedef sprout::pair<ForwardIterator, ForwardIterator> type;
diff --git a/sprout/algorithm/all_of.hpp b/sprout/algorithm/all_of.hpp
index 99189a9b..b3922134 100644
--- a/sprout/algorithm/all_of.hpp
+++ b/sprout/algorithm/all_of.hpp
@@ -84,7 +84,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		all_of(
 			InputIterator first, InputIterator last, Predicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, bool> type;
diff --git a/sprout/algorithm/all_of_equal.hpp b/sprout/algorithm/all_of_equal.hpp
index 54b2e520..6bfa6e02 100644
--- a/sprout/algorithm/all_of_equal.hpp
+++ b/sprout/algorithm/all_of_equal.hpp
@@ -84,7 +84,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		all_of_equal(
 			InputIterator first, InputIterator last, T const& value,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, bool> type;
diff --git a/sprout/algorithm/any_of.hpp b/sprout/algorithm/any_of.hpp
index 1aae2bf4..98f2549d 100644
--- a/sprout/algorithm/any_of.hpp
+++ b/sprout/algorithm/any_of.hpp
@@ -84,7 +84,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		any_of(
 			InputIterator first, InputIterator last, Predicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, bool> type;
diff --git a/sprout/algorithm/any_of_equal.hpp b/sprout/algorithm/any_of_equal.hpp
index 243f82b5..f3a9c81f 100644
--- a/sprout/algorithm/any_of_equal.hpp
+++ b/sprout/algorithm/any_of_equal.hpp
@@ -84,7 +84,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		any_of_equal(
 			InputIterator first, InputIterator last, T const& value,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, bool> type;
diff --git a/sprout/algorithm/count.hpp b/sprout/algorithm/count.hpp
index 5b0ce271..4d57c15d 100644
--- a/sprout/algorithm/count.hpp
+++ b/sprout/algorithm/count.hpp
@@ -84,7 +84,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR typename std::iterator_traits<InputIterator>::difference_type
 		count(
 			InputIterator first, InputIterator last, T const& value,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, typename std::iterator_traits<InputIterator>::difference_type> type;
diff --git a/sprout/algorithm/count_if.hpp b/sprout/algorithm/count_if.hpp
index c34b2cfa..3bb2968c 100644
--- a/sprout/algorithm/count_if.hpp
+++ b/sprout/algorithm/count_if.hpp
@@ -84,7 +84,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR typename std::iterator_traits<InputIterator>::difference_type
 		count_if(
 			InputIterator first, InputIterator last, Predicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, typename std::iterator_traits<InputIterator>::difference_type> type;
diff --git a/sprout/algorithm/equal.hpp b/sprout/algorithm/equal.hpp
index 3b13d9f1..b5ef4f0f 100644
--- a/sprout/algorithm/equal.hpp
+++ b/sprout/algorithm/equal.hpp
@@ -91,7 +91,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		equal(
 			InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, BinaryPredicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::tuples::tuple<InputIterator1, InputIterator2, bool> type;
diff --git a/sprout/algorithm/find.hpp b/sprout/algorithm/find.hpp
index 0d041ba3..7acd117b 100644
--- a/sprout/algorithm/find.hpp
+++ b/sprout/algorithm/find.hpp
@@ -86,7 +86,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR InputIterator
 		find(
 			InputIterator first, InputIterator last, T const& value,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, bool> type;
diff --git a/sprout/algorithm/find_end.hpp b/sprout/algorithm/find_end.hpp
index 04cb79cb..5da0066b 100644
--- a/sprout/algorithm/find_end.hpp
+++ b/sprout/algorithm/find_end.hpp
@@ -134,7 +134,7 @@ namespace sprout {
 			ForwardIterator1 first1, ForwardIterator1 last1,
 			ForwardIterator2 first2, ForwardIterator2 last2,
 			BinaryPredicate pred,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			typedef sprout::pair<ForwardIterator1, ForwardIterator1> type;
diff --git a/sprout/algorithm/find_first_of.hpp b/sprout/algorithm/find_first_of.hpp
index b889ad2e..060d234e 100644
--- a/sprout/algorithm/find_first_of.hpp
+++ b/sprout/algorithm/find_first_of.hpp
@@ -102,7 +102,7 @@ namespace sprout {
 			InputIterator1 first1, InputIterator1 last1,
 			ForwardIterator2 first2, ForwardIterator2 last2,
 			BinaryPredicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator1, bool> type;
diff --git a/sprout/algorithm/find_if.hpp b/sprout/algorithm/find_if.hpp
index ffc8088c..e6356d97 100644
--- a/sprout/algorithm/find_if.hpp
+++ b/sprout/algorithm/find_if.hpp
@@ -86,7 +86,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR InputIterator
 		find_if(
 			InputIterator first, InputIterator last, Predicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, bool> type;
diff --git a/sprout/algorithm/find_if_not.hpp b/sprout/algorithm/find_if_not.hpp
index 1676aa4a..0d97c045 100644
--- a/sprout/algorithm/find_if_not.hpp
+++ b/sprout/algorithm/find_if_not.hpp
@@ -86,7 +86,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR InputIterator
 		find_if_not(
 			InputIterator first, InputIterator last, Predicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, bool> type;
diff --git a/sprout/algorithm/fixed/clamp_range_copy.hpp b/sprout/algorithm/fixed/clamp_range_copy.hpp
index de7d237d..6ef3c0be 100644
--- a/sprout/algorithm/fixed/clamp_range_copy.hpp
+++ b/sprout/algorithm/fixed/clamp_range_copy.hpp
@@ -105,7 +105,7 @@ namespace sprout {
 				typename std::iterator_traits<InputIterator>::value_type const& low,
 				typename std::iterator_traits<InputIterator>::value_type const& high,
 				Compare comp,
-				void*
+				std::input_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::clamp_range_copy_impl(first, last, result, low, high, comp, sprout::size(result));
diff --git a/sprout/algorithm/fixed/copy.hpp b/sprout/algorithm/fixed/copy.hpp
index 4fb3f6c3..5cb8b2e1 100644
--- a/sprout/algorithm/fixed/copy.hpp
+++ b/sprout/algorithm/fixed/copy.hpp
@@ -83,7 +83,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			copy(
 				InputIterator first, InputIterator last, Result const& result,
-				void*
+				std::input_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::copy_impl(first, last, result, sprout::size(result));
diff --git a/sprout/algorithm/fixed/copy_backward.hpp b/sprout/algorithm/fixed/copy_backward.hpp
index a21b74d3..98d35595 100644
--- a/sprout/algorithm/fixed/copy_backward.hpp
+++ b/sprout/algorithm/fixed/copy_backward.hpp
@@ -82,7 +82,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			copy_backward(
 				BidirectionalIterator first, BidirectionalIterator last, Result const& result,
-				void*
+				std::bidirectional_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::copy_backward_impl(
diff --git a/sprout/algorithm/fixed/copy_n.hpp b/sprout/algorithm/fixed/copy_n.hpp
index 0aa27498..366b9c8d 100644
--- a/sprout/algorithm/fixed/copy_n.hpp
+++ b/sprout/algorithm/fixed/copy_n.hpp
@@ -59,7 +59,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			copy_n(
 				InputIterator first, Size n, Result const& result,
-				void*
+				std::input_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::copy_n_impl(first, n, result, sprout::size(result));
diff --git a/sprout/algorithm/fixed/copy_until.hpp b/sprout/algorithm/fixed/copy_until.hpp
index 7a1cab17..3f61db41 100644
--- a/sprout/algorithm/fixed/copy_until.hpp
+++ b/sprout/algorithm/fixed/copy_until.hpp
@@ -61,7 +61,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			copy_until(
 				InputIterator first, InputIterator last, Result const& result, Predicate pred,
-				void*
+				std::input_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::copy_until_impl(first, last, result, pred, sprout::size(result));
diff --git a/sprout/algorithm/fixed/copy_while.hpp b/sprout/algorithm/fixed/copy_while.hpp
index 12f4e13c..960fd971 100644
--- a/sprout/algorithm/fixed/copy_while.hpp
+++ b/sprout/algorithm/fixed/copy_while.hpp
@@ -60,7 +60,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			copy_while(
 				InputIterator first, InputIterator last, Result const& result, Predicate pred,
-				void*
+				std::input_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::copy_while_impl(first, last, result, pred, sprout::size(result));
diff --git a/sprout/algorithm/fixed/replace_copy.hpp b/sprout/algorithm/fixed/replace_copy.hpp
index c7ad9058..3aa425e7 100644
--- a/sprout/algorithm/fixed/replace_copy.hpp
+++ b/sprout/algorithm/fixed/replace_copy.hpp
@@ -93,7 +93,7 @@ namespace sprout {
 			replace_copy(
 				InputIterator first, InputIterator last,
 				Result const& result, T const& old_value, T const& new_value,
-				void*
+				std::input_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::replace_copy_impl(first, last, result, old_value, new_value, sprout::size(result));
diff --git a/sprout/algorithm/fixed/replace_copy_if.hpp b/sprout/algorithm/fixed/replace_copy_if.hpp
index 1227a1b1..e39cd701 100644
--- a/sprout/algorithm/fixed/replace_copy_if.hpp
+++ b/sprout/algorithm/fixed/replace_copy_if.hpp
@@ -92,7 +92,7 @@ namespace sprout {
 			replace_copy_if(
 				InputIterator first, InputIterator last,
 				Result const& result, Predicate pred, T const& new_value,
-				void*
+				std::input_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::replace_copy_if_impl(first, last, result, pred, new_value, sprout::size(result));
diff --git a/sprout/algorithm/fixed/reverse_copy.hpp b/sprout/algorithm/fixed/reverse_copy.hpp
index 26f128a5..9f69f8e1 100644
--- a/sprout/algorithm/fixed/reverse_copy.hpp
+++ b/sprout/algorithm/fixed/reverse_copy.hpp
@@ -84,7 +84,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			reverse_copy(
 				BidirectionalIterator first, BidirectionalIterator last, Result const& result,
-				void*
+				std::bidirectional_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::reverse_copy_impl(first, last, result, sprout::size(result));
diff --git a/sprout/algorithm/fixed/rotate_copy.hpp b/sprout/algorithm/fixed/rotate_copy.hpp
index 2e6fd008..9163859d 100644
--- a/sprout/algorithm/fixed/rotate_copy.hpp
+++ b/sprout/algorithm/fixed/rotate_copy.hpp
@@ -125,7 +125,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			rotate_copy(
 				ForwardIterator first, ForwardIterator middle, ForwardIterator last, Result const& result,
-				void*
+				std::forward_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::rotate_copy_impl(first, middle, last, result, sprout::size(result));
diff --git a/sprout/algorithm/fixed/swap_element_copy.hpp b/sprout/algorithm/fixed/swap_element_copy.hpp
index c1d1186e..675c13cd 100644
--- a/sprout/algorithm/fixed/swap_element_copy.hpp
+++ b/sprout/algorithm/fixed/swap_element_copy.hpp
@@ -102,7 +102,7 @@ namespace sprout {
 				ForwardIterator first, ForwardIterator last,
 				Result const& result,
 				ForwardIterator pos1, ForwardIterator pos2,
-				void*
+				std::forward_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::swap_element_copy_impl(first, last, result, sprout::size(result));
diff --git a/sprout/algorithm/fixed/transform.hpp b/sprout/algorithm/fixed/transform.hpp
index d9c88c3d..22b61c15 100644
--- a/sprout/algorithm/fixed/transform.hpp
+++ b/sprout/algorithm/fixed/transform.hpp
@@ -90,7 +90,7 @@ namespace sprout {
 			transform(
 				InputIterator first, InputIterator last,
 				Result const& result, UnaryOperation op,
-				void*
+				std::input_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::transform_impl(first, last, result, op, sprout::size(result));
@@ -207,7 +207,7 @@ namespace sprout {
 			transform(
 				InputIterator1 first1, InputIterator1 last1, InputIterator2 first2,
 				Result const& result, BinaryOperation op,
-				void*
+				std::input_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::transform_impl(first1, last1, first2, result, op, sprout::size(result));
diff --git a/sprout/algorithm/includes.hpp b/sprout/algorithm/includes.hpp
index d3564bce..bf0af765 100644
--- a/sprout/algorithm/includes.hpp
+++ b/sprout/algorithm/includes.hpp
@@ -99,7 +99,7 @@ namespace sprout {
 			InputIterator1 first1, InputIterator1 last1,
 			InputIterator2 first2, InputIterator2 last2,
 			Compare comp,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator1, InputIterator2> type;
diff --git a/sprout/algorithm/is_partitioned.hpp b/sprout/algorithm/is_partitioned.hpp
index 727f79b6..3a037e68 100644
--- a/sprout/algorithm/is_partitioned.hpp
+++ b/sprout/algorithm/is_partitioned.hpp
@@ -78,7 +78,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		is_partitioned(
 			InputIterator first, InputIterator last, Predicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, typename std::iterator_traits<InputIterator>::difference_type> type;
diff --git a/sprout/algorithm/is_permutation.hpp b/sprout/algorithm/is_permutation.hpp
index f91d8557..8961e37d 100644
--- a/sprout/algorithm/is_permutation.hpp
+++ b/sprout/algorithm/is_permutation.hpp
@@ -99,7 +99,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		is_permutation(
 			ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, BinaryPredicate pred,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			typedef sprout::pair<ForwardIterator1, bool> type;
diff --git a/sprout/algorithm/is_sorted.hpp b/sprout/algorithm/is_sorted.hpp
index f7767eb5..6f69e56f 100644
--- a/sprout/algorithm/is_sorted.hpp
+++ b/sprout/algorithm/is_sorted.hpp
@@ -52,7 +52,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		is_sorted(
 			ForwardIterator first, ForwardIterator last, Compare comp,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			return sprout::is_sorted_until(first, last, comp) == last;
diff --git a/sprout/algorithm/is_sorted_until.hpp b/sprout/algorithm/is_sorted_until.hpp
index 537ce75b..ef46c92c 100644
--- a/sprout/algorithm/is_sorted_until.hpp
+++ b/sprout/algorithm/is_sorted_until.hpp
@@ -94,7 +94,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR ForwardIterator
 		is_sorted_until(
 			ForwardIterator first, ForwardIterator last, Compare comp,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			typedef sprout::pair<ForwardIterator, ForwardIterator> type;
diff --git a/sprout/algorithm/lexicographical_compare.hpp b/sprout/algorithm/lexicographical_compare.hpp
index bbd7d081..fcc23f47 100644
--- a/sprout/algorithm/lexicographical_compare.hpp
+++ b/sprout/algorithm/lexicographical_compare.hpp
@@ -126,7 +126,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		lexicographical_compare(
 			InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, Compare comp,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator1, InputIterator2> type;
diff --git a/sprout/algorithm/max_element.hpp b/sprout/algorithm/max_element.hpp
index aa1a2a56..37d1ad7c 100644
--- a/sprout/algorithm/max_element.hpp
+++ b/sprout/algorithm/max_element.hpp
@@ -97,7 +97,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR ForwardIterator
 		max_element(
 			ForwardIterator first, ForwardIterator last, Compare comp,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			typedef sprout::pair<ForwardIterator, ForwardIterator> type;
diff --git a/sprout/algorithm/min_element.hpp b/sprout/algorithm/min_element.hpp
index 89ba91e1..7523168f 100644
--- a/sprout/algorithm/min_element.hpp
+++ b/sprout/algorithm/min_element.hpp
@@ -97,7 +97,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR ForwardIterator
 		min_element(
 			ForwardIterator first, ForwardIterator last, Compare comp,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			typedef sprout::pair<ForwardIterator, ForwardIterator> type;
diff --git a/sprout/algorithm/minmax_element.hpp b/sprout/algorithm/minmax_element.hpp
index 39a92df2..10417972 100644
--- a/sprout/algorithm/minmax_element.hpp
+++ b/sprout/algorithm/minmax_element.hpp
@@ -109,7 +109,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR sprout::pair<ForwardIterator, ForwardIterator>
 		minmax_element(
 			ForwardIterator first, ForwardIterator last, Compare comp,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			typedef sprout::pair<ForwardIterator, sprout::pair<ForwardIterator, ForwardIterator> > type;
diff --git a/sprout/algorithm/mismatch.hpp b/sprout/algorithm/mismatch.hpp
index 99179331..0463a502 100644
--- a/sprout/algorithm/mismatch.hpp
+++ b/sprout/algorithm/mismatch.hpp
@@ -112,7 +112,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR sprout::pair<InputIterator1, InputIterator2>
 		mismatch(
 			InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, BinaryPredicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::tuples::tuple<InputIterator1, InputIterator2, bool> type;
diff --git a/sprout/algorithm/none_of.hpp b/sprout/algorithm/none_of.hpp
index 2548da8e..6cfae9aa 100644
--- a/sprout/algorithm/none_of.hpp
+++ b/sprout/algorithm/none_of.hpp
@@ -84,7 +84,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		none_of(
 			InputIterator first, InputIterator last, Predicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, bool> type;
diff --git a/sprout/algorithm/none_of_equal.hpp b/sprout/algorithm/none_of_equal.hpp
index e8a851c2..edaa2165 100644
--- a/sprout/algorithm/none_of_equal.hpp
+++ b/sprout/algorithm/none_of_equal.hpp
@@ -84,7 +84,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		none_of_equal(
 			InputIterator first, InputIterator last, T const& value,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, bool> type;
diff --git a/sprout/algorithm/one_of.hpp b/sprout/algorithm/one_of.hpp
index 016d972c..60fd6283 100644
--- a/sprout/algorithm/one_of.hpp
+++ b/sprout/algorithm/one_of.hpp
@@ -108,7 +108,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		one_of(
 			InputIterator first, InputIterator last, Predicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, typename std::iterator_traits<InputIterator>::difference_type> type;
diff --git a/sprout/algorithm/one_of_equal.hpp b/sprout/algorithm/one_of_equal.hpp
index 0309b9af..094d702d 100644
--- a/sprout/algorithm/one_of_equal.hpp
+++ b/sprout/algorithm/one_of_equal.hpp
@@ -108,7 +108,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR bool
 		one_of_equal(
 			InputIterator first, InputIterator last, T const& value,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, typename std::iterator_traits<InputIterator>::difference_type> type;
diff --git a/sprout/algorithm/search.hpp b/sprout/algorithm/search.hpp
index 8fa3f1dd..8941216e 100644
--- a/sprout/algorithm/search.hpp
+++ b/sprout/algorithm/search.hpp
@@ -110,7 +110,7 @@ namespace sprout {
 			ForwardIterator1 first1, ForwardIterator1 last1,
 			ForwardIterator2 first2, ForwardIterator2 last2,
 			BinaryPredicate pred,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			typedef sprout::pair<ForwardIterator1, bool> type;
diff --git a/sprout/algorithm/search_n.hpp b/sprout/algorithm/search_n.hpp
index 7bc5db81..408117e3 100644
--- a/sprout/algorithm/search_n.hpp
+++ b/sprout/algorithm/search_n.hpp
@@ -18,9 +18,10 @@ namespace sprout {
 	inline SPROUT_CONSTEXPR ForwardIterator
 	search_n(ForwardIterator first, ForwardIterator last, Size count, T const& value, BinaryPredicate pred) {
 		typedef sprout::value_iterator<T const&> iterator;
+		typedef typename std::iterator_traits<iterator>::difference_type difference_type;
 		return sprout::search(
 			first, last,
-			iterator(value, static_cast<typename std::iterator_traits<iterator>::difference_type>(count)), iterator(value, 0),
+			iterator(value, static_cast<difference_type>(count)), iterator(value, 0),
 			pred
 			);
 	}
diff --git a/sprout/algorithm/tristate_lexicographical_compare.hpp b/sprout/algorithm/tristate_lexicographical_compare.hpp
index af41d638..ba96c174 100644
--- a/sprout/algorithm/tristate_lexicographical_compare.hpp
+++ b/sprout/algorithm/tristate_lexicographical_compare.hpp
@@ -129,7 +129,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR int
 		tristate_lexicographical_compare(
 			InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, Compare comp,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator1, InputIterator2> type;
@@ -298,7 +298,7 @@ namespace sprout {
 			InputIterator1 first1, InputIterator1 last1, T1 const& delim1,
 			InputIterator2 first2, InputIterator2 last2, T2 const& delim2,
 			Compare comp,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator1, InputIterator2> type;
diff --git a/sprout/detail/algorithm/count_n.hpp b/sprout/detail/algorithm/count_n.hpp
index b032c2d5..3f26f3ce 100644
--- a/sprout/detail/algorithm/count_n.hpp
+++ b/sprout/detail/algorithm/count_n.hpp
@@ -46,7 +46,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR typename std::iterator_traits<InputIterator>::difference_type
 		count_n(
 			InputIterator first, typename std::iterator_traits<InputIterator>::difference_type n, T const& value,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, typename std::iterator_traits<InputIterator>::difference_type> type;
diff --git a/sprout/detail/algorithm/count_n_if.hpp b/sprout/detail/algorithm/count_n_if.hpp
index 791477e7..d7cb6404 100644
--- a/sprout/detail/algorithm/count_n_if.hpp
+++ b/sprout/detail/algorithm/count_n_if.hpp
@@ -46,7 +46,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR typename std::iterator_traits<InputIterator>::difference_type
 		count_n_if(
 			InputIterator first, typename std::iterator_traits<InputIterator>::difference_type n, Predicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, typename std::iterator_traits<InputIterator>::difference_type> type;
diff --git a/sprout/detail/algorithm/overlap_count.hpp b/sprout/detail/algorithm/overlap_count.hpp
index ca149a89..2323a4bf 100644
--- a/sprout/detail/algorithm/overlap_count.hpp
+++ b/sprout/detail/algorithm/overlap_count.hpp
@@ -91,7 +91,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR typename std::iterator_traits<InputIterator>::difference_type
 		overlap_count(
 			InputIterator first, InputIterator last, BinaryPredicate pred,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::tuples::tuple<InputIterator, typename std::iterator_traits<InputIterator>::value_type, typename std::iterator_traits<InputIterator>::difference_type> type;
diff --git a/sprout/detail/algorithm/search_one.hpp b/sprout/detail/algorithm/search_one.hpp
index 16ef1ca1..6c9eddb5 100644
--- a/sprout/detail/algorithm/search_one.hpp
+++ b/sprout/detail/algorithm/search_one.hpp
@@ -86,7 +86,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR ForwardIterator1
 		search_one(
 			ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			typedef sprout::tuples::tuple<ForwardIterator1, ForwardIterator2, bool> type;
diff --git a/sprout/iterator/bytes_iterator.hpp b/sprout/iterator/bytes_iterator.hpp
index a4ef983b..23a52899 100644
--- a/sprout/iterator/bytes_iterator.hpp
+++ b/sprout/iterator/bytes_iterator.hpp
@@ -28,7 +28,7 @@ namespace sprout {
 		>
 	{
 	public:
-		typedef Iterator base_type;
+		typedef Iterator iterator_type;
 		typedef Traits traits_type;
 	private:
 		typedef std::iterator<
@@ -37,34 +37,34 @@ namespace sprout {
 			std::ptrdiff_t,
 			void,
 			unsigned char
-		> iterator_type;
+		> base_type;
 	public:
-		typedef typename iterator_type::iterator_category iterator_category;
-		typedef typename iterator_type::value_type value_type;
-		typedef typename iterator_type::difference_type difference_type;
-		typedef typename iterator_type::pointer pointer;
-		typedef typename iterator_type::reference reference;
+		typedef typename base_type::iterator_category iterator_category;
+		typedef typename base_type::value_type value_type;
+		typedef typename base_type::difference_type difference_type;
+		typedef typename base_type::pointer pointer;
+		typedef typename base_type::reference reference;
 	private:
 		struct next_tag {};
 		struct prev_tag {};
 		struct ra_tag {};
 	private:
-		base_type it_;
+		iterator_type it_;
 		difference_type i_;
 	private:
-		SPROUT_CONSTEXPR bytes_iterator(base_type it, difference_type i)
+		SPROUT_CONSTEXPR bytes_iterator(iterator_type it, difference_type i)
 			: it_(it)
 			, i_(i)
 		{}
-		SPROUT_CONSTEXPR bytes_iterator(base_type it, difference_type i, next_tag)
+		SPROUT_CONSTEXPR bytes_iterator(iterator_type it, difference_type i, next_tag)
 			: it_(i / traits_type::size() == 0 ? it : sprout::next(it))
 			, i_(i % traits_type::size())
 		{}
-		SPROUT_CONSTEXPR bytes_iterator(base_type it, difference_type i, prev_tag)
+		SPROUT_CONSTEXPR bytes_iterator(iterator_type it, difference_type i, prev_tag)
 			: it_((i + 1 - traits_type::size()) / traits_type::size() == 0 ? it : sprout::prev(it))
 			, i_(i % traits_type::size() + traits_type::size())
 		{}
-		SPROUT_CONSTEXPR bytes_iterator(base_type it, difference_type i, ra_tag)
+		SPROUT_CONSTEXPR bytes_iterator(iterator_type it, difference_type i, ra_tag)
 			: it_(i >= 0
 				? sprout::next(it, i / traits_type::size())
 				: sprout::next(it, (i + 1 - traits_type::size()) / traits_type::size())
@@ -76,14 +76,18 @@ namespace sprout {
 		{}
 	public:
 		SPROUT_CONSTEXPR bytes_iterator()
-			: it_()
-			, i_()
+			: it_(), i_()
 		{}
 		bytes_iterator(bytes_iterator const&) = default;
-		explicit SPROUT_CONSTEXPR bytes_iterator(base_type it)
-			: it_(it)
-			, i_()
+		explicit SPROUT_CONSTEXPR bytes_iterator(iterator_type it)
+			: it_(it), i_()
 		{}
+		SPROUT_CONSTEXPR iterator_type const& base() const {
+			return it_;
+		}
+		SPROUT_CONSTEXPR difference_type index() const {
+			return i_;
+		}
 		SPROUT_CONSTEXPR bytes_iterator next() const {
 			return bytes_iterator(it_, i_ + 1, next_tag());
 		}
@@ -96,24 +100,6 @@ namespace sprout {
 			sprout::swap(it_, other.it_);
 			sprout::swap(i_, other.i_);
 		}
-		friend SPROUT_CONSTEXPR bool operator==(bytes_iterator const& lhs, bytes_iterator const& rhs) {
-			return lhs.it_ == rhs.it_ && lhs.i_ == rhs.i_;
-		}
-		friend SPROUT_CONSTEXPR bool operator!=(bytes_iterator const& lhs, bytes_iterator const& rhs) {
-			return !(lhs == rhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator<(bytes_iterator const& lhs, bytes_iterator const& rhs) {
-			return lhs.it_ < rhs.it_ || (lhs.it_ == rhs.it_ && lhs.i_ < rhs.i_);
-		}
-		friend SPROUT_CONSTEXPR bool operator>(bytes_iterator const& lhs, bytes_iterator const& rhs) {
-			return rhs < lhs;
-		}
-		friend SPROUT_CONSTEXPR bool operator<=(bytes_iterator const& lhs, bytes_iterator const& rhs) {
-			return !(rhs < lhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator>=(bytes_iterator const& lhs, bytes_iterator const& rhs) {
-			return !(lhs < rhs);
-		}
 		SPROUT_CONSTEXPR reference operator*() const {
 			return traits_type::get_byte(*it_, i_);
 		}
@@ -156,14 +142,60 @@ namespace sprout {
 		SPROUT_CONSTEXPR reference operator[](difference_type n) const {
 			return *(*this + n);
 		}
-		friend SPROUT_CONSTEXPR difference_type operator-(bytes_iterator const& lhs, bytes_iterator const& rhs) {
-			return (lhs.it_ - rhs.it_) * traits_type::size() + (lhs.i_ - rhs.i_);
-		}
-		friend SPROUT_CONSTEXPR bytes_iterator operator+(difference_type n, bytes_iterator const& it) {
-			return it + n;
-		}
 	};
 
+	template<typename Iterator1, typename Iterator2, typename Traits>
+	inline SPROUT_CONSTEXPR bool
+	operator==(sprout::bytes_iterator<Iterator1, Traits> const& lhs, sprout::bytes_iterator<Iterator2, Traits> const& rhs) {
+		return lhs.base() == rhs.base() && lhs.index() == rhs.index();
+	}
+	template<typename Iterator1, typename Iterator2, typename Traits>
+	inline SPROUT_CONSTEXPR bool
+	operator!=(sprout::bytes_iterator<Iterator1, Traits> const& lhs, sprout::bytes_iterator<Iterator2, Traits> const& rhs) {
+		return !(lhs == rhs);
+	}
+	template<typename Iterator1, typename Iterator2, typename Traits>
+	inline SPROUT_CONSTEXPR bool
+	operator<(sprout::bytes_iterator<Iterator1, Traits> const& lhs, sprout::bytes_iterator<Iterator2, Traits> const& rhs) {
+		return lhs.base() < rhs.base() || (lhs.base() == rhs.base() && lhs.index() < rhs.index());
+	}
+	template<typename Iterator1, typename Iterator2, typename Traits>
+	inline SPROUT_CONSTEXPR bool
+	operator>(sprout::bytes_iterator<Iterator1, Traits> const& lhs, sprout::bytes_iterator<Iterator2, Traits> const& rhs) {
+		return rhs < lhs;
+	}
+	template<typename Iterator1, typename Iterator2, typename Traits>
+	inline SPROUT_CONSTEXPR bool
+	operator<=(sprout::bytes_iterator<Iterator1, Traits> const& lhs, sprout::bytes_iterator<Iterator2, Traits> const& rhs) {
+		return !(rhs < lhs);
+	}
+	template<typename Iterator1, typename Iterator2, typename Traits>
+	inline SPROUT_CONSTEXPR bool
+	operator>=(sprout::bytes_iterator<Iterator1, Traits> const& lhs, sprout::bytes_iterator<Iterator2, Traits> const& rhs) {
+		return !(lhs < rhs);
+	}
+	template<typename Iterator1, typename Iterator2, typename Traits>
+	inline SPROUT_CONSTEXPR decltype(std::declval<Iterator1>() - std::declval<Iterator2>())
+	operator-(sprout::bytes_iterator<Iterator1, Traits> const& lhs, sprout::bytes_iterator<Iterator2, Traits> const& rhs) {
+		return (lhs.base() - rhs.base()) * Traits::size() + (lhs.index() - rhs.index());
+	}
+	template<typename Iterator, typename Traits>
+	inline SPROUT_CONSTEXPR sprout::bytes_iterator<Iterator, Traits>
+	operator+(typename sprout::bytes_iterator<Iterator, Traits>::difference_type n, sprout::bytes_iterator<Iterator, Traits> const& it) {
+		return it + n;
+	}
+
+	//
+	// swap
+	//
+	template<typename Iterator, typename Traits>
+	inline void
+	swap(sprout::bytes_iterator<Iterator, Traits>& lhs, sprout::bytes_iterator<Iterator, Traits>& rhs)
+	SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(lhs.swap(rhs)))
+	{
+		lhs.swap(rhs);
+	}
+
 	//
 	// make_bytes_iterator
 	//
@@ -179,14 +211,18 @@ namespace sprout {
 	}
 
 	//
-	// swap
+	// make_big_bytes_iterator
+	// make_little_bytes_iterator
 	//
-	template<typename Iterator, typename Traits>
-	inline void
-	swap(sprout::bytes_iterator<Iterator, Traits>& lhs, sprout::bytes_iterator<Iterator, Traits>& rhs)
-	SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(lhs.swap(rhs)))
-	{
-		lhs.swap(rhs);
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR sprout::bytes_iterator<Iterator, sprout::big_endian_traits<typename std::iterator_traits<Iterator>::value_type> >
+	make_big_bytes_iterator(Iterator it) {
+		return sprout::bytes_iterator<Iterator, sprout::big_endian_traits<typename std::iterator_traits<Iterator>::value_type> >(it);
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR sprout::bytes_iterator<Iterator, sprout::little_endian_traits<typename std::iterator_traits<Iterator>::value_type> >
+	make_little_bytes_iterator(Iterator it) {
+		return sprout::bytes_iterator<Iterator, sprout::little_endian_traits<typename std::iterator_traits<Iterator>::value_type> >(it);
 	}
 
 	//
diff --git a/sprout/iterator/dft_iterator.hpp b/sprout/iterator/dft_iterator.hpp
index baff8a02..c270b37f 100644
--- a/sprout/iterator/dft_iterator.hpp
+++ b/sprout/iterator/dft_iterator.hpp
@@ -76,24 +76,6 @@ namespace sprout {
 			sprout::swap(index_, other.index_);
 			sprout::swap(size_, other.size_);
 		}
-		friend SPROUT_CONSTEXPR bool operator==(dft_iterator const& lhs, dft_iterator const& rhs) {
-			return lhs.index_ == rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR bool operator!=(dft_iterator const& lhs, dft_iterator const& rhs) {
-			return !(lhs == rhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator<(dft_iterator const& lhs, dft_iterator const& rhs) {
-			return lhs.index_ < rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR bool operator>(dft_iterator const& lhs, dft_iterator const& rhs) {
-			return rhs < lhs;
-		}
-		friend SPROUT_CONSTEXPR bool operator<=(dft_iterator const& lhs, dft_iterator const& rhs) {
-			return !(rhs < lhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator>=(dft_iterator const& lhs, dft_iterator const& rhs) {
-			return !(lhs < rhs);
-		}
 		SPROUT_CONSTEXPR reference operator*() const {
 			return sprout::detail::dft_element_impl(first_, last_, index_, size_);
 		}
@@ -136,14 +118,49 @@ namespace sprout {
 		SPROUT_CONSTEXPR reference operator[](difference_type n) const {
 			return sprout::detail::dft_element_impl(first_, last_, index_ + n, size_);
 		}
-		friend SPROUT_CONSTEXPR difference_type operator-(dft_iterator const& lhs, dft_iterator const& rhs) {
-			return lhs.index_ - rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR dft_iterator operator+(difference_type n, dft_iterator const& it) {
-			return it + n;
-		}
 	};
 
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator==(sprout::dft_iterator<Iterator> const& lhs, sprout::dft_iterator<Iterator> const& rhs) {
+		return lhs.index() == rhs.index();
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator!=(sprout::dft_iterator<Iterator> const& lhs, sprout::dft_iterator<Iterator> const& rhs) {
+		return !(lhs == rhs);
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator<(sprout::dft_iterator<Iterator> const& lhs, sprout::dft_iterator<Iterator> const& rhs) {
+		return lhs.index_ < rhs.index_;
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator>(sprout::dft_iterator<Iterator> const& lhs, sprout::dft_iterator<Iterator> const& rhs) {
+		return rhs < lhs;
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator<=(sprout::dft_iterator<Iterator> const& lhs, sprout::dft_iterator<Iterator> const& rhs) {
+		return !(rhs < lhs);
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator>=(sprout::dft_iterator<Iterator> const& lhs, sprout::dft_iterator<Iterator> const& rhs) {
+		return !(lhs < rhs);
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR typename sprout::dft_iterator<Iterator>::difference_type
+	operator-(sprout::dft_iterator<Iterator> const& lhs, sprout::dft_iterator<Iterator> const& rhs) {
+		return lhs.index() - rhs.index();
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR sprout::dft_iterator<Iterator>
+	operator+(typename sprout::dft_iterator<Iterator>::difference_type n, sprout::dft_iterator<Iterator> const& it) {
+		return it + n;
+	}
+
 	//
 	// make_dft_iterator
 	//
diff --git a/sprout/iterator/distance.hpp b/sprout/iterator/distance.hpp
index f65e9404..e66559a2 100644
--- a/sprout/iterator/distance.hpp
+++ b/sprout/iterator/distance.hpp
@@ -63,7 +63,7 @@ namespace sprout {
 		}
 		template<typename InputIterator>
 		inline SPROUT_CONSTEXPR typename std::iterator_traits<InputIterator>::difference_type
-		iterator_distance(InputIterator first, InputIterator last, void*) {
+		iterator_distance(InputIterator first, InputIterator last, std::input_iterator_tag*) {
 			typedef sprout::pair<InputIterator, typename std::iterator_traits<InputIterator>::difference_type> type;
 			return sprout::iterator_detail::iterator_distance_impl(type(first, 0), last, 1).second;
 		}
diff --git a/sprout/iterator/generator_iterator.hpp b/sprout/iterator/generator_iterator.hpp
index 5e3387db..25890ad5 100644
--- a/sprout/iterator/generator_iterator.hpp
+++ b/sprout/iterator/generator_iterator.hpp
@@ -5,7 +5,6 @@
 #include <limits>
 #include <iterator>
 #include <utility>
-#include <stdexcept>
 #include <type_traits>
 #include <sprout/config.hpp>
 #include <sprout/generator/functions.hpp>
@@ -34,16 +33,16 @@ namespace sprout {
 		typedef value_type* pointer;
 	private:
 		generator_type gen_;
-		difference_type count_;
+		difference_type index_;
 	public:
 		SPROUT_CONSTEXPR generator_iterator()
-			: gen_(), count_()
+			: gen_(), index_()
 		{}
 		explicit SPROUT_CONSTEXPR generator_iterator(
 			generator_type const& gen,
-			difference_type count = std::numeric_limits<difference_type>::max()
+			difference_type index = std::numeric_limits<difference_type>::max()
 			)
-			: gen_(gen), count_(count)
+			: gen_(gen), index_(index)
 		{}
 		generator_type& generator() {
 			return gen_;
@@ -51,14 +50,11 @@ namespace sprout {
 		SPROUT_CONSTEXPR generator_type const& generator() const {
 			return gen_;
 		}
-		SPROUT_CONSTEXPR difference_type count() const {
-			return count_;
+		SPROUT_CONSTEXPR difference_type index() const {
+			return index_;
 		}
 		SPROUT_CONSTEXPR generator_iterator operator()() const {
-			return count_ != 0
-				? generator_iterator(sprout::generators::next_generator(gen_)(), count_ > 0 ? count_ - 1 : count_)
-				: throw std::out_of_range("generator_iterator<>: increment at out of range")
-				;
+			return generator_iterator(sprout::generators::next_generator(gen_)(), index_ - 1);
 		}
 		SPROUT_CONSTEXPR generator_iterator next() const {
 			return (*this)();
@@ -73,35 +69,10 @@ namespace sprout {
 		SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(sprout::swap(gen_, other.gen_)))
 		{
 			sprout::swap(gen_, other.gen_);
-			sprout::swap(count_, other.count_);
-		}
-		friend SPROUT_CONSTEXPR bool operator==(generator_iterator const& lhs, generator_iterator const& rhs) {
-			return lhs.count_ == rhs.count_;
-		}
-		friend SPROUT_CONSTEXPR bool operator!=(generator_iterator const& lhs, generator_iterator const& rhs) {
-			return !(lhs == rhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator<(generator_iterator const& lhs, generator_iterator const& rhs) {
-			typedef typename std::make_unsigned<difference_type>::type unsigned_type;
-			return static_cast<unsigned_type>(lhs.count_) > static_cast<unsigned_type>(rhs.count_);
-		}
-		friend SPROUT_CONSTEXPR bool operator>(generator_iterator const& lhs, generator_iterator const& rhs) {
-			return rhs < lhs;
-		}
-		friend SPROUT_CONSTEXPR bool operator<=(generator_iterator const& lhs, generator_iterator const& rhs) {
-			return !(rhs < lhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator>=(generator_iterator const& lhs, generator_iterator const& rhs) {
-			return !(lhs < rhs);
+			sprout::swap(index_, other.index_);
 		}
 		SPROUT_CONSTEXPR reference operator*() const {
-			return count_ != 0
-				? sprout::generators::generated_value(gen_)
-				: (
-					throw std::out_of_range("generator_iterator<>: dereference at out of range"),
-					sprout::generators::generated_value(gen_)
-					)
-				;
+			return sprout::generators::generated_value(gen_);
 		}
 		SPROUT_CONSTEXPR pointer operator->() const {
 			return &*(*this);
@@ -116,11 +87,44 @@ namespace sprout {
 			++*this;
 			return result;
 		}
-		friend SPROUT_CONSTEXPR difference_type operator-(generator_iterator const& lhs, generator_iterator const& rhs) {
-			return rhs.count_ - lhs.count_;
-		}
 	};
 
+	template<typename Generator>
+	inline SPROUT_CONSTEXPR bool
+	operator==(sprout::generator_iterator<Generator> const& lhs, sprout::generator_iterator<Generator> const& rhs) {
+		return lhs.index() == rhs.index();
+	}
+	template<typename Generator>
+	inline SPROUT_CONSTEXPR bool
+	operator!=(sprout::generator_iterator<Generator> const& lhs, sprout::generator_iterator<Generator> const& rhs) {
+		return !(lhs == rhs);
+	}
+	template<typename Generator>
+	inline SPROUT_CONSTEXPR bool
+	operator<(sprout::generator_iterator<Generator> const& lhs, sprout::generator_iterator<Generator> const& rhs) {
+		return rhs.index() < lhs.index();
+	}
+	template<typename Generator>
+	inline SPROUT_CONSTEXPR bool
+	operator>(sprout::generator_iterator<Generator> const& lhs, sprout::generator_iterator<Generator> const& rhs) {
+		return rhs < lhs;
+	}
+	template<typename Generator>
+	inline SPROUT_CONSTEXPR bool
+	operator<=(sprout::generator_iterator<Generator> const& lhs, sprout::generator_iterator<Generator> const& rhs) {
+		return !(rhs < lhs);
+	}
+	template<typename Generator>
+	inline SPROUT_CONSTEXPR bool
+	operator>=(sprout::generator_iterator<Generator> const& lhs, sprout::generator_iterator<Generator> const& rhs) {
+		return !(lhs < rhs);
+	}
+	template<typename Generator>
+	inline SPROUT_CONSTEXPR typename sprout::generator_iterator<Generator>::difference_type
+	operator-(sprout::generator_iterator<Generator> const& lhs, sprout::generator_iterator<Generator> const& rhs) {
+		return rhs.index() - lhs.index();
+	}
+
 	//
 	// swap
 	//
diff --git a/sprout/iterator/idft_iterator.hpp b/sprout/iterator/idft_iterator.hpp
index a3949897..2ce5d95f 100644
--- a/sprout/iterator/idft_iterator.hpp
+++ b/sprout/iterator/idft_iterator.hpp
@@ -76,24 +76,6 @@ namespace sprout {
 			sprout::swap(index_, other.index_);
 			sprout::swap(size_, other.size_);
 		}
-		friend SPROUT_CONSTEXPR bool operator==(idft_iterator const& lhs, idft_iterator const& rhs) {
-			return lhs.index_ == rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR bool operator!=(idft_iterator const& lhs, idft_iterator const& rhs) {
-			return !(lhs == rhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator<(idft_iterator const& lhs, idft_iterator const& rhs) {
-			return lhs.index_ < rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR bool operator>(idft_iterator const& lhs, idft_iterator const& rhs) {
-			return rhs < lhs;
-		}
-		friend SPROUT_CONSTEXPR bool operator<=(idft_iterator const& lhs, idft_iterator const& rhs) {
-			return !(rhs < lhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator>=(idft_iterator const& lhs, idft_iterator const& rhs) {
-			return !(lhs < rhs);
-		}
 		SPROUT_CONSTEXPR reference operator*() const {
 			return sprout::detail::idft_element_impl(first_, last_, index_, size_);
 		}
@@ -136,14 +118,49 @@ namespace sprout {
 		SPROUT_CONSTEXPR reference operator[](difference_type n) const {
 			return sprout::detail::idft_element_impl(first_, last_, index_ + n, size_);
 		}
-		friend SPROUT_CONSTEXPR difference_type operator-(idft_iterator const& lhs, idft_iterator const& rhs) {
-			return lhs.index_ - rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR idft_iterator operator+(difference_type n, idft_iterator const& it) {
-			return it + n;
-		}
 	};
 
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator==(sprout::idft_iterator<Iterator> const& lhs, sprout::idft_iterator<Iterator> const& rhs) {
+		return lhs.index() == rhs.index();
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator!=(sprout::idft_iterator<Iterator> const& lhs, sprout::idft_iterator<Iterator> const& rhs) {
+		return !(lhs == rhs);
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator<(sprout::idft_iterator<Iterator> const& lhs, sprout::idft_iterator<Iterator> const& rhs) {
+		return lhs.index_ < rhs.index_;
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator>(sprout::idft_iterator<Iterator> const& lhs, sprout::idft_iterator<Iterator> const& rhs) {
+		return rhs < lhs;
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator<=(sprout::idft_iterator<Iterator> const& lhs, sprout::idft_iterator<Iterator> const& rhs) {
+		return !(rhs < lhs);
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR bool
+	operator>=(sprout::idft_iterator<Iterator> const& lhs, sprout::idft_iterator<Iterator> const& rhs) {
+		return !(lhs < rhs);
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR typename sprout::idft_iterator<Iterator>::difference_type
+	operator-(sprout::idft_iterator<Iterator> const& lhs, sprout::idft_iterator<Iterator> const& rhs) {
+		return lhs.index() - rhs.index();
+	}
+	template<typename Iterator>
+	inline SPROUT_CONSTEXPR sprout::idft_iterator<Iterator>
+	operator+(typename sprout::idft_iterator<Iterator>::difference_type n, sprout::idft_iterator<Iterator> const& it) {
+		return it + n;
+	}
+
 	//
 	// make_idft_iterator
 	//
diff --git a/sprout/iterator/index_iterator.hpp b/sprout/iterator/index_iterator.hpp
index 92432954..cf95dd25 100644
--- a/sprout/iterator/index_iterator.hpp
+++ b/sprout/iterator/index_iterator.hpp
@@ -78,25 +78,21 @@ namespace sprout {
 		typedef typename traits_type::size_type size_type;
 	private:
 		holder_type holder_;
-		size_type index_;
+		difference_type index_;
 	private:
-		SPROUT_CONSTEXPR index_iterator(holder_type const& r, size_type index)
-			: holder_(r)
-			, index_(index)
+		SPROUT_CONSTEXPR index_iterator(holder_type const& r, difference_type index)
+			: holder_(r), index_(index)
 		{}
 	public:
 		SPROUT_CONSTEXPR index_iterator()
-			: holder_()
-			, index_()
+			: holder_(), index_()
 		{}
 		index_iterator(index_iterator const&) = default;
 		explicit SPROUT_CONSTEXPR index_iterator(typename holder_type::param_type p)
-			: holder_(p)
-			, index_(0)
+			: holder_(p), index_(0)
 		{}
-		SPROUT_CONSTEXPR index_iterator(typename holder_type::param_type p, size_type index)
-			: holder_(p)
-			, index_(index)
+		SPROUT_CONSTEXPR index_iterator(typename holder_type::param_type p, difference_type index)
+			: holder_(p), index_(index)
 		{}
 		operator index_iterator<const_container_type>() const {
 			return index_iterator<const_container_type>(holder_.get(), index_);
@@ -104,7 +100,7 @@ namespace sprout {
 		SPROUT_CONSTEXPR typename holder_type::mutable_or_const_reference base() const {
 			return holder_.get();
 		}
-		SPROUT_CONSTEXPR size_type index() const {
+		SPROUT_CONSTEXPR difference_type index() const {
 			return index_;
 		}
 		SPROUT_CONSTEXPR index_iterator next() const {
@@ -119,24 +115,6 @@ namespace sprout {
 			sprout::swap(holder_, other.holder_);
 			sprout::swap(index_, other.index_);
 		}
-		friend SPROUT_CONSTEXPR bool operator==(index_iterator const& lhs, index_iterator const& rhs) {
-			return lhs.index_ == rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR bool operator!=(index_iterator const& lhs, index_iterator const& rhs) {
-			return !(lhs == rhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator<(index_iterator const& lhs, index_iterator const& rhs) {
-			return lhs.index_ < rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR bool operator>(index_iterator const& lhs, index_iterator const& rhs) {
-			return rhs < lhs;
-		}
-		friend SPROUT_CONSTEXPR bool operator<=(index_iterator const& lhs, index_iterator const& rhs) {
-			return !(rhs < lhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator>=(index_iterator const& lhs, index_iterator const& rhs) {
-			return !(lhs < rhs);
-		}
 		SPROUT_CONSTEXPR reference operator*() const {
 			return holder_.get()[index_];
 		}
@@ -182,14 +160,73 @@ namespace sprout {
 		SPROUT_CONSTEXPR reference operator[](difference_type n) const {
 			return holder_.get()[index_ + n];
 		}
-		friend SPROUT_CONSTEXPR difference_type operator-(index_iterator const& lhs, index_iterator const& rhs) {
-			return static_cast<difference_type>(lhs.index_) - static_cast<difference_type>(rhs.index_);
-		}
-		friend SPROUT_CONSTEXPR index_iterator operator+(difference_type n, index_iterator const& it) {
-			return it + n;
-		}
 	};
 
+	template<typename Container1, typename Container2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<Container1>::type, typename std::decay<Container2>::type>::value,
+		bool
+	>::type
+	operator==(sprout::index_iterator<Container1, C> const& lhs, sprout::index_iterator<Container2, C> const& rhs) {
+		return lhs.index() == rhs.index();
+	}
+	template<typename Container1, typename Container2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<Container1>::type, typename std::decay<Container2>::type>::value,
+		bool
+	>::type
+	operator!=(sprout::index_iterator<Container1, C> const& lhs, sprout::index_iterator<Container2, C> const& rhs) {
+		return !(lhs == rhs);
+	}
+	template<typename Container1, typename Container2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<Container1>::type, typename std::decay<Container2>::type>::value,
+		bool
+	>::type
+	operator<(sprout::index_iterator<Container1, C> const& lhs, sprout::index_iterator<Container2, C> const& rhs) {
+		return lhs.index() < rhs.index();
+	}
+	template<typename Container1, typename Container2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<Container1>::type, typename std::decay<Container2>::type>::value,
+		bool
+	>::type
+	operator>(sprout::index_iterator<Container1, C> const& lhs, sprout::index_iterator<Container2, C> const& rhs) {
+		return rhs < lhs;
+	}
+	template<typename Container1, typename Container2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<Container1>::type, typename std::decay<Container2>::type>::value,
+		bool
+	>::type
+	operator<=(sprout::index_iterator<Container1, C> const& lhs, sprout::index_iterator<Container2, C> const& rhs) {
+		return !(rhs < lhs);
+	}
+	template<typename Container1, typename Container2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<Container1>::type, typename std::decay<Container2>::type>::value,
+		bool
+	>::type
+	operator>=(sprout::index_iterator<Container1, C> const& lhs, sprout::index_iterator<Container2, C> const& rhs) {
+		return !(lhs < rhs);
+	}
+	template<typename Container1, typename Container2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<Container1>::type, typename std::decay<Container2>::type>::value,
+		decltype(
+			std::declval<typename std::iterator_traits<sprout::index_iterator<Container1, C> >::difference_type>()
+				- std::declval<typename std::iterator_traits<sprout::index_iterator<Container2, C> >::difference_type>()
+			)
+	>::type
+	operator-(sprout::index_iterator<Container1, C> const& lhs, sprout::index_iterator<Container2, C> const& rhs) {
+		return lhs.index() - rhs.index();
+	}
+	template<typename Container, bool C>
+	inline SPROUT_CONSTEXPR sprout::index_iterator<Container, C>
+	operator+(typename sprout::index_iterator<Container, C>::difference_type n, sprout::index_iterator<Container, C> const& it) {
+		return it + n;
+	}
+
 	//
 	// swap
 	//
diff --git a/sprout/iterator/next.hpp b/sprout/iterator/next.hpp
index 3d74f2ee..e72ddc34 100644
--- a/sprout/iterator/next.hpp
+++ b/sprout/iterator/next.hpp
@@ -25,7 +25,7 @@ namespace sprout {
 		}
 		template<typename ForwardIterator>
 		inline SPROUT_CONSTEXPR ForwardIterator
-		next_impl(ForwardIterator const& it, void*) {
+		next_impl(ForwardIterator const& it, std::forward_iterator_tag*) {
 			return std::next(it);
 		}
 
@@ -79,7 +79,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR ForwardIterator
 		next_impl_1(
 			ForwardIterator const& it, typename std::iterator_traits<ForwardIterator>::difference_type n,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			return n == 0 ? it
@@ -95,7 +95,7 @@ namespace sprout {
 		>::type
 		next_impl(
 			ForwardIterator const& it, typename std::iterator_traits<ForwardIterator>::difference_type n,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			typedef typename std::iterator_traits<ForwardIterator>::iterator_category* category;
@@ -108,7 +108,7 @@ namespace sprout {
 		>::type
 		next_impl(
 			ForwardIterator const& it, typename std::iterator_traits<ForwardIterator>::difference_type n,
-			void*
+			std::forward_iterator_tag*
 			)
 		{
 			return std::next(it, n);
diff --git a/sprout/iterator/prev.hpp b/sprout/iterator/prev.hpp
index 4dd65906..48794810 100644
--- a/sprout/iterator/prev.hpp
+++ b/sprout/iterator/prev.hpp
@@ -24,7 +24,7 @@ namespace sprout {
 		}
 		template<typename BidirectionalIterator>
 		inline SPROUT_CONSTEXPR BidirectionalIterator
-		prev_impl(BidirectionalIterator const& it, void*) {
+		prev_impl(BidirectionalIterator const& it, std::bidirectional_iterator_tag*) {
 			return std::prev(it);
 		}
 
@@ -69,7 +69,7 @@ namespace sprout {
 		>::type
 		prev_impl(
 			BidirectionalIterator const& it, typename std::iterator_traits<BidirectionalIterator>::difference_type n,
-			void*
+			std::bidirectional_iterator_tag*
 			)
 		{
 			return n == 0 ? it
@@ -84,7 +84,7 @@ namespace sprout {
 		>::type
 		prev_impl(
 			BidirectionalIterator const& it, typename std::iterator_traits<BidirectionalIterator>::difference_type n,
-			void*
+			std::bidirectional_iterator_tag*
 			)
 		{
 			return std::prev(it, n);
diff --git a/sprout/iterator/ptr_index_iterator.hpp b/sprout/iterator/ptr_index_iterator.hpp
index 5f6ff3af..d4375dd7 100644
--- a/sprout/iterator/ptr_index_iterator.hpp
+++ b/sprout/iterator/ptr_index_iterator.hpp
@@ -52,17 +52,14 @@ namespace sprout {
 		difference_type index_;
 	public:
 		SPROUT_CONSTEXPR ptr_index_iterator()
-			: p_()
-			, index_()
+			: p_(), index_()
 		{}
 		ptr_index_iterator(ptr_index_iterator const&) = default;
 		explicit SPROUT_CONSTEXPR ptr_index_iterator(pointer p)
-			: p_(p)
-			, index_(0)
+			: p_(p), index_(0)
 		{}
 		SPROUT_CONSTEXPR ptr_index_iterator(pointer p, difference_type index)
-			: p_(p)
-			, index_(index)
+			: p_(p), index_(index)
 		{}
 		operator ptr_index_iterator<const_pointer>() const {
 			return ptr_index_iterator<const_pointer>(p_, index_);
@@ -88,24 +85,6 @@ namespace sprout {
 			sprout::swap(p_, other.p_);
 			sprout::swap(index_, other.index_);
 		}
-		friend SPROUT_CONSTEXPR bool operator==(ptr_index_iterator const& lhs, ptr_index_iterator const& rhs) {
-			return lhs.index_ == rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR bool operator!=(ptr_index_iterator const& lhs, ptr_index_iterator const& rhs) {
-			return !(lhs == rhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator<(ptr_index_iterator const& lhs, ptr_index_iterator const& rhs) {
-			return lhs.index_ < rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR bool operator>(ptr_index_iterator const& lhs, ptr_index_iterator const& rhs) {
-			return rhs < lhs;
-		}
-		friend SPROUT_CONSTEXPR bool operator<=(ptr_index_iterator const& lhs, ptr_index_iterator const& rhs) {
-			return !(rhs < lhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator>=(ptr_index_iterator const& lhs, ptr_index_iterator const& rhs) {
-			return !(lhs < rhs);
-		}
 		SPROUT_CONSTEXPR reference operator*() const {
 			return p_[index_];
 		}
@@ -151,14 +130,73 @@ namespace sprout {
 		SPROUT_CONSTEXPR reference operator[](difference_type n) const {
 			return p_[index_ + n];
 		}
-		friend SPROUT_CONSTEXPR difference_type operator-(ptr_index_iterator const& lhs, ptr_index_iterator const& rhs) {
-			return lhs.index_ - rhs.index_;
-		}
-		friend SPROUT_CONSTEXPR ptr_index_iterator operator+(difference_type n, ptr_index_iterator const& it) {
-			return it + n;
-		}
 	};
 
+	template<typename T1, typename T2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<T1>::type, typename std::decay<T2>::type>::value,
+		bool
+	>::type
+	operator==(sprout::ptr_index_iterator<T1, C> const& lhs, sprout::ptr_index_iterator<T2, C> const& rhs) {
+		return lhs.index() == rhs.index();
+	}
+	template<typename T1, typename T2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<T1>::type, typename std::decay<T2>::type>::value,
+		bool
+	>::type
+	operator!=(sprout::ptr_index_iterator<T1, C> const& lhs, sprout::ptr_index_iterator<T2, C> const& rhs) {
+		return !(lhs == rhs);
+	}
+	template<typename T1, typename T2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<T1>::type, typename std::decay<T2>::type>::value,
+		bool
+	>::type
+	operator<(sprout::ptr_index_iterator<T1, C> const& lhs, sprout::ptr_index_iterator<T2, C> const& rhs) {
+		return lhs.index() < rhs.index();
+	}
+	template<typename T1, typename T2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<T1>::type, typename std::decay<T2>::type>::value,
+		bool
+	>::type
+	operator>(sprout::ptr_index_iterator<T1, C> const& lhs, sprout::ptr_index_iterator<T2, C> const& rhs) {
+		return rhs < lhs;
+	}
+	template<typename T1, typename T2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<T1>::type, typename std::decay<T2>::type>::value,
+		bool
+	>::type
+	operator<=(sprout::ptr_index_iterator<T1, C> const& lhs, sprout::ptr_index_iterator<T2, C> const& rhs) {
+		return !(rhs < lhs);
+	}
+	template<typename T1, typename T2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<T1>::type, typename std::decay<T2>::type>::value,
+		bool
+	>::type
+	operator>=(sprout::ptr_index_iterator<T1, C> const& lhs, sprout::ptr_index_iterator<T2, C> const& rhs) {
+		return !(lhs < rhs);
+	}
+	template<typename T1, typename T2, bool C>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		std::is_same<typename std::decay<T1>::type, typename std::decay<T2>::type>::value,
+		decltype(
+			std::declval<typename std::iterator_traits<sprout::ptr_index_iterator<T1, C> >::difference_type>()
+				- std::declval<typename std::iterator_traits<sprout::ptr_index_iterator<T2, C> >::difference_type>()
+			)
+	>::type
+	operator-(sprout::ptr_index_iterator<T1, C> const& lhs, sprout::ptr_index_iterator<T2, C> const& rhs) {
+		return lhs.index() - rhs.index();
+	}
+	template<typename T, bool C>
+	inline SPROUT_CONSTEXPR sprout::ptr_index_iterator<T, C>
+	operator+(typename sprout::ptr_index_iterator<T, C>::difference_type n, sprout::ptr_index_iterator<T, C> const& it) {
+		return it + n;
+	}
+
 	//
 	// swap
 	//
diff --git a/sprout/iterator/value_iterator.hpp b/sprout/iterator/value_iterator.hpp
index 04eb0ed2..00747288 100644
--- a/sprout/iterator/value_iterator.hpp
+++ b/sprout/iterator/value_iterator.hpp
@@ -5,7 +5,6 @@
 #include <limits>
 #include <iterator>
 #include <utility>
-#include <stdexcept>
 #include <type_traits>
 #include <sprout/config.hpp>
 #include <sprout/iterator/next.hpp>
@@ -51,66 +50,45 @@ namespace sprout {
 		typedef typename base_type::reference reference;
 	private:
 		sprout::value_holder<T> holder_;
-		difference_type count_;
+		difference_type index_;
 	private:
-		SPROUT_CONSTEXPR value_iterator(sprout::value_holder<T> const& r, difference_type count)
-			: holder_(r), count_(count)
+		SPROUT_CONSTEXPR value_iterator(sprout::value_holder<T> const& r, difference_type index)
+			: holder_(r), index_(index)
 		{}
 	public:
 		SPROUT_CONSTEXPR value_iterator()
-			: holder_(), count_()
+			: holder_(), index_()
 		{}
 		value_iterator(value_iterator const&) = default;
 		explicit SPROUT_CONSTEXPR value_iterator(
 			typename sprout::value_holder<T>::param_type p,
-			difference_type count = std::numeric_limits<difference_type>::max()
+			difference_type index = std::numeric_limits<difference_type>::max()
 			)
-			: holder_(p), count_(count)
+			: holder_(p), index_(index)
 		{}
 		operator value_iterator<const_type>() const {
-			return value_iterator<const_type>(holder_.get(), count_);
+			return value_iterator<const_type>(holder_.get(), index_);
+		}
+		SPROUT_CONSTEXPR difference_type index() const {
+			return index_;
 		}
 		SPROUT_CONSTEXPR value_iterator next() const {
-			return value_iterator(holder_, count_ != 0 ? count_ - 1 : count_);
+			return value_iterator(holder_, index_ - 1);
 		}
 		SPROUT_CONSTEXPR value_iterator prev() const {
-			return value_iterator(holder_, count_ != 0 ? count_ + 1 : count_);
+			return value_iterator(holder_, index_ + 1);
 		}
 		void swap(value_iterator& other)
 		SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(sprout::swap(holder_, other.holder_)))
 		{
 			sprout::swap(holder_, other.holder_);
-			sprout::swap(count_, other.count_);
-		}
-		friend SPROUT_CONSTEXPR bool operator==(value_iterator const& lhs, value_iterator const& rhs) {
-			return lhs.count_ == rhs.count_ && (lhs.count_ == 0 || lhs.holder_.get() == rhs.holder_.get());
-		}
-		friend SPROUT_CONSTEXPR bool operator!=(value_iterator const& lhs, value_iterator const& rhs) {
-			return !(lhs == rhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator<(value_iterator const& lhs, value_iterator const& rhs) {
-			return lhs.count_ > rhs.count_;
-		}
-		friend SPROUT_CONSTEXPR bool operator>(value_iterator const& lhs, value_iterator const& rhs) {
-			return rhs < lhs;
-		}
-		friend SPROUT_CONSTEXPR bool operator<=(value_iterator const& lhs, value_iterator const& rhs) {
-			return !(rhs < lhs);
-		}
-		friend SPROUT_CONSTEXPR bool operator>=(value_iterator const& lhs, value_iterator const& rhs) {
-			return !(lhs < rhs);
+			sprout::swap(index_, other.index_);
 		}
 		SPROUT_CONSTEXPR reference operator*() const {
-			return count_ != 0
-				? holder_.get()
-				: (throw std::out_of_range("value_iterator<>: dereference at out of range"), holder_.get())
-				;
+			return holder_.get();
 		}
 		SPROUT_CONSTEXPR pointer operator->() const {
-			return count_ != 0
-				? holder_.get_pointer()
-				: throw std::out_of_range("value_iterator<>: dereference at out of range")
-				;
+			return holder_.get_pointer();
 		}
 		value_iterator& operator++() {
 			value_iterator temp(next());
@@ -133,32 +111,67 @@ namespace sprout {
 			return result;
 		}
 		SPROUT_CONSTEXPR value_iterator operator+(difference_type n) const {
-			return value_iterator(holder_, count_ - n);
+			return value_iterator(holder_, index_ - n);
 		}
 		SPROUT_CONSTEXPR value_iterator operator-(difference_type n) const {
-			return value_iterator(holder_, count_ + n);
+			return value_iterator(holder_, index_ + n);
 		}
 		value_iterator& operator+=(difference_type n) {
-			value_iterator temp(holder_, count_ - n);
+			value_iterator temp(holder_, index_ - n);
 			temp.swap(*this);
 			return *this;
 		}
 		value_iterator& operator-=(difference_type n) {
-			value_iterator temp(holder_, count_ + n);
+			value_iterator temp(holder_, index_ + n);
 			temp.swap(*this);
 			return *this;
 		}
 		SPROUT_CONSTEXPR reference operator[](difference_type n) const {
 			return holder_.get();
 		}
-		friend SPROUT_CONSTEXPR difference_type operator-(value_iterator const& lhs, value_iterator const& rhs) {
-			return rhs.count_ - lhs.count_;
-		}
-		friend SPROUT_CONSTEXPR value_iterator operator+(difference_type n, value_iterator const& it) {
-			return it + n;
-		}
 	};
 
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator==(sprout::value_iterator<T> const& lhs, sprout::value_iterator<T> const& rhs) {
+		return lhs.index() == rhs.index();
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator!=(sprout::value_iterator<T> const& lhs, sprout::value_iterator<T> const& rhs) {
+		return !(lhs == rhs);
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator<(sprout::value_iterator<T> const& lhs, sprout::value_iterator<T> const& rhs) {
+		return rhs.index() < lhs.index();
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator>(sprout::value_iterator<T> const& lhs, sprout::value_iterator<T> const& rhs) {
+		return rhs < lhs;
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator<=(sprout::value_iterator<T> const& lhs, sprout::value_iterator<T> const& rhs) {
+		return !(rhs < lhs);
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR bool
+	operator>=(sprout::value_iterator<T> const& lhs, sprout::value_iterator<T> const& rhs) {
+		return !(lhs < rhs);
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR typename sprout::value_iterator<T>::difference_type
+	operator-(sprout::value_iterator<T> const& lhs, sprout::value_iterator<T> const& rhs) {
+		return rhs.index() - lhs.index();
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR sprout::value_iterator<T>
+	operator+(typename sprout::value_iterator<T>::difference_type n, sprout::value_iterator<T> const& it) {
+		return it + n;
+	}
+
 	//
 	// swap
 	//
diff --git a/sprout/numeric/accumulate.hpp b/sprout/numeric/accumulate.hpp
index 13941078..c23ec1f2 100644
--- a/sprout/numeric/accumulate.hpp
+++ b/sprout/numeric/accumulate.hpp
@@ -90,7 +90,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR T
 		accumulate(
 			InputIterator first, InputIterator last, T init, BinaryOperation binary_op,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::pair<InputIterator, T> type;
diff --git a/sprout/numeric/dft/detail/dft_element_gen.hpp b/sprout/numeric/dft/detail/dft_element_gen.hpp
index 1312e300..1a08fd4c 100644
--- a/sprout/numeric/dft/detail/dft_element_gen.hpp
+++ b/sprout/numeric/dft/detail/dft_element_gen.hpp
@@ -75,7 +75,7 @@ namespace sprout {
 		dft_element_gen_impl(
 			InputIterator first, InputIterator last,
 			typename std::iterator_traits<InputIterator>::value_type::value_type arg,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			return sprout::detail::dft_element_gen_impl_1(first, last, arg);
diff --git a/sprout/numeric/dft/fixed/amplitude_spectrum.hpp b/sprout/numeric/dft/fixed/amplitude_spectrum.hpp
index 33a39295..d3a61ca8 100644
--- a/sprout/numeric/dft/fixed/amplitude_spectrum.hpp
+++ b/sprout/numeric/dft/fixed/amplitude_spectrum.hpp
@@ -88,7 +88,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			amplitude_spectrum(
 				ForwardIterator first, ForwardIterator last, Result const& result,
-				void*
+				std::forward_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::amplitude_spectrum_impl(first, last, result, sprout::size(result));
diff --git a/sprout/numeric/dft/fixed/dft.hpp b/sprout/numeric/dft/fixed/dft.hpp
index 40742cc5..6d02ae43 100644
--- a/sprout/numeric/dft/fixed/dft.hpp
+++ b/sprout/numeric/dft/fixed/dft.hpp
@@ -90,7 +90,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			dft(
 				ForwardIterator first, ForwardIterator last, Result const& result,
-				void*
+				std::forward_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::dft_impl(first, last, result, sprout::size(result), first, 0);
diff --git a/sprout/numeric/dft/fixed/idft.hpp b/sprout/numeric/dft/fixed/idft.hpp
index bf0576a6..efb9b7df 100644
--- a/sprout/numeric/dft/fixed/idft.hpp
+++ b/sprout/numeric/dft/fixed/idft.hpp
@@ -90,7 +90,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			idft(
 				ForwardIterator first, ForwardIterator last, Result const& result,
-				void*
+				std::forward_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::idft_impl(first, last, result, sprout::size(result), first, 0);
diff --git a/sprout/numeric/dft/fixed/phase_spectrum.hpp b/sprout/numeric/dft/fixed/phase_spectrum.hpp
index 88d69a5d..720c4612 100644
--- a/sprout/numeric/dft/fixed/phase_spectrum.hpp
+++ b/sprout/numeric/dft/fixed/phase_spectrum.hpp
@@ -88,7 +88,7 @@ namespace sprout {
 			inline SPROUT_CONSTEXPR typename sprout::fixed::result_of::algorithm<Result>::type
 			phase_spectrum(
 				ForwardIterator first, ForwardIterator last, Result const& result,
-				void*
+				std::forward_iterator_tag*
 				)
 			{
 				return sprout::fixed::detail::phase_spectrum_impl(first, last, result, sprout::size(result));
diff --git a/sprout/numeric/inner_product.hpp b/sprout/numeric/inner_product.hpp
index 730b887d..3b516c05 100644
--- a/sprout/numeric/inner_product.hpp
+++ b/sprout/numeric/inner_product.hpp
@@ -99,7 +99,7 @@ namespace sprout {
 		inner_product(
 			InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, T init,
 			BinaryOperation1 binary_op1, BinaryOperation2 binary_op2,
-			void*
+			std::input_iterator_tag*
 			)
 		{
 			typedef sprout::tuples::tuple<InputIterator1, InputIterator2, T> type;
diff --git a/sprout/type_traits.hpp b/sprout/type_traits.hpp
index 88d26a6b..a0bf0855 100644
--- a/sprout/type_traits.hpp
+++ b/sprout/type_traits.hpp
@@ -8,6 +8,7 @@
 #include <sprout/type_traits/is_c_str.hpp>
 #include <sprout/type_traits/lvalue_reference.hpp>
 #include <sprout/type_traits/const_reference.hpp>
+#include <sprout/type_traits/common_decay.hpp>
 #include <sprout/type_traits/arithmetic_promote.hpp>
 #include <sprout/type_traits/float_promote.hpp>
 #include <sprout/type_traits/enabler_if.hpp>
diff --git a/sprout/type_traits/common_decay.hpp b/sprout/type_traits/common_decay.hpp
new file mode 100644
index 00000000..4b22ade9
--- /dev/null
+++ b/sprout/type_traits/common_decay.hpp
@@ -0,0 +1,17 @@
+#ifndef SPROUT_TYPE_TRAITS_COMMON_DECAY_HPP
+#define SPROUT_TYPE_TRAITS_COMMON_DECAY_HPP
+
+#include <type_traits>
+#include <sprout/config.hpp>
+
+namespace sprout {
+	//
+	// common_decay
+	//
+	template<typename... Types>
+	struct common_decay
+		: public std::decay<typename std::common_type<Types...>::type>
+	{};
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_TYPE_TRAITS_COMMON_DECAY_HPP