diff --git a/sprout/checksum/md5.hpp b/sprout/checksum/md5.hpp
index efa178ce..04318459 100644
--- a/sprout/checksum/md5.hpp
+++ b/sprout/checksum/md5.hpp
@@ -372,7 +372,7 @@ namespace sprout {
 				static_cast<std::size_t>(bit_count_ % 32)
 				);
 		}
-		SPROUT_CONSTEXPR md5 const process_bits(std::uint8_t bits, std::size_t bit_count) {
+		SPROUT_CONSTEXPR md5 const process_bits(std::uint8_t bits, std::size_t bit_count) const {
 			return process_bits_impl(bits, bit_count);
 		}
 		SPROUT_CONSTEXPR md5 const process_byte(std::uint8_t byte) const {
diff --git a/sprout/compost/effects/auto_pan.hpp b/sprout/compost/effects/auto_pan.hpp
index 365a247d..4a5e1b0b 100644
--- a/sprout/compost/effects/auto_pan.hpp
+++ b/sprout/compost/effects/auto_pan.hpp
@@ -97,12 +97,12 @@ namespace sprout {
 			public:
 				template<typename T, typename IntType>
 				SPROUT_CONSTEXPR sprout::compost::effects::auto_pan_holder<T, IntType>
-				operator()(T const& depth, T const& rate, IntType samples_per_sec) {
+				operator()(T const& depth, T const& rate, IntType samples_per_sec) const {
 					return sprout::compost::effects::auto_pan_holder<T, IntType>(depth, rate, samples_per_sec);
 				}
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::auto_pan_holder<T>
-				operator()(T const& depth, T const& rate) {
+				operator()(T const& depth, T const& rate) const {
 					return sprout::compost::effects::auto_pan_holder<T>(depth, rate);
 				}
 			};
diff --git a/sprout/compost/effects/changed_volume.hpp b/sprout/compost/effects/changed_volume.hpp
index ae9a9603..6a48e45e 100644
--- a/sprout/compost/effects/changed_volume.hpp
+++ b/sprout/compost/effects/changed_volume.hpp
@@ -37,7 +37,7 @@ namespace sprout {
 			public:
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::change_volume_holder<T>
-				operator()(T const& value) {
+				operator()(T const& value) const {
 					return sprout::compost::effects::change_volume_holder<T>(value);
 				}
 			};
diff --git a/sprout/compost/effects/chorus.hpp b/sprout/compost/effects/chorus.hpp
index e0acd5f5..0746ce28 100644
--- a/sprout/compost/effects/chorus.hpp
+++ b/sprout/compost/effects/chorus.hpp
@@ -105,12 +105,12 @@ namespace sprout {
 			public:
 				template<typename T, typename IntType>
 				SPROUT_CONSTEXPR sprout::compost::effects::chorus_holder<T, IntType>
-				operator()(T const& d, T const& depth, T const& rate, IntType samples_per_sec) {
+				operator()(T const& d, T const& depth, T const& rate, IntType samples_per_sec) const {
 					return sprout::compost::effects::chorus_holder<T, IntType>(d, depth, rate, samples_per_sec);
 				}
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::chorus_holder<T>
-				operator()(T const& d, T const& depth, T const& rate) {
+				operator()(T const& d, T const& depth, T const& rate) const {
 					return sprout::compost::effects::chorus_holder<T>(d, depth, rate);
 				}
 			};
diff --git a/sprout/compost/effects/clipped.hpp b/sprout/compost/effects/clipped.hpp
index 41c94ab3..eb701202 100644
--- a/sprout/compost/effects/clipped.hpp
+++ b/sprout/compost/effects/clipped.hpp
@@ -39,7 +39,7 @@ namespace sprout {
 			public:
 				template<typename T = double>
 				SPROUT_CONSTEXPR sprout::compost::effects::clip_holder<T>
-				operator()(T const& low = -1, T const& up = 1) {
+				operator()(T const& low = -1, T const& up = 1) const {
 					return sprout::compost::effects::clip_holder<T>(low, up);
 				}
 			};
diff --git a/sprout/compost/effects/compressed.hpp b/sprout/compost/effects/compressed.hpp
index 5ec6b3d6..30ba3d52 100644
--- a/sprout/compost/effects/compressed.hpp
+++ b/sprout/compost/effects/compressed.hpp
@@ -92,7 +92,7 @@ namespace sprout {
 			public:
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::compress_holder<T>
-				operator()(T const& threshold, T const& ratio) {
+				operator()(T const& threshold, T const& ratio) const {
 					return sprout::compost::effects::compress_holder<T>(threshold, ratio);
 				}
 			};
diff --git a/sprout/compost/effects/distorted.hpp b/sprout/compost/effects/distorted.hpp
index a15748dd..e8771c94 100644
--- a/sprout/compost/effects/distorted.hpp
+++ b/sprout/compost/effects/distorted.hpp
@@ -40,7 +40,7 @@ namespace sprout {
 			public:
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::distort_holder<T>
-				operator()(T const& gain, T const& level) {
+				operator()(T const& gain, T const& level) const {
 					return sprout::compost::effects::distort_holder<T>(gain, level);
 				}
 			};
diff --git a/sprout/compost/effects/fuzzed.hpp b/sprout/compost/effects/fuzzed.hpp
index ac85c9cc..c2308271 100644
--- a/sprout/compost/effects/fuzzed.hpp
+++ b/sprout/compost/effects/fuzzed.hpp
@@ -41,7 +41,7 @@ namespace sprout {
 			public:
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::fuzz_holder<T>
-				operator()(T const& gain, T const& level) {
+				operator()(T const& gain, T const& level) const {
 					return sprout::compost::effects::fuzz_holder<T>(gain, level);
 				}
 			};
diff --git a/sprout/compost/effects/noise_gated.hpp b/sprout/compost/effects/noise_gated.hpp
index 232c398b..09cfbdb6 100644
--- a/sprout/compost/effects/noise_gated.hpp
+++ b/sprout/compost/effects/noise_gated.hpp
@@ -82,7 +82,7 @@ namespace sprout {
 			public:
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::noise_gate_holder<T>
-				operator()(T const& threshold) {
+				operator()(T const& threshold) const {
 					return sprout::compost::effects::noise_gate_holder<T>(threshold);
 				}
 			};
diff --git a/sprout/compost/effects/overdriven.hpp b/sprout/compost/effects/overdriven.hpp
index 2145f83c..b9e00659 100644
--- a/sprout/compost/effects/overdriven.hpp
+++ b/sprout/compost/effects/overdriven.hpp
@@ -68,7 +68,7 @@ namespace sprout {
 			public:
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::overdrive_holder<T>
-				operator()(T const& gain, T const& level) {
+				operator()(T const& gain, T const& level) const {
 					return sprout::compost::effects::overdrive_holder<T>(gain, level);
 				}
 			};
diff --git a/sprout/compost/effects/pseudo_stereo.hpp b/sprout/compost/effects/pseudo_stereo.hpp
index eddac748..6d7a4d92 100644
--- a/sprout/compost/effects/pseudo_stereo.hpp
+++ b/sprout/compost/effects/pseudo_stereo.hpp
@@ -93,12 +93,12 @@ namespace sprout {
 			public:
 				template<typename T, typename IntType>
 				SPROUT_CONSTEXPR sprout::compost::effects::pseudo_stereo_holder<T, IntType>
-				operator()(T const& delay, IntType samples_per_sec) {
+				operator()(T const& delay, IntType samples_per_sec) const {
 					return sprout::compost::effects::pseudo_stereo_holder<T, IntType>(delay, samples_per_sec);
 				}
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::pseudo_stereo_holder<T>
-				operator()(T const& delay) {
+				operator()(T const& delay) const {
 					return sprout::compost::effects::pseudo_stereo_holder<T>(delay);
 				}
 			};
diff --git a/sprout/compost/effects/reverbed.hpp b/sprout/compost/effects/reverbed.hpp
index 4624839e..53130c81 100644
--- a/sprout/compost/effects/reverbed.hpp
+++ b/sprout/compost/effects/reverbed.hpp
@@ -100,12 +100,12 @@ namespace sprout {
 			public:
 				template<typename T, typename IntType>
 				SPROUT_CONSTEXPR sprout::compost::effects::reverb_holder<T, IntType>
-				operator()(T const& attenuation, T const& delay, std::size_t repeat, IntType samples_per_sec) {
+				operator()(T const& attenuation, T const& delay, std::size_t repeat, IntType samples_per_sec) const {
 					return sprout::compost::effects::reverb_holder<T, IntType>(attenuation, delay, repeat, samples_per_sec);
 				}
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::reverb_holder<T>
-				operator()(T const& attenuation, T const& delay, std::size_t repeat = 2) {
+				operator()(T const& attenuation, T const& delay, std::size_t repeat = 2) const {
 					return sprout::compost::effects::reverb_holder<T>(attenuation, delay, repeat);
 				}
 			};
diff --git a/sprout/compost/effects/superposed.hpp b/sprout/compost/effects/superposed.hpp
index 26767381..810ea261 100644
--- a/sprout/compost/effects/superposed.hpp
+++ b/sprout/compost/effects/superposed.hpp
@@ -41,7 +41,7 @@ namespace sprout {
 				SPROUT_CONSTEXPR sprout::compost::effects::superpose_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 				>
-				operator()(RRange&& range) {
+				operator()(RRange&& range) const {
 					return sprout::compost::effects::superpose_holder<
 						typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 					>(
diff --git a/sprout/compost/effects/tremolo.hpp b/sprout/compost/effects/tremolo.hpp
index 3f37a04c..e97944c5 100644
--- a/sprout/compost/effects/tremolo.hpp
+++ b/sprout/compost/effects/tremolo.hpp
@@ -72,12 +72,12 @@ namespace sprout {
 			public:
 				template<typename T, typename IntType>
 				SPROUT_CONSTEXPR sprout::compost::effects::tremolo_holder<T, IntType>
-				operator()(T const& depth, T const& rate, IntType samples_per_sec) {
+				operator()(T const& depth, T const& rate, IntType samples_per_sec) const {
 					return sprout::compost::effects::tremolo_holder<T, IntType>(depth, rate, samples_per_sec);
 				}
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::tremolo_holder<T>
-				operator()(T const& depth, T const& rate) {
+				operator()(T const& depth, T const& rate) const {
 					return sprout::compost::effects::tremolo_holder<T>(depth, rate);
 				}
 			};
diff --git a/sprout/compost/effects/vibrato.hpp b/sprout/compost/effects/vibrato.hpp
index 4f171b44..34f53d2b 100644
--- a/sprout/compost/effects/vibrato.hpp
+++ b/sprout/compost/effects/vibrato.hpp
@@ -104,12 +104,12 @@ namespace sprout {
 			public:
 				template<typename T, typename IntType>
 				SPROUT_CONSTEXPR sprout::compost::effects::vibrato_holder<T, IntType>
-				operator()(T const& d, T const& depth, T const& rate, IntType samples_per_sec) {
+				operator()(T const& d, T const& depth, T const& rate, IntType samples_per_sec) const {
 					return sprout::compost::effects::vibrato_holder<T, IntType>(d, depth, rate, samples_per_sec);
 				}
 				template<typename T>
 				SPROUT_CONSTEXPR sprout::compost::effects::vibrato_holder<T>
-				operator()(T const& d, T const& depth, T const& rate) {
+				operator()(T const& d, T const& depth, T const& rate) const {
 					return sprout::compost::effects::vibrato_holder<T>(d, depth, rate);
 				}
 			};
diff --git a/sprout/compost/formats/effected_each.hpp b/sprout/compost/formats/effected_each.hpp
index 4c5e5513..e4ae3302 100644
--- a/sprout/compost/formats/effected_each.hpp
+++ b/sprout/compost/formats/effected_each.hpp
@@ -54,12 +54,12 @@ namespace sprout {
 			public:
 				template<typename LAdaptor, typename RAdaptor>
 				SPROUT_CONSTEXPR sprout::compost::formats::effect_each_holder<LAdaptor, RAdaptor>
-				operator()(LAdaptor const& left_adaptor, RAdaptor const& right_adaptor) {
+				operator()(LAdaptor const& left_adaptor, RAdaptor const& right_adaptor) const {
 					return sprout::compost::formats::effect_each_holder<LAdaptor, RAdaptor>(left_adaptor, right_adaptor);
 				}
 				template<typename Adaptor>
 				SPROUT_CONSTEXPR sprout::compost::formats::effect_each_holder<Adaptor>
-				operator()(Adaptor const& adaptor) {
+				operator()(Adaptor const& adaptor) const {
 					return sprout::compost::formats::effect_each_holder<Adaptor>(adaptor);
 				}
 			};
diff --git a/sprout/compost/formats/effected_left.hpp b/sprout/compost/formats/effected_left.hpp
index db36c7cd..556313fc 100644
--- a/sprout/compost/formats/effected_left.hpp
+++ b/sprout/compost/formats/effected_left.hpp
@@ -35,7 +35,7 @@ namespace sprout {
 			public:
 				template<typename Adaptor>
 				SPROUT_CONSTEXPR sprout::compost::formats::effect_left_holder<Adaptor>
-				operator()(Adaptor const& adaptor) {
+				operator()(Adaptor const& adaptor) const {
 					return sprout::compost::formats::effect_left_holder<Adaptor>(adaptor);
 				}
 			};
diff --git a/sprout/compost/formats/effected_right.hpp b/sprout/compost/formats/effected_right.hpp
index 093da954..10a73371 100644
--- a/sprout/compost/formats/effected_right.hpp
+++ b/sprout/compost/formats/effected_right.hpp
@@ -35,7 +35,7 @@ namespace sprout {
 			public:
 				template<typename Adaptor>
 				SPROUT_CONSTEXPR sprout::compost::formats::effect_right_holder<Adaptor>
-				operator()(Adaptor const& adaptor) {
+				operator()(Adaptor const& adaptor) const {
 					return sprout::compost::formats::effect_right_holder<Adaptor>(adaptor);
 				}
 			};
diff --git a/sprout/range/adaptor/adapted_dropped.hpp b/sprout/range/adaptor/adapted_dropped.hpp
index b47f3735..d411b23c 100644
--- a/sprout/range/adaptor/adapted_dropped.hpp
+++ b/sprout/range/adaptor/adapted_dropped.hpp
@@ -39,7 +39,7 @@ namespace sprout {
 		public:
 			template<typename Adaptor, typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::adapt_drop_holder<Adaptor, Difference>
-			operator()(Adaptor const& adaptor, Difference distance) {
+			operator()(Adaptor const& adaptor, Difference distance) const {
 				return sprout::adaptors::adapt_drop_holder<Adaptor, Difference>(adaptor, distance);
 			}
 		};
diff --git a/sprout/range/adaptor/adapted_dropped_end.hpp b/sprout/range/adaptor/adapted_dropped_end.hpp
index e5740868..dcfc6806 100644
--- a/sprout/range/adaptor/adapted_dropped_end.hpp
+++ b/sprout/range/adaptor/adapted_dropped_end.hpp
@@ -39,7 +39,7 @@ namespace sprout {
 		public:
 			template<typename Adaptor, typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::adapt_drop_end_holder<Adaptor, Difference>
-			operator()(Adaptor const& adaptor, Difference distance) {
+			operator()(Adaptor const& adaptor, Difference distance) const {
 				return sprout::adaptors::adapt_drop_end_holder<Adaptor, Difference>(adaptor, distance);
 			}
 		};
diff --git a/sprout/range/adaptor/adapted_offset.hpp b/sprout/range/adaptor/adapted_offset.hpp
index fc9202e0..747e9dc6 100644
--- a/sprout/range/adaptor/adapted_offset.hpp
+++ b/sprout/range/adaptor/adapted_offset.hpp
@@ -64,12 +64,12 @@ namespace sprout {
 		public:
 			template<typename Adaptor, typename Difference1, typename Difference2>
 			SPROUT_CONSTEXPR sprout::adaptors::adapt_offset_holder<Adaptor, Difference1, Difference2>
-			operator()(Adaptor const& adaptor, Difference1 from_begin, Difference2 from_end) {
+			operator()(Adaptor const& adaptor, Difference1 from_begin, Difference2 from_end) const {
 				return sprout::adaptors::adapt_offset_holder<Adaptor, Difference1, Difference2>(adaptor, from_begin, from_end);
 			}
 			template<typename Adaptor, typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::adapt_offset_holder<Adaptor, Difference>
-			operator()(Adaptor const& adaptor, Difference from_begin) {
+			operator()(Adaptor const& adaptor, Difference from_begin) const {
 				return sprout::adaptors::adapt_offset_holder<Adaptor, Difference>(adaptor, from_begin);
 			}
 		};
diff --git a/sprout/range/adaptor/adapted_taken.hpp b/sprout/range/adaptor/adapted_taken.hpp
index 4b235de4..3b267838 100644
--- a/sprout/range/adaptor/adapted_taken.hpp
+++ b/sprout/range/adaptor/adapted_taken.hpp
@@ -39,7 +39,7 @@ namespace sprout {
 		public:
 			template<typename Adaptor, typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::adapt_take_holder<Adaptor, Difference>
-			operator()(Adaptor const& adaptor, Difference distance) {
+			operator()(Adaptor const& adaptor, Difference distance) const {
 				return sprout::adaptors::adapt_take_holder<Adaptor, Difference>(adaptor, distance);
 			}
 		};
diff --git a/sprout/range/adaptor/adapted_taken_end.hpp b/sprout/range/adaptor/adapted_taken_end.hpp
index 4b171f9d..f3464592 100644
--- a/sprout/range/adaptor/adapted_taken_end.hpp
+++ b/sprout/range/adaptor/adapted_taken_end.hpp
@@ -39,7 +39,7 @@ namespace sprout {
 		public:
 			template<typename Adaptor, typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::adapt_take_end_holder<Adaptor, Difference>
-			operator()(Adaptor const& adaptor, Difference distance) {
+			operator()(Adaptor const& adaptor, Difference distance) const {
 				return sprout::adaptors::adapt_take_end_holder<Adaptor, Difference>(adaptor, distance);
 			}
 		};
diff --git a/sprout/range/adaptor/adapted_window.hpp b/sprout/range/adaptor/adapted_window.hpp
index 7878db9e..e0a6558c 100644
--- a/sprout/range/adaptor/adapted_window.hpp
+++ b/sprout/range/adaptor/adapted_window.hpp
@@ -64,12 +64,12 @@ namespace sprout {
 		public:
 			template<typename Adaptor, typename Difference1, typename Difference2>
 			SPROUT_CONSTEXPR sprout::adaptors::adapt_window_holder<Adaptor, Difference1, Difference2>
-			operator()(Adaptor const& adaptor, Difference1 to_first, Difference2 to_last) {
+			operator()(Adaptor const& adaptor, Difference1 to_first, Difference2 to_last) const {
 				return sprout::adaptors::adapt_window_holder<Adaptor, Difference1, Difference2>(adaptor, to_first, to_last);
 			}
 			template<typename Adaptor, typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::adapt_window_holder<Adaptor, Difference>
-			operator()(Adaptor const& adaptor, Difference to_first) {
+			operator()(Adaptor const& adaptor, Difference to_first) const {
 				return sprout::adaptors::adapt_window_holder<Adaptor, Difference>(adaptor, to_first);
 			}
 		};
diff --git a/sprout/range/adaptor/adjacent_filtered.hpp b/sprout/range/adaptor/adjacent_filtered.hpp
index d61c92a8..2bc2e83c 100644
--- a/sprout/range/adaptor/adjacent_filtered.hpp
+++ b/sprout/range/adaptor/adjacent_filtered.hpp
@@ -75,7 +75,7 @@ namespace sprout {
 		public:
 			template<typename Predicate>
 			SPROUT_CONSTEXPR sprout::adaptors::adjacent_filter_holder<Predicate>
-			operator()(Predicate pred) {
+			operator()(Predicate pred) const {
 				return sprout::adaptors::adjacent_filter_holder<Predicate>(pred);
 			}
 		};
diff --git a/sprout/range/adaptor/alternated.hpp b/sprout/range/adaptor/alternated.hpp
index 37c8d592..3dcf02ad 100644
--- a/sprout/range/adaptor/alternated.hpp
+++ b/sprout/range/adaptor/alternated.hpp
@@ -83,7 +83,7 @@ namespace sprout {
 			SPROUT_CONSTEXPR sprout::adaptors::alternate_holder<
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 			>
-			operator()(RRange&& range) {
+			operator()(RRange&& range) const {
 				return sprout::adaptors::alternate_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 				>(
diff --git a/sprout/range/adaptor/clamped.hpp b/sprout/range/adaptor/clamped.hpp
index 0522378e..2c104c3e 100644
--- a/sprout/range/adaptor/clamped.hpp
+++ b/sprout/range/adaptor/clamped.hpp
@@ -95,12 +95,12 @@ namespace sprout {
 		public:
 			template<typename Value, typename Compare>
 			SPROUT_CONSTEXPR sprout::adaptors::clamp_holder<Value, Compare>
-			operator()(Value const& low, Value const& up, Compare comp) {
+			operator()(Value const& low, Value const& up, Compare comp) const {
 				return sprout::adaptors::clamp_holder<Value, Compare>(low, up, comp);
 			}
 			template<typename Value>
 			SPROUT_CONSTEXPR sprout::adaptors::clamp_holder<Value>
-			operator()(Value const& low, Value const& up) {
+			operator()(Value const& low, Value const& up) const {
 				return sprout::adaptors::clamp_holder<Value>(low, up);
 			}
 		};
diff --git a/sprout/range/adaptor/copied.hpp b/sprout/range/adaptor/copied.hpp
index b4b78e97..62651894 100644
--- a/sprout/range/adaptor/copied.hpp
+++ b/sprout/range/adaptor/copied.hpp
@@ -67,7 +67,7 @@ namespace sprout {
 		public:
 			template<typename Result>
 			SPROUT_CONSTEXPR typename sprout::adaptors::copy_holder<Result const>
-			operator()(Result const& result) {
+			operator()(Result const& result) const {
 				return typename sprout::adaptors::copy_holder<Result const>(result);
 			}
 		};
diff --git a/sprout/range/adaptor/dropped.hpp b/sprout/range/adaptor/dropped.hpp
index 2a8e73c9..cbeab499 100644
--- a/sprout/range/adaptor/dropped.hpp
+++ b/sprout/range/adaptor/dropped.hpp
@@ -38,7 +38,7 @@ namespace sprout {
 		public:
 			template<typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::drop_holder<Difference>
-			operator()(Difference distance) {
+			operator()(Difference distance) const {
 				return sprout::adaptors::drop_holder<Difference>(distance);
 			}
 		};
diff --git a/sprout/range/adaptor/dropped_end.hpp b/sprout/range/adaptor/dropped_end.hpp
index 43b88f10..d2c98742 100644
--- a/sprout/range/adaptor/dropped_end.hpp
+++ b/sprout/range/adaptor/dropped_end.hpp
@@ -38,7 +38,7 @@ namespace sprout {
 		public:
 			template<typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::drop_end_holder<Difference>
-			operator()(Difference distance) {
+			operator()(Difference distance) const {
 				return sprout::adaptors::drop_end_holder<Difference>(distance);
 			}
 		};
diff --git a/sprout/range/adaptor/filtered.hpp b/sprout/range/adaptor/filtered.hpp
index 59102ca5..c2e37726 100644
--- a/sprout/range/adaptor/filtered.hpp
+++ b/sprout/range/adaptor/filtered.hpp
@@ -75,7 +75,7 @@ namespace sprout {
 		public:
 			template<typename Predicate>
 			SPROUT_CONSTEXPR sprout::adaptors::filter_holder<Predicate>
-			operator()(Predicate pred) {
+			operator()(Predicate pred) const {
 				return sprout::adaptors::filter_holder<Predicate>(pred);
 			}
 		};
diff --git a/sprout/range/adaptor/indexed.hpp b/sprout/range/adaptor/indexed.hpp
index 9e954885..a418bd9c 100644
--- a/sprout/range/adaptor/indexed.hpp
+++ b/sprout/range/adaptor/indexed.hpp
@@ -75,7 +75,7 @@ namespace sprout {
 		public:
 			template<typename Index>
 			SPROUT_CONSTEXPR sprout::adaptors::indexed_holder<Index>
-			operator()(Index index) {
+			operator()(Index index) const {
 				return sprout::adaptors::indexed_holder<Index>(index);
 			}
 		};
diff --git a/sprout/range/adaptor/jointed.hpp b/sprout/range/adaptor/jointed.hpp
index f5161fd4..e19a9655 100644
--- a/sprout/range/adaptor/jointed.hpp
+++ b/sprout/range/adaptor/jointed.hpp
@@ -78,7 +78,7 @@ namespace sprout {
 			SPROUT_CONSTEXPR sprout::adaptors::joint_holder<
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 			>
-			operator()(RRange&& range) {
+			operator()(RRange&& range) const {
 				return sprout::adaptors::joint_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 				>(
diff --git a/sprout/range/adaptor/merged.hpp b/sprout/range/adaptor/merged.hpp
index 5dd93b7c..5fb88d49 100644
--- a/sprout/range/adaptor/merged.hpp
+++ b/sprout/range/adaptor/merged.hpp
@@ -87,7 +87,7 @@ namespace sprout {
 			SPROUT_CONSTEXPR sprout::adaptors::merge_holder<
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 			>
-			operator()(RRange&& range) {
+			operator()(RRange&& range) const {
 				return sprout::adaptors::merge_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 				>(
@@ -99,7 +99,7 @@ namespace sprout {
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type,
 				Compare
 			>
-			operator()(RRange&& range, Compare comp) {
+			operator()(RRange&& range, Compare comp) const {
 				return sprout::adaptors::merge_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type,
 					Compare
diff --git a/sprout/range/adaptor/piped.hpp b/sprout/range/adaptor/piped.hpp
index b1ad7d11..1db44c5d 100644
--- a/sprout/range/adaptor/piped.hpp
+++ b/sprout/range/adaptor/piped.hpp
@@ -103,7 +103,7 @@ namespace sprout {
 		public:
 			template<typename... Adaptors>
 			SPROUT_CONSTEXPR sprout::adaptors::pipe_holder<Adaptors...>
-			operator()(Adaptors const&... adaptors) {
+			operator()(Adaptors const&... adaptors) const {
 				return sprout::adaptors::pipe_holder<Adaptors...>(adaptors...);
 			}
 		};
diff --git a/sprout/range/adaptor/removed.hpp b/sprout/range/adaptor/removed.hpp
index c29ac171..eeb32f17 100644
--- a/sprout/range/adaptor/removed.hpp
+++ b/sprout/range/adaptor/removed.hpp
@@ -74,7 +74,7 @@ namespace sprout {
 		public:
 			template<typename T>
 			SPROUT_CONSTEXPR sprout::adaptors::remove_holder<T>
-			operator()(T const& value) {
+			operator()(T const& value) const {
 				return sprout::adaptors::remove_holder<T>(value);
 			}
 		};
diff --git a/sprout/range/adaptor/removed_if.hpp b/sprout/range/adaptor/removed_if.hpp
index 9826837b..ba23db57 100644
--- a/sprout/range/adaptor/removed_if.hpp
+++ b/sprout/range/adaptor/removed_if.hpp
@@ -74,7 +74,7 @@ namespace sprout {
 		public:
 			template<typename Predicate>
 			SPROUT_CONSTEXPR sprout::adaptors::remove_if_holder<Predicate>
-			operator()(Predicate pred) {
+			operator()(Predicate pred) const {
 				return sprout::adaptors::remove_if_holder<Predicate>(pred);
 			}
 		};
diff --git a/sprout/range/adaptor/replaced.hpp b/sprout/range/adaptor/replaced.hpp
index 7e288ee3..41705076 100644
--- a/sprout/range/adaptor/replaced.hpp
+++ b/sprout/range/adaptor/replaced.hpp
@@ -79,7 +79,7 @@ namespace sprout {
 		public:
 			template<typename T>
 			SPROUT_CONSTEXPR sprout::adaptors::replace_holder<T>
-			operator()(T const& old_value, T const& new_value) {
+			operator()(T const& old_value, T const& new_value) const {
 				return sprout::adaptors::replace_holder<T>(old_value, new_value);
 			}
 		};
diff --git a/sprout/range/adaptor/replaced_if.hpp b/sprout/range/adaptor/replaced_if.hpp
index 73282de2..210cb08c 100644
--- a/sprout/range/adaptor/replaced_if.hpp
+++ b/sprout/range/adaptor/replaced_if.hpp
@@ -81,7 +81,7 @@ namespace sprout {
 		public:
 			template<typename Predicate, typename T>
 			SPROUT_CONSTEXPR sprout::adaptors::replace_if_holder<Predicate, T>
-			operator()(Predicate pred, T const& new_value) {
+			operator()(Predicate pred, T const& new_value) const {
 				return sprout::adaptors::replace_if_holder<Predicate, T>(pred, new_value);
 			}
 		};
diff --git a/sprout/range/adaptor/sawtooth_wave.hpp b/sprout/range/adaptor/sawtooth_wave.hpp
index e5a05edc..b6205c4b 100644
--- a/sprout/range/adaptor/sawtooth_wave.hpp
+++ b/sprout/range/adaptor/sawtooth_wave.hpp
@@ -106,7 +106,7 @@ namespace sprout {
 				Value const& frequency = 1,
 				Value const& amplitude = 1,
 				Value const& phase = 0
-				)
+				) const
 			{
 				return sprout::adaptors::sawtooth_wave_range<Value>(frequency, amplitude, phase);
 			}
diff --git a/sprout/range/adaptor/set_difference.hpp b/sprout/range/adaptor/set_difference.hpp
index 78f45dc9..eb22029d 100644
--- a/sprout/range/adaptor/set_difference.hpp
+++ b/sprout/range/adaptor/set_difference.hpp
@@ -87,7 +87,7 @@ namespace sprout {
 			SPROUT_CONSTEXPR sprout::adaptors::set_difference_holder<
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 			>
-			operator()(RRange&& range) {
+			operator()(RRange&& range) const {
 				return sprout::adaptors::set_difference_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 				>(
@@ -99,7 +99,7 @@ namespace sprout {
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type,
 				Compare
 			>
-			operator()(RRange&& range, Compare comp) {
+			operator()(RRange&& range, Compare comp) const {
 				return sprout::adaptors::set_difference_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type,
 					Compare
diff --git a/sprout/range/adaptor/set_intersection.hpp b/sprout/range/adaptor/set_intersection.hpp
index 6ccf9c1c..adda50c1 100644
--- a/sprout/range/adaptor/set_intersection.hpp
+++ b/sprout/range/adaptor/set_intersection.hpp
@@ -87,7 +87,7 @@ namespace sprout {
 			SPROUT_CONSTEXPR sprout::adaptors::set_intersection_holder<
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 			>
-			operator()(RRange&& range) {
+			operator()(RRange&& range) const {
 				return sprout::adaptors::set_intersection_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 				>(
@@ -99,7 +99,7 @@ namespace sprout {
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type,
 				Compare
 			>
-			operator()(RRange&& range, Compare comp) {
+			operator()(RRange&& range, Compare comp) const {
 				return sprout::adaptors::set_intersection_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type,
 					Compare
diff --git a/sprout/range/adaptor/set_symmetric_difference.hpp b/sprout/range/adaptor/set_symmetric_difference.hpp
index 625bed23..92be4ddc 100644
--- a/sprout/range/adaptor/set_symmetric_difference.hpp
+++ b/sprout/range/adaptor/set_symmetric_difference.hpp
@@ -87,7 +87,7 @@ namespace sprout {
 			SPROUT_CONSTEXPR sprout::adaptors::set_symmetric_difference_holder<
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 			>
-			operator()(RRange&& range) {
+			operator()(RRange&& range) const {
 				return sprout::adaptors::set_symmetric_difference_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 				>(
@@ -99,7 +99,7 @@ namespace sprout {
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type,
 				Compare
 			>
-			operator()(RRange&& range, Compare comp) {
+			operator()(RRange&& range, Compare comp) const {
 				return sprout::adaptors::set_symmetric_difference_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type,
 					Compare
diff --git a/sprout/range/adaptor/set_union.hpp b/sprout/range/adaptor/set_union.hpp
index 45e5af27..237dee0a 100644
--- a/sprout/range/adaptor/set_union.hpp
+++ b/sprout/range/adaptor/set_union.hpp
@@ -87,7 +87,7 @@ namespace sprout {
 			SPROUT_CONSTEXPR sprout::adaptors::set_union_holder<
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 			>
-			operator()(RRange&& range) {
+			operator()(RRange&& range) const {
 				return sprout::adaptors::set_union_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 				>(
@@ -99,7 +99,7 @@ namespace sprout {
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type,
 				Compare
 			>
-			operator()(RRange&& range, Compare comp) {
+			operator()(RRange&& range, Compare comp) const {
 				return sprout::adaptors::set_union_holder<
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type,
 					Compare
diff --git a/sprout/range/adaptor/sinusoidal.hpp b/sprout/range/adaptor/sinusoidal.hpp
index 43b66bc8..dcd83323 100644
--- a/sprout/range/adaptor/sinusoidal.hpp
+++ b/sprout/range/adaptor/sinusoidal.hpp
@@ -106,7 +106,7 @@ namespace sprout {
 				Value const& frequency = 1,
 				Value const& amplitude = 1,
 				Value const& phase = 0
-				)
+				) const
 			{
 				return sprout::adaptors::sinusoidal_range<Value>(frequency, amplitude, phase);
 			}
diff --git a/sprout/range/adaptor/size_enumed.hpp b/sprout/range/adaptor/size_enumed.hpp
index 29ab1c2d..643b2d3e 100644
--- a/sprout/range/adaptor/size_enumed.hpp
+++ b/sprout/range/adaptor/size_enumed.hpp
@@ -115,7 +115,7 @@ namespace sprout {
 		public:
 			template<typename Size>
 			SPROUT_CONSTEXPR typename sprout::adaptors::size_enum_holder<Size>
-			operator()(Size size, bool omit_last = false) {
+			operator()(Size size, bool omit_last = false) const {
 				return typename sprout::adaptors::size_enum_holder<Size>(size, omit_last);
 			}
 		};
diff --git a/sprout/range/adaptor/square_wave.hpp b/sprout/range/adaptor/square_wave.hpp
index 3c32d2f7..4d77de69 100644
--- a/sprout/range/adaptor/square_wave.hpp
+++ b/sprout/range/adaptor/square_wave.hpp
@@ -115,7 +115,7 @@ namespace sprout {
 				Value const& amplitude = 1,
 				Value const& phase = 0,
 				Value const& duty = 0.5
-				)
+				) const
 			{
 				return sprout::adaptors::square_wave_range<Value>(frequency, amplitude, phase, duty);
 			}
diff --git a/sprout/range/adaptor/steps.hpp b/sprout/range/adaptor/steps.hpp
index 0cbf57fb..c8516f71 100644
--- a/sprout/range/adaptor/steps.hpp
+++ b/sprout/range/adaptor/steps.hpp
@@ -86,12 +86,12 @@ namespace sprout {
 		public:
 			template<typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::step_holder<Difference>
-			operator()(Difference width) {
+			operator()(Difference width) const {
 				return sprout::adaptors::step_holder<Difference>(width);
 			}
 			template<typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::step_holder<Difference>
-			operator()(Difference width, typename std::common_type<Difference>::type init) {
+			operator()(Difference width, typename std::common_type<Difference>::type init) const {
 				return sprout::adaptors::step_holder<Difference>(width, init);
 			}
 		};
diff --git a/sprout/range/adaptor/taken.hpp b/sprout/range/adaptor/taken.hpp
index 019b688d..df0e8c04 100644
--- a/sprout/range/adaptor/taken.hpp
+++ b/sprout/range/adaptor/taken.hpp
@@ -38,7 +38,7 @@ namespace sprout {
 		public:
 			template<typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::take_holder<Difference>
-			operator()(Difference distance) {
+			operator()(Difference distance) const {
 				return sprout::adaptors::take_holder<Difference>(distance);
 			}
 		};
diff --git a/sprout/range/adaptor/taken_end.hpp b/sprout/range/adaptor/taken_end.hpp
index f8c84048..cec229cb 100644
--- a/sprout/range/adaptor/taken_end.hpp
+++ b/sprout/range/adaptor/taken_end.hpp
@@ -38,7 +38,7 @@ namespace sprout {
 		public:
 			template<typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::take_end_holder<Difference>
-			operator()(Difference distance) {
+			operator()(Difference distance) const {
 				return sprout::adaptors::take_end_holder<Difference>(distance);
 			}
 		};
diff --git a/sprout/range/adaptor/transformed.hpp b/sprout/range/adaptor/transformed.hpp
index 5ba44e90..d3f1af8e 100644
--- a/sprout/range/adaptor/transformed.hpp
+++ b/sprout/range/adaptor/transformed.hpp
@@ -136,7 +136,7 @@ namespace sprout {
 				BinaryFunction,
 				typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
 			>
-			operator()(RRange&& range, BinaryFunction func) {
+			operator()(RRange&& range, BinaryFunction func) const {
 				return sprout::adaptors::transform_holder<
 					BinaryFunction,
 					typename std::remove_reference<typename sprout::lvalue_reference<RRange>::type>::type
@@ -147,7 +147,7 @@ namespace sprout {
 			}
 			template<typename UnaryFunction>
 			SPROUT_CONSTEXPR sprout::adaptors::transform_holder<UnaryFunction>
-			operator()(UnaryFunction func) {
+			operator()(UnaryFunction func) const {
 				return sprout::adaptors::transform_holder<UnaryFunction>(func);
 			}
 		};
diff --git a/sprout/range/adaptor/triangle_wave.hpp b/sprout/range/adaptor/triangle_wave.hpp
index 3050fa80..c1516a64 100644
--- a/sprout/range/adaptor/triangle_wave.hpp
+++ b/sprout/range/adaptor/triangle_wave.hpp
@@ -106,7 +106,7 @@ namespace sprout {
 				Value const& frequency = 1,
 				Value const& amplitude = 1,
 				Value const& phase = 0
-				)
+				) const
 			{
 				return sprout::adaptors::triangle_wave_range<Value>(frequency, amplitude, phase);
 			}
diff --git a/sprout/range/adaptor/uniqued.hpp b/sprout/range/adaptor/uniqued.hpp
index dc8023ca..4ae894cd 100644
--- a/sprout/range/adaptor/uniqued.hpp
+++ b/sprout/range/adaptor/uniqued.hpp
@@ -78,7 +78,7 @@ namespace sprout {
 		public:
 			template<typename Predicate>
 			SPROUT_CONSTEXPR sprout::adaptors::unique_holder<Predicate>
-			operator()(Predicate pred) {
+			operator()(Predicate pred) const {
 				return sprout::adaptors::unique_holder<Predicate>(pred);
 			}
 		};
diff --git a/sprout/range/adaptor/valued.hpp b/sprout/range/adaptor/valued.hpp
index 0a0e7c88..6165fdb7 100644
--- a/sprout/range/adaptor/valued.hpp
+++ b/sprout/range/adaptor/valued.hpp
@@ -72,7 +72,7 @@ namespace sprout {
 		public:
 			template<typename T>
 			SPROUT_CONSTEXPR sprout::adaptors::valued_holder<T>
-			operator()(T const& param) {
+			operator()(T const& param) const {
 				return sprout::adaptors::valued_holder<T>(param);
 			}
 		};
diff --git a/sprout/range/adaptor/window.hpp b/sprout/range/adaptor/window.hpp
index 9ee517d5..3a5568dd 100644
--- a/sprout/range/adaptor/window.hpp
+++ b/sprout/range/adaptor/window.hpp
@@ -57,12 +57,12 @@ namespace sprout {
 		public:
 			template<typename Difference1, typename Difference2>
 			SPROUT_CONSTEXPR sprout::adaptors::window_holder<Difference1, Difference2>
-			operator()(Difference1 to_first, Difference2 to_last) {
+			operator()(Difference1 to_first, Difference2 to_last) const {
 				return sprout::adaptors::window_holder<Difference1, Difference2>(to_first, to_last);
 			}
 			template<typename Difference>
 			SPROUT_CONSTEXPR sprout::adaptors::window_holder<Difference>
-			operator()(Difference to_first) {
+			operator()(Difference to_first) const {
 				return sprout::adaptors::window_holder<Difference>(to_first);
 			}
 		};
diff --git a/sprout/string/npos.hpp b/sprout/string/npos.hpp
index 61968484..13b70554 100644
--- a/sprout/string/npos.hpp
+++ b/sprout/string/npos.hpp
@@ -1,6 +1,7 @@
 #ifndef SPROUT_STRING_NPOS_HPP
 #define SPROUT_STRING_NPOS_HPP
 
+#include <type_traits>
 #include <sprout/config.hpp>
 #include <sprout/type_traits/is_uint.hpp>
 #include <sprout/type_traits/enabler_if.hpp>
@@ -11,6 +12,11 @@ namespace sprout {
 	// npos
 	//
 	struct npos_t {
+	public:
+		template<typename UIntType, typename sprout::enabler_if<sprout::is_uint<UIntType>::value>::type = sprout::enabler>
+		struct get
+			: public std::integral_constant<UIntType, UIntType(-1)>
+		{};
 	public:
 		template<typename UIntType, typename sprout::enabler_if<sprout::is_uint<UIntType>::value>::type = sprout::enabler>
 		SPROUT_CONSTEXPR operator UIntType() const {
@@ -18,7 +24,7 @@ namespace sprout {
 		}
 	};
 	namespace {
-		SPROUT_STATIC_CONSTEXPR sprout::npos_t npos{};
+		SPROUT_STATIC_CONSTEXPR sprout::npos_t npos = {};
 	}	// anonymous-namespace
 
 	//
diff --git a/sprout/string/string.hpp b/sprout/string/string.hpp
index e0f75657..364294d7 100644
--- a/sprout/string/string.hpp
+++ b/sprout/string/string.hpp
@@ -76,7 +76,7 @@ namespace sprout {
 		{};
 #endif
 	public:
-		SPROUT_STATIC_CONSTEXPR size_type npos = sprout::npos;
+		SPROUT_STATIC_CONSTEXPR size_type npos = sprout::npos_t::get<size_type>::value;
 		SPROUT_STATIC_CONSTEXPR size_type static_size = N;
 	private:
 		template<sprout::index_t... Indexes>
diff --git a/sprout/utility/string_ref/string_ref.hpp b/sprout/utility/string_ref/string_ref.hpp
index e5b204dc..32e1260f 100644
--- a/sprout/utility/string_ref/string_ref.hpp
+++ b/sprout/utility/string_ref/string_ref.hpp
@@ -66,7 +66,7 @@ namespace sprout {
 		{};
 #endif
 	public:
-		SPROUT_STATIC_CONSTEXPR size_type npos = sprout::npos;
+		SPROUT_STATIC_CONSTEXPR size_type npos = sprout::npos_t::get<size_type>::value;
 	private:
 		const_pointer ptr_;
 		size_type len_;
@@ -211,7 +211,7 @@ namespace sprout {
 			len_ -= n;
 		}
 		SPROUT_CONSTEXPR basic_string_ref
-		remove_prefix(size_type n) {
+		remove_prefix(size_type n) const {
 			return n > size() ? basic_string_ref(data() + size(), 0)
 				: basic_string_ref(data() + n, size() - n)
 				;
@@ -224,7 +224,7 @@ namespace sprout {
 			len_ -= n;
 		}
 		SPROUT_CONSTEXPR basic_string_ref
-		remove_suffix(size_type n) {
+		remove_suffix(size_type n) const {
 			return n > size() ? basic_string_ref(data(), 0)
 				: basic_string_ref(data(), size() - n)
 				;