diff --git a/sprout/optional.hpp b/sprout/optional.hpp
index bf137694..f832d0a8 100644
--- a/sprout/optional.hpp
+++ b/sprout/optional.hpp
@@ -9,5 +9,6 @@
 #include <sprout/optional/make_optional.hpp>
 #include <sprout/optional/get.hpp>
 #include <sprout/optional/hash.hpp>
+#include <sprout/optional/exceptions.hpp>
 
 #endif	// #ifndef SPROUT_OPTIONAL_HPP
diff --git a/sprout/optional/exceptions.hpp b/sprout/optional/exceptions.hpp
new file mode 100644
index 00000000..3dcb40f7
--- /dev/null
+++ b/sprout/optional/exceptions.hpp
@@ -0,0 +1,25 @@
+#ifndef SPROUT_OPTIONAL_EXCEPTIONS_HPP
+#define SPROUT_OPTIONAL_EXCEPTIONS_HPP
+
+#include <string>
+#include <stdexcept>
+#include <sprout/config.hpp>
+
+namespace sprout {
+	//
+	// bad_optional_access
+	//
+	class bad_optional_access
+		: public std::logic_error
+	{
+	public:
+		explicit bad_optional_access(std::string const& what_arg)
+			: std::logic_error(what_arg)
+		{}
+		explicit bad_optional_access(char const* what_arg)
+			: std::logic_error(what_arg)
+		{}
+	};
+}	// namespace sprout
+
+#endif	// SPROUT_OPTIONAL_EXCEPTIONS_HPP
diff --git a/sprout/optional/optional.hpp b/sprout/optional/optional.hpp
index 7420b2e8..d2b061af 100644
--- a/sprout/optional/optional.hpp
+++ b/sprout/optional/optional.hpp
@@ -6,6 +6,7 @@
 #include <sprout/utility/swap.hpp>
 #include <sprout/none.hpp>
 #include <sprout/optional/nullopt.hpp>
+#include <sprout/optional/exceptions.hpp>
 #include <sprout/assert.hpp>
 
 namespace sprout {
@@ -53,12 +54,12 @@ namespace sprout {
 		{}
 		SPROUT_CONSTEXPR optional(optional const& v)
 			: init(v.init)
-			, val(v.val)
+			, val(v.is_initialized() ? holder_type(*v) : holder_type())
 		{}
 		template<typename U>
 		explicit SPROUT_CONSTEXPR optional(optional<U> const& v)
 			: init(v.is_initialized())
-			, val(v.is_initialized() ? v.get() : holder_type())
+			, val(v.is_initialized() ? holder_type(*v) : holder_type())
 		{}
 
 		optional& operator=(sprout::nullopt_t v) SPROUT_NOEXCEPT {
@@ -114,21 +115,25 @@ namespace sprout {
 		}
 
 		SPROUT_CONSTEXPR reference_const_type operator*() const {
-			return get();
-		}
-		reference_type operator*() {
-			return get();
-		}
-		SPROUT_CONSTEXPR reference_const_type get() const {
 			return (SPROUT_ASSERT(is_initialized()), true) ? val.get()
 				: val.get()
 				;
 		}
-		reference_type get() {
+		reference_type operator*() {
 			return (SPROUT_ASSERT(is_initialized()), true) ? val.get()
 				: val.get()
 				;
 		}
+		SPROUT_CONSTEXPR reference_const_type get() const {
+			return is_initialized() ? val.get()
+				: (throw sprout::bad_optional_access("optional<>: bad optional access"), val.get())
+				;
+		}
+		reference_type get() {
+			return is_initialized() ? val.get()
+				: (throw sprout::bad_optional_access("optional<>: bad optional access"), val.get())
+				;
+		}
 		SPROUT_CONSTEXPR reference_const_type get_value_or(reference_const_type& v) const {
 			return is_initialized() ? val.get()
 				: v
@@ -140,6 +145,19 @@ namespace sprout {
 				;
 		}
 
+		SPROUT_CONSTEXPR reference_const_type value() const {
+			return get();
+		}
+		reference_type value() {
+			return get();
+		}
+		SPROUT_CONSTEXPR reference_const_type value_or(reference_const_type& v) const {
+			return get_value_or(v);
+		}
+		reference_type value_or(reference_type& v) {
+			return get_value_or(v);
+		}
+
 		SPROUT_CONSTEXPR pointer_const_type operator->() const {
 			return SPROUT_ASSERT(is_initialized()),
 				val.get_pointer()
@@ -151,13 +169,13 @@ namespace sprout {
 				;
 		}
 		SPROUT_CONSTEXPR pointer_const_type get_pointer() const {
-			return SPROUT_ASSERT(is_initialized()),
-				val.get_pointer()
+			return is_initialized() ? val.get_pointer()
+				: 0
 				;
 		}
 		pointer_type get_pointer() {
-			return SPROUT_ASSERT(is_initialized()),
-				val.get_pointer()
+			return is_initialized() ? val.get_pointer()
+				: 0
 				;
 		}
 		SPROUT_CONSTEXPR pointer_const_type get_ptr() const {
diff --git a/sprout/tuple/tuple/flexibly_construct.hpp b/sprout/tuple/flexibly_construct.hpp
similarity index 68%
rename from sprout/tuple/tuple/flexibly_construct.hpp
rename to sprout/tuple/flexibly_construct.hpp
index 7ebffbf4..2a71fc5b 100644
--- a/sprout/tuple/tuple/flexibly_construct.hpp
+++ b/sprout/tuple/flexibly_construct.hpp
@@ -1,5 +1,5 @@
-#ifndef SPROUT_TUPLE_TUPLE_FREXIBLY_CONSTRUCT_HPP
-#define SPROUT_TUPLE_TUPLE_FREXIBLY_CONSTRUCT_HPP
+#ifndef SPROUT_TUPLE_FREXIBLY_CONSTRUCT_HPP
+#define SPROUT_TUPLE_FREXIBLY_CONSTRUCT_HPP
 
 #include <sprout/config.hpp>
 
@@ -17,4 +17,4 @@ namespace sprout {
 	using sprout::tuples::flexibly_construct;
 }	// namespace sprout
 
-#endif	// #ifndef SPROUT_TUPLE_TUPLE_FREXIBLY_CONSTRUCT_HPP
+#endif	// #ifndef SPROUT_TUPLE_FREXIBLY_CONSTRUCT_HPP
diff --git a/sprout/tuple/tuple.hpp b/sprout/tuple/tuple.hpp
index 448fb9f2..32363633 100644
--- a/sprout/tuple/tuple.hpp
+++ b/sprout/tuple/tuple.hpp
@@ -11,6 +11,6 @@
 #include <sprout/tuple/tuple/ignore.hpp>
 #include <sprout/tuple/tuple/make_tuple.hpp>
 #include <sprout/tuple/tuple/type_traits.hpp>
-#include <sprout/tuple/tuple/flexibly_construct.hpp>
+#include <sprout/tuple/flexibly_construct.hpp>
 
 #endif	// #ifndef SPROUT_TUPLE_TUPLE_HPP
diff --git a/sprout/tuple/tuple/tuple.hpp b/sprout/tuple/tuple/tuple.hpp
index 15e7841d..814f02b4 100644
--- a/sprout/tuple/tuple/tuple.hpp
+++ b/sprout/tuple/tuple/tuple.hpp
@@ -11,7 +11,7 @@
 #include <sprout/type_traits/is_convert_constructible.hpp>
 #include <sprout/tpp/algorithm/all_of.hpp>
 #include <sprout/tuple/tuple/tuple_decl.hpp>
-#include <sprout/tuple/tuple/flexibly_construct.hpp>
+#include <sprout/tuple/flexibly_construct.hpp>
 
 namespace sprout {
 	namespace tuples {
@@ -71,6 +71,7 @@ namespace sprout {
 		inline SPROUT_CONSTEXPR sprout::tuples::tuple<Types...>::tuple(sprout::pair<UTypes...>&& t)
 			: impl_type(sprout::forward<typename sprout::pair<UTypes...>::first_type>(t.first), sprout::forward<typename sprout::pair<UTypes...>::second_type>(t.second))
 		{}
+
 		template<typename... Types>
 		template<
 			typename... UTypes,
diff --git a/sprout/tuple/tuple/tuple_decl.hpp b/sprout/tuple/tuple/tuple_decl.hpp
index 8ef46639..a0bcaaf2 100644
--- a/sprout/tuple/tuple/tuple_decl.hpp
+++ b/sprout/tuple/tuple/tuple_decl.hpp
@@ -15,7 +15,7 @@
 #include <sprout/type_traits/is_convert_constructible.hpp>
 #include <sprout/tpp/algorithm/all_of.hpp>
 #include <sprout/tuple/tuple/tuple_fwd.hpp>
-#include <sprout/tuple/tuple/flexibly_construct.hpp>
+#include <sprout/tuple/flexibly_construct.hpp>
 
 namespace sprout {
 	namespace tuples {
@@ -245,9 +245,9 @@ namespace sprout {
 			typedef sprout::tuples::detail::tuple_impl<0, Types...> impl_type;
 		private:
 			template<typename TndexTuple, typename... Utypes>
-			struct is_flexibly_constructible_impl;
+			struct is_flexibly_convert_constructible_impl;
 			template<sprout::index_t... Indexes, typename... Utypes>
-			struct is_flexibly_constructible_impl<sprout::index_tuple<Indexes...>, Utypes...>
+			struct is_flexibly_convert_constructible_impl<sprout::index_tuple<Indexes...>, Utypes...>
 				: public sprout::tpp::all_of<
 					sprout::is_convert_constructible<
 						typename sprout::tppack_at<Indexes, Types...>::type,
@@ -262,19 +262,19 @@ namespace sprout {
 				>
 			{};
 			template<typename... UTypes>
-			struct is_flexibly_constructible
-				: public is_flexibly_constructible_impl<
+			struct is_flexibly_convert_constructible
+				: public is_flexibly_convert_constructible_impl<
 					typename sprout::index_range<0, (sizeof...(UTypes) < sizeof...(Types) ? sizeof...(UTypes) : sizeof...(Types))>::type,
 					UTypes...
 				>
 			{};
 			template<typename... UTypes>
-			struct is_rvref_flexibly_constructible
-				: public is_flexibly_constructible<UTypes&&...>
+			struct is_rvref_flexibly_convert_constructible
+				: public is_flexibly_convert_constructible<UTypes&&...>
 			{};
 			template<typename... UTypes>
-			struct is_clvref_flexibly_constructible
-				: public is_flexibly_constructible<UTypes const&...>
+			struct is_clvref_flexibly_convert_constructible
+				: public is_flexibly_convert_constructible<UTypes const&...>
 			{};
 		public:
 			// tuple construction
@@ -317,38 +317,39 @@ namespace sprout {
 				>::type
 			>
 			SPROUT_CONSTEXPR tuple(sprout::pair<UTypes...>&& t);
+
 			template<
 				typename... UTypes,
 				typename = typename std::enable_if<
-					is_rvref_flexibly_constructible<UTypes...>::value
+					is_rvref_flexibly_convert_constructible<UTypes...>::value
 				>::type
 			>
 			explicit SPROUT_CONSTEXPR tuple(sprout::tuples::flexibly_construct_t, UTypes&&... elements);
 			template<
 				typename... UTypes,
 				typename = typename std::enable_if<
-					is_clvref_flexibly_constructible<UTypes...>::value
+					is_clvref_flexibly_convert_constructible<UTypes...>::value
 				>::type
 			>
 			SPROUT_CONSTEXPR tuple(sprout::tuples::flexibly_construct_t, sprout::tuples::tuple<UTypes...> const& t);
 			template<
 				typename... UTypes,
 				typename = typename std::enable_if<
-					is_rvref_flexibly_constructible<UTypes...>::value
+					is_rvref_flexibly_convert_constructible<UTypes...>::value
 				>::type
 			>
 			SPROUT_CONSTEXPR tuple(sprout::tuples::flexibly_construct_t, sprout::tuples::tuple<UTypes...>&& t);
 			template<
 				typename... UTypes,
 				typename = typename std::enable_if<
-					is_clvref_flexibly_constructible<UTypes...>::value
+					is_clvref_flexibly_convert_constructible<UTypes...>::value
 				>::type
 			>
 			SPROUT_CONSTEXPR tuple(sprout::tuples::flexibly_construct_t, sprout::pair<UTypes...> const& t);
 			template<
 				typename... UTypes,
 				typename = typename std::enable_if<
-					is_rvref_flexibly_constructible<UTypes...>::value
+					is_rvref_flexibly_convert_constructible<UTypes...>::value
 				>::type
 			>
 			SPROUT_CONSTEXPR tuple(sprout::tuples::flexibly_construct_t, sprout::pair<UTypes...>&& t);
diff --git a/sprout/utility/pair/pair.hpp b/sprout/utility/pair/pair.hpp
index 1110d6f3..de6d9c27 100644
--- a/sprout/utility/pair/pair.hpp
+++ b/sprout/utility/pair/pair.hpp
@@ -1,11 +1,11 @@
 #ifndef SPROUT_UTILITY_PAIR_PAIR_HPP
 #define SPROUT_UTILITY_PAIR_PAIR_HPP
 
+#include <utility>
 #include <type_traits>
 #include <sprout/config.hpp>
 #include <sprout/index_tuple.hpp>
 #include <sprout/utility/forward.hpp>
-#include <sprout/utility/move.hpp>
 #include <sprout/utility/swap.hpp>
 #include <sprout/utility/pair/pair_decl.hpp>
 #include <sprout/tuple/tuple/tuple.hpp>
@@ -41,29 +41,41 @@ namespace sprout {
 		, second(y)
 	{}
 	template <typename T1, typename T2>
-	template<typename U, typename V>
+	template<
+		typename U, typename V,
+		typename
+	>
 	inline SPROUT_CONSTEXPR sprout::pair<T1, T2>::pair(U&& x, V&& y)
 		: first(sprout::forward<U>(x))
 		, second(sprout::forward<V>(y))
 	{}
 	template <typename T1, typename T2>
-	template<typename U, typename V>
+	template<
+		typename U, typename V,
+		typename
+	>
 	inline SPROUT_CONSTEXPR sprout::pair<T1, T2>::pair(sprout::pair<U, V> const& other)
 		: first(other.first)
 		, second(other.second)
 	{}
 	template <typename T1, typename T2>
-	template<typename U, typename V>
+	template<
+		typename U, typename V,
+		typename
+	>
 	inline SPROUT_CONSTEXPR sprout::pair<T1, T2>::pair(sprout::pair<U, V>&& other)
-		: first(sprout::move(other.first))
-		, second(sprout::move(other.second))
+		: first(sprout::forward<U>(other.first))
+		, second(sprout::forward<V>(other.second))
 	{}
 #if SPROUT_USE_DELEGATING_CONSTRUCTORS
 	template <typename T1, typename T2>
-	template <typename... Args1, typename... Args2>
+	template<
+		typename... Args1, typename... Args2,
+		typename
+	>
 	inline SPROUT_CONSTEXPR sprout::pair<T1, T2>::pair(
-		sprout::tuples::tuple<Args1...> first_args,
-		sprout::tuples::tuple<Args2...> second_args
+		std::piecewise_construct_t,
+		sprout::tuples::tuple<Args1...> first_args, sprout::tuples::tuple<Args2...> second_args
 		)
 		: pair(
 			first_args,
@@ -74,29 +86,75 @@ namespace sprout {
 	{}
 #endif	// #if SPROUT_USE_DELEGATING_CONSTRUCTORS
 	template <typename T1, typename T2>
-	inline sprout::pair<T1, T2>& sprout::pair<T1, T2>::operator=(pair const& other) = default;
+	template<
+		typename U, typename V,
+		typename
+	>
+	inline SPROUT_CONSTEXPR sprout::pair<T1, T2>::pair(sprout::tuples::tuple<U, V> const& other)
+		: first(sprout::tuples::get<0>(other))
+		, second(sprout::tuples::get<1>(other))
+	{}
 	template <typename T1, typename T2>
-	template<typename U, typename V>
-	inline sprout::pair<T1, T2>& sprout::pair<T1, T2>::operator=(sprout::pair<U, V> const& other) {
-		first = other.first;
-		second = other.second;
-		return *this;
-	}
+	template<
+		typename U, typename V,
+		typename
+	>
+	inline SPROUT_CONSTEXPR sprout::pair<T1, T2>::pair(sprout::tuples::tuple<U, V>&& other)
+		: first(sprout::forward<U>(sprout::tuples::get<0>(other)))
+		, second(sprout::forward<V>(sprout::tuples::get<1>(other)))
+	{}
+
 	template <typename T1, typename T2>
-	inline sprout::pair<T1, T2>& sprout::pair<T1, T2>::operator=(pair&& other)
+	inline sprout::pair<T1, T2>& sprout::pair<T1, T2>::operator=(pair const& rhs) = default;
+	template <typename T1, typename T2>
+	inline sprout::pair<T1, T2>& sprout::pair<T1, T2>::operator=(pair&& rhs)
 	SPROUT_NOEXCEPT_EXPR(std::is_nothrow_move_assignable<T1>::value && std::is_nothrow_move_assignable<T2>::value)
 	{
-		first = sprout::move(other.first);
-		second = sprout::move(other.second);
+		first = sprout::forward<T1>(rhs.first);
+		second = sprout::forward<T2>(rhs.second);
 		return *this;
 	}
 	template <typename T1, typename T2>
-	template<typename U, typename V>
-	inline sprout::pair<T1, T2>& sprout::pair<T1, T2>::operator=(sprout::pair<U, V>&& other) {
-		first = sprout::move(other.first);
-		second = sprout::move(other.second);
+	template<
+		typename U, typename V,
+		typename
+	>
+	inline sprout::pair<T1, T2>& sprout::pair<T1, T2>::operator=(sprout::pair<U, V> const& rhs) {
+		first = rhs.first;
+		second = rhs.second;
 		return *this;
 	}
+	template <typename T1, typename T2>
+	template<
+		typename U, typename V,
+		typename
+	>
+	inline sprout::pair<T1, T2>& sprout::pair<T1, T2>::operator=(sprout::pair<U, V>&& rhs) {
+		first = sprout::forward<U>(rhs.first);
+		second = sprout::forward<V>(rhs.second);
+		return *this;
+	}
+	template <typename T1, typename T2>
+	template<
+		typename U, typename V,
+		typename
+	>
+	inline sprout::pair<T1, T2>& sprout::pair<T1, T2>::operator=(sprout::tuples::tuple<U, V> const& rhs) {
+		first = sprout::tuples::get<0>(rhs);
+		second = sprout::tuples::get<0>(rhs);
+		return *this;
+	}
+	template <typename T1, typename T2>
+	template<
+		typename U, typename V,
+		typename
+	>
+	inline sprout::pair<T1, T2>& sprout::pair<T1, T2>::operator=(sprout::tuples::tuple<U, V>&& rhs) {
+		first = sprout::forward<U>(sprout::tuples::get<0>(rhs));
+		second = sprout::forward<V>(sprout::tuples::get<1>(rhs));
+		return *this;
+	}
+
 	template <typename T1, typename T2>
 	inline void sprout::pair<T1, T2>::swap(pair& other)
 	SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(sprout::swap(first, other.first)) && SPROUT_NOEXCEPT_EXPR(sprout::swap(second, other.second))) {
diff --git a/sprout/utility/pair/pair_decl.hpp b/sprout/utility/pair/pair_decl.hpp
index 124fae1b..0062c4ff 100644
--- a/sprout/utility/pair/pair_decl.hpp
+++ b/sprout/utility/pair/pair_decl.hpp
@@ -1,6 +1,7 @@
 #ifndef SPROUT_UTILITY_PAIR_PAIR_DECL_HPP
 #define SPROUT_UTILITY_PAIR_PAIR_DECL_HPP
 
+#include <utility>
 #include <type_traits>
 #include <sprout/config.hpp>
 #include <sprout/index_tuple.hpp>
@@ -34,24 +35,85 @@ namespace sprout {
 		SPROUT_CONSTEXPR pair(pair&&);
 		SPROUT_CONSTEXPR pair();
 		SPROUT_CONSTEXPR pair(T1 const& x, T2 const& y);
-		template<typename U, typename V>
+		template<
+			typename U, typename V,
+			typename = typename std::enable_if<
+				std::is_constructible<first_type, U&&>::value && std::is_constructible<second_type, V&&>::value
+			>::type
+		>
 		SPROUT_CONSTEXPR pair(U&& x, V&& y);
-		template<typename U, typename V>
+		template<
+			typename U, typename V,
+			typename = typename std::enable_if<
+				std::is_constructible<first_type, U const&>::value && std::is_constructible<second_type, V const&>::value
+			>::type
+		>
 		SPROUT_CONSTEXPR pair(sprout::pair<U, V> const& other);
-		template<typename U, typename V>
+		template<
+			typename U, typename V,
+			typename = typename std::enable_if<
+				std::is_constructible<first_type, U&&>::value && std::is_constructible<second_type, V&&>::value
+			>::type
+		>
 		SPROUT_CONSTEXPR pair(sprout::pair<U, V>&& other);
-		template <typename... Args1, typename... Args2>
+		template<
+			typename... Args1, typename... Args2,
+			typename = typename std::enable_if<
+				std::is_constructible<first_type, Args1&&...>::value && std::is_constructible<second_type, Args2&&...>::value
+			>::type
+		>
 		SPROUT_CONSTEXPR pair(
-			sprout::tuples::tuple<Args1...> first_args,
-			sprout::tuples::tuple<Args2...> second_args
+			std::piecewise_construct_t,
+			sprout::tuples::tuple<Args1...> first_args, sprout::tuples::tuple<Args2...> second_args
 			);
-		pair& operator=(pair const& other);
-		template<typename U, typename V>
-		pair& operator=(sprout::pair<U, V> const& other);
-		pair& operator=(pair&& other)
+
+		template<
+			typename U, typename V,
+			typename = typename std::enable_if<
+				std::is_constructible<first_type, U const&>::value && std::is_constructible<second_type, V const&>::value
+			>::type
+		>
+		SPROUT_CONSTEXPR pair(sprout::tuples::tuple<U, V> const& other);
+		template<
+			typename U, typename V,
+			typename = typename std::enable_if<
+				std::is_constructible<first_type, U&&>::value && std::is_constructible<second_type, V&&>::value
+			>::type
+		>
+		SPROUT_CONSTEXPR pair(sprout::tuples::tuple<U, V>&& other);
+
+		pair& operator=(pair const& rhs);
+		pair& operator=(pair&& rhs)
 		SPROUT_NOEXCEPT_EXPR(std::is_nothrow_move_assignable<T1>::value && std::is_nothrow_move_assignable<T2>::value);
-		template<typename U, typename V>
-		pair& operator=(sprout::pair<U, V>&& other);
+		template<
+			typename U, typename V,
+			typename = typename std::enable_if<
+				std::is_assignable<first_type&, U const&>::value && std::is_assignable<second_type&, V const&>::value
+			>::type
+		>
+		pair& operator=(sprout::pair<U, V> const& rhs);
+		template<
+			typename U, typename V,
+			typename = typename std::enable_if<
+				std::is_assignable<first_type&, U&&>::value && std::is_assignable<second_type&, V&&>::value
+			>::type
+		>
+		pair& operator=(sprout::pair<U, V>&& rhs);
+		template<
+			typename U, typename V,
+			typename = typename std::enable_if<
+				std::is_assignable<first_type&, U const&>::value && std::is_assignable<second_type&, V const&>::value
+			>::type
+		>
+		pair& operator=(sprout::tuples::tuple<U, V> const& rhs);
+		template<
+			typename U, typename V,
+			typename = typename std::enable_if<
+				std::is_assignable<first_type&, U&&>::value && std::is_assignable<second_type&, V&&>::value
+			>::type
+		>
+		pair& operator=(sprout::tuples::tuple<U, V>&& rhs);
+
 		void swap(pair& other)
 		SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(sprout::swap(first, other.first)) && SPROUT_NOEXCEPT_EXPR(sprout::swap(second, other.second)));
 	};