diff --git a/sprout/cstdlib.hpp b/sprout/cstdlib.hpp
index 961c26b9..42e5a085 100644
--- a/sprout/cstdlib.hpp
+++ b/sprout/cstdlib.hpp
@@ -2,7 +2,7 @@
 #define SPROUT_CSTDLIB_HPP
 
 #include <sprout/config.hpp>
-#include <sprout/cstdlib/abs.hpp>
-#include <sprout/cstdlib/div.hpp>
+#include <sprout/cstdlib/conversion.hpp>
+#include <sprout/cstdlib/arithmetic.hpp>
 
 #endif	// #ifndef SPROUT_CSTDLIB_HPP
diff --git a/sprout/cstdlib/arithmetic.hpp b/sprout/cstdlib/arithmetic.hpp
new file mode 100644
index 00000000..7cb35ca9
--- /dev/null
+++ b/sprout/cstdlib/arithmetic.hpp
@@ -0,0 +1,8 @@
+#ifndef SPROUT_CSTDLIB_ARITHMETIC_HPP
+#define SPROUT_CSTDLIB_ARITHMETIC_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/cstdlib/abs.hpp>
+#include <sprout/cstdlib/div.hpp>
+
+#endif	// #ifndef SPROUT_CSTDLIB_ARITHMETIC_HPP
diff --git a/sprout/cstdlib/atoi.hpp b/sprout/cstdlib/atoi.hpp
new file mode 100644
index 00000000..d8745310
--- /dev/null
+++ b/sprout/cstdlib/atoi.hpp
@@ -0,0 +1,20 @@
+#ifndef SPROUT_CSTDLIB_ATOI_HPP
+#define SPROUT_CSTDLIB_ATOI_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/cstdlib/decimal_to_int.hpp>
+
+namespace sprout {
+	//
+	// atoi
+	//
+	inline SPROUT_CONSTEXPR int atoi(char const* str) {
+		return sprout::decimal_to_int<int>(str);
+	}
+	template<typename Char>
+	inline SPROUT_CONSTEXPR int atoi(Char const* str) {
+		return sprout::decimal_to_int<int>(str);
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_CSTDLIB_ATOI_HPP
diff --git a/sprout/cstdlib/atol.hpp b/sprout/cstdlib/atol.hpp
new file mode 100644
index 00000000..81cbf6bd
--- /dev/null
+++ b/sprout/cstdlib/atol.hpp
@@ -0,0 +1,20 @@
+#ifndef SPROUT_CSTDLIB_ATOL_HPP
+#define SPROUT_CSTDLIB_ATOL_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/cstdlib/decimal_to_int.hpp>
+
+namespace sprout {
+	//
+	// atol
+	//
+	inline SPROUT_CONSTEXPR long atol(char const* str) {
+		return sprout::decimal_to_int<long>(str);
+	}
+	template<typename Char>
+	inline SPROUT_CONSTEXPR int atol(Char const* str) {
+		return sprout::decimal_to_int<int>(str);
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_CSTDLIB_ATOL_HPP
diff --git a/sprout/cstdlib/conversion.hpp b/sprout/cstdlib/conversion.hpp
new file mode 100644
index 00000000..a4ad07a5
--- /dev/null
+++ b/sprout/cstdlib/conversion.hpp
@@ -0,0 +1,11 @@
+#ifndef SPROUT_CSTDLIB_CONVERSION_HPP
+#define SPROUT_CSTDLIB_CONVERSION_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/cstdlib/decimal_to_int.hpp>
+#include <sprout/cstdlib/str_to_int.hpp>
+#include <sprout/cstdlib/atoi.hpp>
+#include <sprout/cstdlib/atol.hpp>
+#include <sprout/cstdlib/strtol.hpp>
+
+#endif	// #ifndef SPROUT_CSTDLIB_CONVERSION_HPP
diff --git a/sprout/cstdlib/decimal_to_int.hpp b/sprout/cstdlib/decimal_to_int.hpp
new file mode 100644
index 00000000..7ef60f27
--- /dev/null
+++ b/sprout/cstdlib/decimal_to_int.hpp
@@ -0,0 +1,51 @@
+#ifndef SPROUT_CSTDLIB_DECIMAL_TO_INT_HPP
+#define SPROUT_CSTDLIB_DECIMAL_TO_INT_HPP
+
+#include <iterator>
+#include <limits>
+#include <type_traits>
+#include <sprout/config.hpp>
+#include <sprout/iterator/operation.hpp>
+#include <sprout/ctype/ascii.hpp>
+#include <sprout/utility/enabler_if.hpp>
+
+namespace sprout {
+	namespace detail {
+		// Copyright (c) 2011 osyo-manga : http://d.hatena.ne.jp/osyo-manga/
+
+		template<typename IntType, typename CStrIterator>
+		inline SPROUT_CONSTEXPR IntType decimal_to_int_impl(CStrIterator str, IntType val, bool negative) {
+			return !sprout::ascii::isdigit(*str)
+				? negative ? -val : val
+				: val > (std::numeric_limits<IntType>::max() - (*str - static_cast<typename std::iterator_traits<CStrIterator>::value_type>('0')) - (negative ? 1 : 0)) / 10
+				? (negative ? std::numeric_limits<IntType>::min() : std::numeric_limits<IntType>::max())
+				: sprout::detail::decimal_to_int_impl<IntType>(
+					sprout::next(str),
+					val * 10 + (*str - static_cast<typename std::iterator_traits<CStrIterator>::value_type>('0')),
+					negative
+					)
+				;
+		}
+		template<typename IntType, typename CStrIterator>
+		inline SPROUT_CONSTEXPR IntType decimal_to_int(CStrIterator str) {
+			return sprout::ascii::isspace(*str)
+				? sprout::detail::decimal_to_int<IntType>(sprout::next(str))
+				: *str == static_cast<typename std::iterator_traits<CStrIterator>::value_type>('-')
+				? sprout::detail::decimal_to_int_impl<IntType>(sprout::next(str), IntType(), true)
+				: *str == static_cast<typename std::iterator_traits<CStrIterator>::value_type>('+')
+				? sprout::detail::decimal_to_int_impl<IntType>(sprout::next(str), IntType(), false)
+				: sprout::detail::decimal_to_int_impl<IntType>(str, IntType(), false)
+				;
+		}
+	}	// namespace detail
+
+	//
+	// decimal_to_int
+	//
+	template<typename IntType, typename Char, typename sprout::enabler_if<std::is_integral<IntType>::value>::type = sprout::enabler>
+	inline SPROUT_CONSTEXPR IntType decimal_to_int(Char const* str) {
+		return sprout::detail::decimal_to_int<IntType>(str);
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_CSTDLIB_DECIMAL_TO_INT_HPP
diff --git a/sprout/cstdlib/str_to_int.hpp b/sprout/cstdlib/str_to_int.hpp
new file mode 100644
index 00000000..3f899b51
--- /dev/null
+++ b/sprout/cstdlib/str_to_int.hpp
@@ -0,0 +1,95 @@
+#ifndef SPROUT_CSTDLIB_STR_TO_INT_HPP
+#define SPROUT_CSTDLIB_STR_TO_INT_HPP
+
+#include <cstddef>
+#include <cstdlib>
+#include <limits>
+#include <sprout/config.hpp>
+#include <sprout/iterator/operation.hpp>
+#include <sprout/utility/enabler_if.hpp>
+#include <sprout/detail/char_conversion.hpp>
+
+namespace sprout {
+	namespace detail {
+		// Copyright (c) 2011 osyo-manga : http://d.hatena.ne.jp/osyo-manga/
+
+		template<typename IntType, typename CStrIterator>
+		inline SPROUT_CONSTEXPR IntType str_to_int_impl_1(CStrIterator str, int base, IntType val, IntType x, bool negative) {
+			return x == static_cast<IntType>(-1)
+				? (negative ? -val : val)
+				: val > (std::numeric_limits<IntType>::max() - x - (negative ? 1 : 0)) / base
+				? (negative ? std::numeric_limits<IntType>::min() : std::numeric_limits<IntType>::max())
+				: sprout::detail::str_to_int_impl_1<IntType>(
+					sprout::next(str),
+					base,
+					val * base + x,
+					sprout::detail::char_to_int<IntType>(*sprout::next(str), base),
+					negative
+					)
+				;
+		}
+		template<typename IntType, typename CStrIterator>
+		inline SPROUT_CONSTEXPR IntType str_to_int_impl(CStrIterator str, int base, bool negative) {
+			return *str == static_cast<typename std::iterator_traits<CStrIterator>::value_type>('0')
+				? *sprout::next(str) == static_cast<typename std::iterator_traits<CStrIterator>::value_type>('x')
+					|| *sprout::next(str) == static_cast<typename std::iterator_traits<CStrIterator>::value_type>('X')
+					? sprout::detail::str_to_int_impl_1<IntType>(
+						sprout::next(str, 2),
+						base ? base : 16,
+						IntType(),
+						sprout::detail::char_to_int<IntType>(*sprout::next(str, 2), base),
+						negative
+						)
+					: sprout::detail::str_to_int_impl_1<IntType>(
+						sprout::next(str),
+						base ? base : 8,
+						IntType(),
+						sprout::detail::char_to_int<IntType>(*sprout::next(str), base),
+						negative
+						)
+				: sprout::detail::str_to_int_impl_1<IntType>(
+					str,
+					base ? base : 10,
+					IntType(),
+					sprout::detail::char_to_int<IntType>(*str, base),
+					negative
+					)
+				;
+		}
+		template<typename IntType, typename CStrIterator>
+		inline SPROUT_CONSTEXPR IntType str_to_int(CStrIterator str, int base) {
+			return sprout::ascii::isspace(*str)
+				? sprout::detail::str_to_int<IntType>(sprout::next(str), base)
+				: *str == static_cast<typename std::iterator_traits<CStrIterator>::value_type>('-')
+				? sprout::detail::str_to_int_impl<IntType>(sprout::next(str), base, true)
+				: *str == static_cast<typename std::iterator_traits<CStrIterator>::value_type>('+')
+				? sprout::detail::str_to_int_impl<IntType>(sprout::next(str), base, false)
+				: sprout::detail::str_to_int_impl<IntType>(str, base, false)
+				;
+		}
+		template<typename IntType, typename CStrIterator, typename CharPtr>
+		inline SPROUT_CONSTEXPR IntType str_to_int(CStrIterator str, CharPtr* endptr, int base) {
+			return !endptr ? sprout::detail::str_to_int<IntType>(str, base)
+				: std::strtol(str, endptr, base)
+				;
+		}
+	}	// namespace detail
+
+	//
+	// str_to_int
+	//
+	template<typename IntType, typename Char, typename sprout::enabler_if<std::is_integral<IntType>::value>::type = sprout::enabler>
+	inline SPROUT_CONSTEXPR IntType str_to_int(Char const* str, Char** endptr, int base = 10) {
+		return sprout::detail::str_to_int<IntType>(str, endptr, base);
+	}
+	template<typename IntType, typename Char, typename sprout::enabler_if<std::is_integral<IntType>::value>::type = sprout::enabler>
+	inline SPROUT_CONSTEXPR IntType str_to_int(Char const* str, std::nullptr_t endptr, int base = 10) {
+		return sprout::detail::str_to_int<IntType>(str, base);
+	}
+	template<typename IntType, typename Char, typename sprout::enabler_if<std::is_integral<IntType>::value>::type = sprout::enabler>
+	inline SPROUT_CONSTEXPR IntType str_to_int(Char const* str, int base = 10) {
+		return sprout::detail::str_to_int<IntType>(str, base);
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_CSTDLIB_STR_TO_INT_HPP
diff --git a/sprout/cstdlib/strtol.hpp b/sprout/cstdlib/strtol.hpp
new file mode 100644
index 00000000..3ac23064
--- /dev/null
+++ b/sprout/cstdlib/strtol.hpp
@@ -0,0 +1,29 @@
+#ifndef SPROUT_CSTDLIB_STRTOL_HPP
+#define SPROUT_CSTDLIB_STRTOL_HPP
+
+#include <cstddef>
+#include <sprout/config.hpp>
+#include <sprout/cstdlib/str_to_int.hpp>
+
+namespace sprout {
+	//
+	// strtol
+	//
+	inline SPROUT_CONSTEXPR long strtol(char const* str, char** endptr, int base = 10){
+		return sprout::str_to_int<long>(str, endptr, base);
+	}
+	template<typename Char>
+	inline SPROUT_CONSTEXPR long strtol(Char const* str, Char** endptr, int base = 10){
+		return sprout::str_to_int<long>(str, endptr, base);
+	}
+	template<typename Char>
+	inline SPROUT_CONSTEXPR long strtol(Char const* str, std::nullptr_t endptr, int base = 10){
+		return sprout::str_to_int<long>(str, base);
+	}
+	template<typename Char>
+	inline SPROUT_CONSTEXPR long strtol(Char const* str, int base = 10){
+		return sprout::str_to_int<long>(str, base);
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_CSTDLIB_STRTOL_HPP
diff --git a/sprout/detail/char_conversion.hpp b/sprout/detail/char_conversion.hpp
new file mode 100644
index 00000000..de4b64d6
--- /dev/null
+++ b/sprout/detail/char_conversion.hpp
@@ -0,0 +1,41 @@
+#ifndef SPROUT_DETAIL_CHAR_CONVERSION_HPP
+#define SPROUT_DETAIL_CHAR_CONVERSION_HPP
+
+#include <cstddef>
+#include <stdexcept>
+#include <sprout/config.hpp>
+
+namespace sprout {
+	namespace detail {
+		template<typename Elem, typename IntType>
+		inline SPROUT_CONSTEXPR Elem int_to_char(IntType val, std::size_t base){
+			return val >= 0 && val < 10 ? static_cast<Elem>('0') + val
+				: val >= 10 && val < base ? static_cast<Elem>('a') + (val - 10)
+				: throw std::invalid_argument("value out of bounds")
+				;
+		}
+		template<typename Elem, typename IntType>
+		inline SPROUT_CONSTEXPR Elem int_to_char(IntType val){
+			return val >= 0 && val < 10 ? static_cast<Elem>('0') + val
+				: throw std::invalid_argument("value out of bounds")
+				;
+		}
+
+		template<typename IntType, typename Elem>
+		inline SPROUT_CONSTEXPR IntType char_to_int(Elem c, std::size_t base){
+			return c >= static_cast<Elem>('0') && c <= static_cast<Elem>('9') ? c - static_cast<Elem>('0')
+				: c >= static_cast<Elem>('a') && c <= static_cast<Elem>('a' + (base - 11)) ? c - static_cast<Elem>('a') + 10
+				: c >= static_cast<Elem>('A') && c <= static_cast<Elem>('A' + (base - 11)) ? c - static_cast<Elem>('A') + 10
+				: static_cast<IntType>(-1)
+				;
+		}
+		template<typename IntType, typename Elem>
+		inline SPROUT_CONSTEXPR IntType char_to_int(Elem c){
+			return c >= static_cast<Elem>('0') && c <= static_cast<Elem>('9') ? c - static_cast<Elem>('0')
+				: static_cast<IntType>(-1)
+				;
+		}
+	}	// namespace detail
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_DETAIL_CHAR_CONVERSION_HPP
diff --git a/sprout/detail/param_at.hpp b/sprout/detail/param_at.hpp
index 4e3551dc..27b33bc1 100644
--- a/sprout/detail/param_at.hpp
+++ b/sprout/detail/param_at.hpp
@@ -6,32 +6,32 @@
 #include <sprout/config.hpp>
 
 namespace sprout {
-		namespace detail {
-			template<typename R, typename T, typename... Values>
-			inline SPROUT_CONSTEXPR typename std::enable_if<
-				sizeof...(Values) == 0,
-				R
-			>::type param_at(
-				std::size_t n,
-				T const& v,
-				Values const&... values
-				)
-			{
-				return v;
-			}
-			template<typename R, typename T, typename... Values>
-			inline SPROUT_CONSTEXPR typename std::enable_if<
-				sizeof...(Values) != 0,
-				R
-			>::type param_at(
-				std::size_t n,
-				T const& v,
-				Values const&... values
-				)
-			{
-				return n == 0 ? v : sprout::detail::param_at<R>(n - 1, values...);
-			}
-		}	// namespace detail
+	namespace detail {
+		template<typename R, typename T, typename... Values>
+		inline SPROUT_CONSTEXPR typename std::enable_if<
+			sizeof...(Values) == 0,
+			R
+		>::type param_at(
+			std::size_t n,
+			T const& v,
+			Values const&... values
+			)
+		{
+			return v;
+		}
+		template<typename R, typename T, typename... Values>
+		inline SPROUT_CONSTEXPR typename std::enable_if<
+			sizeof...(Values) != 0,
+			R
+		>::type param_at(
+			std::size_t n,
+			T const& v,
+			Values const&... values
+			)
+		{
+			return n == 0 ? v : sprout::detail::param_at<R>(n - 1, values...);
+		}
+	}	// namespace detail
 }	// namespace sprout
 
 #endif	// #ifndef SPROUT_DETAIL_PARAM_AT_HPP
diff --git a/sprout/string/to_string.hpp b/sprout/string/to_string.hpp
index 61cdace5..edb8c748 100644
--- a/sprout/string/to_string.hpp
+++ b/sprout/string/to_string.hpp
@@ -2,39 +2,16 @@
 #define SPROUT_STRING_TO_STRING_HPP
 
 #include <cstddef>
-#include <stdexcept>
 #include <type_traits>
+#include <sprout/config.hpp>
 #include <sprout/string/string.hpp>
 #include <sprout/string/make_string.hpp>
 #include <sprout/integer/integer_digits.hpp>
 #include <sprout/utility/enabler_if.hpp>
+#include <sprout/detail/char_conversion.hpp>
 
 namespace sprout {
 	namespace detail {
-		template<
-			typename Elem,
-			std::size_t Base,
-			typename IntType,
-			typename sprout::enabler_if<(Base < 10)>::type = sprout::enabler
-		>
-		inline SPROUT_CONSTEXPR Elem int_to_char(IntType val){
-			return val >= 0 && val < 10 ? static_cast<Elem>('0') + val
-				: throw std::invalid_argument("value out of bounds")
-				;
-		}
-		template<
-			typename Elem,
-			std::size_t Base,
-			typename IntType,
-			typename sprout::enabler_if<(Base >= 10 && Base < 36)>::type = sprout::enabler
-		>
-		inline SPROUT_CONSTEXPR Elem int_to_char(IntType val){
-			return val >= 0 && val < 10 ? static_cast<Elem>('0') + val
-				: val >= 10 && val < 36 ? static_cast<Elem>('a') + (val - 10)
-				: throw std::invalid_argument("value out of bounds")
-				;
-		}
-
 		template<
 			typename Elem,
 			std::size_t Base,
@@ -64,7 +41,7 @@ namespace sprout {
 				: sprout::detail::int_to_string_impl_1<Elem, Base>(
 					val / Base,
 					negative,
-					sprout::detail::int_to_char<Elem, Base>(val % Base),
+					sprout::detail::int_to_char<Elem>(val % Base, Base),
 					args...
 					)
 				;
@@ -102,7 +79,7 @@ namespace sprout {
 				? sprout::make_string_as<Elem>(args...)
 				: sprout::detail::uint_to_string_impl_1<Elem, Base>(
 					val / Base,
-					sprout::detail::int_to_char<Elem, Base>(val % Base),
+					sprout::detail::int_to_char<Elem>(val % Base, Base),
 					args...
 					)
 				;