diff --git a/sprout/iterator/generator_iterator.hpp b/sprout/iterator/generator_iterator.hpp
index 39e4fee3..16a15326 100644
--- a/sprout/iterator/generator_iterator.hpp
+++ b/sprout/iterator/generator_iterator.hpp
@@ -17,16 +17,16 @@ namespace sprout {
 	class generator_iterator
 		: public std::iterator<
 			std::forward_iterator_tag,
-			typename std::remove_reference<decltype(sprout::generators::generated_value(std::declval<Generator>()))>::type,
+			typename std::remove_reference<decltype(sprout::generators::generated_value(std::declval<Generator const>()))>::type,
 			std::ptrdiff_t,
-			typename std::remove_reference<decltype(sprout::generators::generated_value(std::declval<Generator>()))>::type*,
-			decltype(sprout::generators::generated_value(std::declval<Generator>()))
+			typename std::remove_reference<decltype(sprout::generators::generated_value(std::declval<Generator const>()))>::type*,
+			decltype(sprout::generators::generated_value(std::declval<Generator const>()))
 		>
 	{
 	public:
 		typedef Generator generator_type;
 		typedef std::forward_iterator_tag iterator_category;
-		typedef decltype(sprout::generators::generated_value(std::declval<Generator>())) reference;
+		typedef decltype(sprout::generators::generated_value(std::declval<Generator const>())) reference;
 		typedef typename std::remove_reference<reference>::type value_type;
 		typedef std::ptrdiff_t difference_type;
 		typedef value_type* pointer;
@@ -96,7 +96,10 @@ namespace sprout {
 		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"), gen_.result())
+				: (
+					throw std::out_of_range("generator_iterator<>: dereference at out of range"),
+					sprout::generators::generated_value(gen_)
+					)
 				;
 		}
 		SPROUT_CONSTEXPR pointer operator->() const {
diff --git a/sprout/random/iterator.hpp b/sprout/random/iterator.hpp
index f5fb6f6a..dfad5893 100644
--- a/sprout/random/iterator.hpp
+++ b/sprout/random/iterator.hpp
@@ -1,7 +1,6 @@
 #ifndef SPROUT_RANDOM_ITERATOR_HPP
 #define SPROUT_RANDOM_ITERATOR_HPP
 
-#include <utility>
 #include <type_traits>
 #include <sprout/config.hpp>
 #include <sprout/iterator/generator_iterator.hpp>
@@ -14,26 +13,26 @@ namespace sprout {
 		//
 		template<
 			typename Engine, typename Distribution,
-			typename sprout::enabler_if<!std::is_integral<Distribution>::value>::tyep = sprout::enabler
+			typename sprout::enabler_if<!std::is_integral<Distribution>::value>::type = sprout::enabler
 		>
 		inline SPROUT_CONSTEXPR auto
 		begin(
 			Engine const& engine, Distribution const& distribution,
-			typename sprout::generator_iterator<typename std::decay<decltype(distribution(engine))>::type>::difference_type count = -1
+			typename sprout::generator_iterator<typename std::remove_reference<decltype(distribution(engine))>::type>::difference_type count = -1
 			)
-			-> sprout::generator_iterator<typename std::decay<decltype(distribution(engine))>::type>
+			-> sprout::generator_iterator<typename std::remove_reference<decltype(distribution(engine))>::type>
 		{
-			return sprout::generator_iterator<typename std::decay<decltype(distribution(engine))>::type>(distribution(engine), count);
+			return sprout::generator_iterator<typename std::remove_reference<decltype(distribution(engine))>::type>(distribution(engine), count);
 		}
 		template<typename Engine>
 		inline SPROUT_CONSTEXPR auto
 		begin(
 			Engine const& engine,
-			typename sprout::generator_iterator<typename std::decay<decltype(engine())>::type>::difference_type count = -1
+			typename sprout::generator_iterator<typename std::remove_reference<decltype(engine())>::type>::difference_type count = -1
 			)
-			-> sprout::generator_iterator<typename std::decay<decltype(engine())>::type>
+			-> sprout::generator_iterator<typename std::remove_reference<decltype(engine())>::type>
 		{
-			return sprout::generator_iterator<typename std::decay<decltype(engine())>::type>(engine(), count);
+			return sprout::generator_iterator<typename std::remove_reference<decltype(engine())>::type>(engine(), count);
 		}
 
 		//
@@ -41,20 +40,20 @@ namespace sprout {
 		//
 		template<
 			typename Engine, typename Distribution,
-			typename sprout::enabler_if<!std::is_integral<Distribution>::value>::tyep = sprout::enabler
+			typename sprout::enabler_if<!std::is_integral<Distribution>::value>::type = sprout::enabler
 		>
 		inline SPROUT_CONSTEXPR auto
 		end(Engine const& engine, Distribution const& distribution)
-			-> sprout::generator_iterator<typename std::decay<decltype(distribution(engine))>::type>
+			-> sprout::generator_iterator<typename std::remove_reference<decltype(distribution(engine))>::type>
 		{
-			return sprout::generator_iterator<typename std::decay<decltype(distribution(engine))>::type>();
+			return sprout::generator_iterator<typename std::remove_reference<decltype(distribution(engine))>::type>();
 		}
 		template<typename Engine>
 		inline SPROUT_CONSTEXPR auto
 		end(Engine const& engine)
-			-> sprout::generator_iterator<typename std::decay<decltype(engine())>::type>
+			-> sprout::generator_iterator<typename std::remove_reference<decltype(engine())>::type>
 		{
-			return sprout::generator_iterator<typename std::decay<decltype(engine())>::type>();
+			return sprout::generator_iterator<typename std::remove_reference<decltype(engine())>::type>();
 		}
 	}	// namespace random
 }	// namespace sprout
diff --git a/sprout/random/random_result.hpp b/sprout/random/random_result.hpp
index 3fb21266..a823dab2 100644
--- a/sprout/random/random_result.hpp
+++ b/sprout/random/random_result.hpp
@@ -280,13 +280,13 @@ namespace sprout {
 		// generated_value
 		//
 		template<typename Engine, typename Distribution>
-		inline SPROUT_CONSTEXPR typename sprout::random::random_result<Engine, Distribution>::result_type const&
-		generated_value(sprout::random::random_result<Engine, Distribution> const& t) {
+		inline typename sprout::random::random_result<Engine, Distribution>::result_type&
+		generated_value(sprout::random::random_result<Engine, Distribution>& t) {
 			return t.generated_value();
 		}
 		template<typename Engine, typename Distribution>
-		inline typename sprout::random::random_result<Engine, Distribution>::result_type&
-		generated_value(sprout::random::random_result<Engine, Distribution>& t) {
+		inline SPROUT_CONSTEXPR typename sprout::random::random_result<Engine, Distribution>::result_type const&
+		generated_value(sprout::random::random_result<Engine, Distribution> const& t) {
 			return t.generated_value();
 		}
 
@@ -294,13 +294,13 @@ namespace sprout {
 		// next_generator
 		//
 		template<typename Engine, typename Distribution>
-		inline SPROUT_CONSTEXPR typename sprout::random::random_result<Engine, Distribution> const&
-		next_generator(sprout::random::random_result<Engine, Distribution> const& t) {
+		inline typename sprout::random::random_result<Engine, Distribution>&
+		next_generator(sprout::random::random_result<Engine, Distribution>& t) {
 			return t.next_generator();
 		}
 		template<typename Engine, typename Distribution>
-		inline typename sprout::random::random_result<Engine, Distribution>&
-		next_generator(sprout::random::random_result<Engine, Distribution>& t) {
+		inline SPROUT_CONSTEXPR typename sprout::random::random_result<Engine, Distribution> const&
+		next_generator(sprout::random::random_result<Engine, Distribution> const& t) {
 			return t.next_generator();
 		}
 	}	// namespace random
diff --git a/sprout/random/variate_generator.hpp b/sprout/random/variate_generator.hpp
index 23dc9f8b..208d21b2 100644
--- a/sprout/random/variate_generator.hpp
+++ b/sprout/random/variate_generator.hpp
@@ -67,13 +67,19 @@ namespace sprout {
 		// combine
 		//
 		template<typename Engine, typename Distribution>
-		SPROUT_CONSTEXPR sprout::random::variate_generator<Engine const&, Distribution const&> combine(
-			Engine const& engine,
-			Distribution const& distribution
-			)
-		{
+		inline SPROUT_CONSTEXPR sprout::random::variate_generator<Engine const&, Distribution const&>
+		combine(Engine const& engine, Distribution const& distribution) {
 			return sprout::random::variate_generator<Engine const&, Distribution const&>(engine, distribution);
 		}
+
+		//
+		// combine_copy
+		//
+		template<typename Engine, typename Distribution>
+		inline SPROUT_CONSTEXPR sprout::random::variate_generator<Engine, Distribution>
+		combine_copy(Engine const& engine, Distribution const& distribution) {
+			return sprout::random::variate_generator<Engine, Distribution>(engine, distribution);
+		}
 	}	// namespace random
 
 	using sprout::random::variate_generator;