diff --git a/functools.hpp b/functools.hpp
index c82214b..0843024 100644
--- a/functools.hpp
+++ b/functools.hpp
@@ -44,21 +44,21 @@ namespace sc {
 	// iterator versions
 
 	template<class T, class M, class FwdIter>
-	std::vector<M> map(FwdIter& begin, FwdIter& end, std::function<M(T)> fun) {
+	std::vector<M> map(FwdIter begin, FwdIter end, std::function<M(T)> fun) {
 		std::vector<M> result;
 		std::transform(begin, end, std::back_inserter(result), fun);
 		return result;
 	}
 
 	template<class T, class FwdIter>
-	std::vector<T> filter(FwdIter& begin, FwdIter& end, std::function<bool(T)> fun) {
+	std::vector<T> filter(FwdIter begin, FwdIter end, std::function<bool(T)> fun) {
 		std::vector<T> result;
 		std::copy_if(begin, end, std::back_inserter(result), fun);
 		return result;
 	}
 
 	template<class T, class R, class FwdIter>
-	R reduce(FwdIter& begin, FwdIter& end, R seed, std::function<R(T, R)> fun) {
+	R reduce(FwdIter begin, FwdIter end, R seed, std::function<R(T, R)> fun) {
 		R result {seed};
 		for (FwdIter it = begin; it != end; ++it) {
 			result = fun(*it, result);