--- include/frozen/map.h	2023-07-22 20:20:19.580400347 +0100
+++ include/frozen/map.h	2023-07-22 20:25:44.893704485 +0100
@@ -94,11 +94,11 @@
 
 public:
   /* constructors */
-  constexpr map(container_type items, Compare const &compare)
+  constexpr map(const container_type& items, Compare const &compare)
       : less_than_{compare}
       , items_{bits::quicksort(items, less_than_)} {}
 
-  explicit constexpr map(container_type items)
+  explicit constexpr map(const container_type& items)
       : map{items, Compare{}} {}
 
   constexpr map(std::initializer_list<value_type> items, Compare const &compare)
--- include/frozen/set.h	2023-09-29 09:19:22.208195458 +0100
+++ include/frozen/set.h	2023-09-29 09:20:21.175291240 +0100
@@ -59,12 +59,12 @@
   /* constructors */
   constexpr set(const set &other) = default;
 
-  constexpr set(container_type keys, Compare const & comp)
+  constexpr set(const container_type& keys, Compare const & comp)
       : less_than_{comp}
       , keys_(bits::quicksort(keys, less_than_)) {
       }
 
-  explicit constexpr set(container_type keys)
+  explicit constexpr set(const container_type& keys)
       : set{keys, Compare{}} {}
 
   constexpr set(std::initializer_list<Key> keys, Compare const & comp)