diff --git a/2023/04.cpp b/2023/04.cpp
index eac9bfb..e1395fa 100644
--- a/2023/04.cpp
+++ b/2023/04.cpp
@@ -54,7 +54,7 @@ auto CountWins(std::vector<Card>& cards) -> std::vector<std::size_t>
       std::set_intersection(
         card.mine   .begin(), card.mine   .end(),
         card.winners.begin(), card.winners.end(),
-        Counter<std::size_t>{});
+        Counter{});
     result.push_back(counter);
   }
 
diff --git a/lib/include/aocpp/Counter.hpp b/lib/include/aocpp/Counter.hpp
index 98ca576..0e3e87d 100644
--- a/lib/include/aocpp/Counter.hpp
+++ b/lib/include/aocpp/Counter.hpp
@@ -3,20 +3,20 @@
 #include <cstdint>
 #include <iterator>
 
-template <typename T>
+/// @brief Output iterator that only counts the number of outputs.
 struct Counter
 {
   using difference_type = std::ptrdiff_t;
 
   struct EmptyRef {
-    auto operator=(T const&) const -> void {}
-    auto operator=(T &&    ) const -> void {}
+    template <typename T>
+    auto operator=(T&&) const -> void {}
   };
 
-  std::size_t n;
+  std::size_t n = 0;
 
-  Counter() : n{0} {}
-  Counter(std::size_t n) : n{n} {}
+  Counter() = default;
+  constexpr Counter(std::size_t n) : n{n} {}
 
   // prefix increment
   auto operator++() -> Counter&
@@ -38,7 +38,7 @@ struct Counter
 
   auto operator-(Counter rhs) const -> difference_type
   {
-    return difference_type{n} - difference_type{rhs};
+    return static_cast<difference_type>(n) - static_cast<difference_type>(rhs);
   }
 
   operator std::size_t() const
@@ -47,4 +47,4 @@ struct Counter
   }
 };
 
-static_assert(std::output_iterator<Counter<int>, int>);
+static_assert(std::output_iterator<Counter, int>);