Fix counter postfix++ bug

lib/include/aocpp/Counter.hpp

@@ -6,25 +6,45 @@
 template <typename T>
 struct Counter
 {
-  using value_type        = T;
-  using difference_type   = std::ptrdiff_t;
-  using pointer           = T*;
-  using reference         = T&;
-  using iterator_category = std::output_iterator_tag;
+  using difference_type = std::ptrdiff_t;
 
   struct EmptyRef {
-    auto operator=(T const&) const -> void { }
+    auto operator=(T const&) const -> void {}
+    auto operator=(T &&    ) const -> void {}
   };
 
   std::size_t n;
 
-  auto operator++() -> Counter& { n++; return *this; }
-  auto operator++(int) -> Counter { Counter temp; n++; return temp; }
-  auto operator*() const -> EmptyRef { return EmptyRef{}; }
-  auto operator-(Counter const& rhs) const -> std::ptrdiff_t {
-    return std::ptrdiff_t{n} - std::ptrdiff_t{rhs.n};
+  Counter() : n{0} {}
+  Counter(std::size_t n) : n{n} {}
+
+  auto operator++() -> Counter&
+  {
+    n++;
+    return *this;
+  }
+
+  auto operator++(int) -> Counter
+  {
+    auto temp = *this;
+    n++;
+    return temp;
+  }
+
+  auto operator*() const -> EmptyRef
+  {
+    return {};
+  }
+
+  auto operator-(Counter rhs) const -> difference_type
+  {
+    return difference_type{n} - difference_type{rhs};
+  }
+
+  operator std::size_t() const
+  {
+    return n;
   }
-  operator std::size_t() const { return n; }
 };
 
 static_assert(std::output_iterator<Counter<int>, int>);