iterator: add constant_iterator

iterator.hpp

@@ -804,4 +804,49 @@ namespace cruft {
     private:
         IteratorT m_cursor;
     };
-}
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    namespace iterator {
+        template <typename ValueT>
+        class constant {
+        public:
+            using iterator_category = std::random_access_iterator_tag;
+            using value_type = ValueT;
+            using difference_type = std::size_t;
+            using pointer = value_type*;
+            using reference = value_type&;
+
+            constant () = delete;
+
+            constant (ValueT &&_value) noexcept
+                : m_value (std::move (_value))
+            { ; }
+
+            constant (ValueT const &_value)
+                : m_value (_value)
+            { ; }
+
+            constant (constant const &) = default;
+            constant& operator= (constant const &) = default;
+
+            constant (constant &&) = default;
+            constant& operator= (constant &&) = default;
+
+            ValueT const& operator*  () const& { return  m_value; }
+            ValueT      & operator*  ()      & { return  m_value; }
+
+            ValueT const* operator-> () const & { return &m_value; }
+            ValueT      * operator-> ()       & { return &m_value; }
+
+            constant& operator++ (   ) { return *this; }
+            constant& operator++ (int) { return *this; }
+
+            constant& operator-- (   ) { return *this; }
+            constant& operator-- (int) { return *this; }
+
+        private:
+            ValueT m_value;
+        };
+    }
+};