iterator/cast: add skeleton of a casting iterator

CMakeLists.txt

@@ -407,6 +407,7 @@ list (
     introspection.hpp
     io.cpp
     io.hpp
+    iterator/cast.hpp
     iterator/constant.hpp
     iterator/counting.hpp
     iterator/dereference.hpp

iterator/cast.hpp

@@ -0,0 +1,69 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2020, Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include <iterator>
+
+namespace cruft::iterator {
+    ///////////////////////////////////////////////////////////////////////////
+    /// An iterator that converts the value_type of an underlying iterator to
+    /// a requested type.
+    template <typename ValueT, typename IteratorT>
+    class cast {
+    public:
+        cast (IteratorT _inner)
+            : m_inner (_inner)
+        { ; }
+
+        using iterator_category = typename std::iterator_traits<IteratorT>::iterator_category;
+        using value_type = ValueT;
+        using difference_type = typename std::iterator_traits<IteratorT>::difference_type;
+        using pointer = value_type*;
+        using reference = value_type&;
+
+        class wrapper {
+        public:
+            using inner_value     = typename std::iterator_traits<IteratorT>::value;
+            using inner_pointer   = typename std::iterator_traits<IteratorT>::pointer;
+            using inner_reference = typename std::iterator_traits<IteratorT>::reference;
+
+            reference
+            operator= (ValueT const &val)
+            {
+                _value = static_cast<inner_value> (val);
+                return _value;
+            }
+
+            reference operator= (ValueT &&val)
+            {
+                _value = static_cast<inner_value> (val);
+            }
+
+            inner_pointer
+            operator-> ()
+            { return &_value; }
+
+        private:
+            inner_reference _value;
+        };
+
+
+        wrapper operator*  (void) { return wrapper (*m_inner); };
+        wrapper operator-> (void) { return wrapper (*m_inner); };
+
+        cast& operator++ (void)
+        {
+            ++m_inner;
+            return *this;
+        }
+
+    private:
+        IteratorT m_inner;
+    };
+}