iterator: add dereference_adapter for iterators

iterator.hpp

@@ -645,6 +645,93 @@ namespace cruft {
     private:
         std::size_t m_count = 0;
     };
-};
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// Adapts a supplied iterator by dereferencing any operations that deal
+    /// with the underlying value_type of the supplied iterator.
+    template <typename IteratorT>
+    struct dereference_adapter {
+        //---------------------------------------------------------------------
+        using difference_type = typename std::iterator_traits<IteratorT>::difference_type;
+        using iterator_category = typename std::iterator_traits<IteratorT>::iterator_category;
+
+        using value_type = std::remove_reference_t<
+            decltype(
+                *std::declval<
+                    typename std::iterator_traits<IteratorT>::value_type
+                > ()
+            )
+        >;
+
+        using pointer = value_type*;
+        using reference = value_type&;
+
+
+        //---------------------------------------------------------------------
+        dereference_adapter (IteratorT _cursor)
+           : m_cursor (_cursor)
+        { ; }
+
+
+        //---------------------------------------------------------------------
+        // Trivial conditional operations
+        decltype(auto)
+        operator== (dereference_adapter const &rhs) const
+        {
+            return m_cursor == rhs.m_cursor;
+        }
+
+
+        decltype(auto)
+        operator!= (dereference_adapter const &rhs) const
+        {
+            return !(*this == rhs);
+        }
+
+
+        //---------------------------------------------------------------------
+        // Iterator movement operations
+        dereference_adapter&
+        operator++ ()
+        {
+            ++m_cursor;
+            return *this;
+        }
+
+
+        difference_type
+        operator- (dereference_adapter const &rhs) const
+        {
+            return m_cursor - rhs.m_cursor;
+        }
+
+
+        auto
+        operator+ (difference_type offset) const
+        {
+            return dereference_adapter { m_cursor + offset };
+        }
+
+
+        //---------------------------------------------------------------------
+        // value_type operations
+        decltype(auto)
+        operator= (value_type const &rhs)
+        {
+            return **m_cursor = rhs;
+        }
+
+        decltype(auto) operator*  () { return **m_cursor; }
+        decltype(auto) operator-> () { return **m_cursor; }
+
+        decltype(auto) operator*  () const { return **m_cursor; }
+        decltype(auto) operator-> () const { return **m_cursor; }
+
+
+    private:
+        IteratorT m_cursor;
+    };
+}
 
 #endif

test/iterator.cpp

@@ -19,6 +19,22 @@ test_counting_output_iterator (cruft::TAP::logger &tap)
 }
 
 
+void
+test_dereference_iterator (cruft::TAP::logger &tap)
+{
+    int values[4] { 0, 1, 2, 3 };
+    int const* pointers[4] = { values + 0, values + 1, values + 2, values + 3 };
+
+    auto const sum = std::accumulate (
+        cruft::dereference_adapter (std::cbegin (pointers)),
+        cruft::dereference_adapter (std::cend   (pointers)),
+        0
+    );
+
+    tap.expect_eq (sum, 6, "dereference_adapter used in accumulate");
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 int
 main (int, char**)