Add converting constructors for iterator

src/json.hpp

@@ -8053,42 +8053,58 @@ class basic_json
             }
         }
 
-        /*
-        Use operator `const_iterator` instead of `const_iterator(const iterator&
-        other) noexcept` to avoid two class definitions for @ref iterator and
-        @ref const_iterator.
-
-        This function is only called if this class is an @ref iterator. If this
-        class is a @ref const_iterator this function is not called.
+        /*!
+        @note The conventional copy constructor is not defined. It is replaced
+		      by either of the following two converting constructors.
+              They support copy from iterator to iterator,
+                           copy from const iterator to const iterator, 
+                           and conversion from iterator to const iterator.
+              However conversion from const iterator to iterator is not defined.
         */
-        operator const_iterator() const
-        {
-            const_iterator ret;
 
-            if (m_object)
-            {
-                ret.m_object = m_object;
-                ret.m_it = m_it;
-            }
-
-            return ret;
-        }
+        /*!
+        @brief converting constructor
+        @param[in] other  non-const iterator to copy from
+        @note It is not checked whether @a other is initialized.
+        */
+        iter_impl(const iter_impl<basic_json>& other) noexcept
+            : m_object(other.m_object), m_it(other.m_it)
+        {}
 
         /*!
-        @brief copy constructor
-        @param[in] other  iterator to copy from
+        @brief converting constructor
+        @param[in] other  const iterator to copy from
         @note It is not checked whether @a other is initialized.
         */
-        iter_impl(const iter_impl& other) noexcept
+        iter_impl<const basic_json>(const iter_impl<const basic_json>& other) noexcept
             : m_object(other.m_object), m_it(other.m_it)
         {}
 
         /*!
         @brief copy assignment
-        @param[in,out] other  iterator to copy from
+        @param[in,out] other  non-const iterator to copy from
+		@return const/non-const iterator
+        @note It is not checked whether @a other is initialized.
+        */
+        iter_impl& operator=(iter_impl<basic_json> other) noexcept(
+            std::is_nothrow_move_constructible<pointer>::value and
+            std::is_nothrow_move_assignable<pointer>::value and
+            std::is_nothrow_move_constructible<internal_iterator>::value and
+            std::is_nothrow_move_assignable<internal_iterator>::value
+        )
+        {
+            std::swap(m_object, other.m_object);
+            std::swap(m_it, other.m_it);
+            return *this;
+        }
+
+        /*!
+        @brief copy assignment
+        @param[in,out] other  const iterator to copy from
+		@return const iterator
         @note It is not checked whether @a other is initialized.
         */
-        iter_impl& operator=(iter_impl other) noexcept(
+        iter_impl<const basic_json>& operator=(iter_impl<const basic_json> other) noexcept(
             std::is_nothrow_move_constructible<pointer>::value and
             std::is_nothrow_move_assignable<pointer>::value and
             std::is_nothrow_move_constructible<internal_iterator>::value and