+ /**
+ * Format an object to produce a string. Subclasses should override this
+ * method. This method allows polymorphic formatting of Formattable objects.
+ * If a subclass of Format receives a Formattable object type it doesn't
+ * handle (e.g., if a numeric Formattable is passed to a DateFormat object)
+ * then it returns a failing UErrorCode.
+ *
+ * @param obj The object to format.
+ * @param appendTo Output parameter to receive result.
+ * Result is appended to existing contents.
+ * @param posIter On return, can be used to iterate over positions
+ * of fields generated by this format call.
+ * @param status Output param filled with success/failure status.
+ * @return Reference to 'appendTo' parameter.
+ * @stable ICU 4.4
+ */
+ virtual UnicodeString& format(const Formattable& obj,
+ UnicodeString& appendTo,
+ FieldPositionIterator* posIter,
+ UErrorCode& status) const;