MESSAGE...[IS...] IN

 Synopsis

         Control multiply inherited messages

 Syntax

         MESSAGE <message name> [IS <original name>]
              IN <ancestor class>

 Arguments

         <message name> is the name of the message being declared.

         <original name> is the name of a message defined in an
         ancestor class which the declared message will be mapped to. This
         argument should be omitted if its value is the same as
         <message name>.

         <ancestor class> is the name of the class which contains the
         original message.

 Description

         This command allows the default precedence of multiply inherited
         messages to be overridden. It allows a message to be defined, in a
         multiply inherited class, which corresponds to a message in one of
         the ancestor classes which would not otherwise be available.

 Example

         Assume that a class named BordTextWindow is multiply inherited
         from two classes named BorderedWindow and TextWindow as follows:

           CREATE CLASS BorderedTextWindow ;
                 FROM TextWindow, BorderedWindow
              // ...

         By default, because it is listed first, methods inherited from
         TextWindow will take precedence over methods of the same name
         inherited from BorderedWindow. We can override the default
         precedence as follows:

           MESSAGE show IN BorderedWindow

         Assuming TextWindow also has a method named show, the above
         declaration will cause it to be overridden by the one inherited
         from BorderedWindow. However, this does not necessarily solve all
         problems caused by name conflicts. See the warning below for more
         information.

         In our example so far, special action (use of a scope resolution
         message) would be required to access the show method inherited
         from TextWindow. To resolve this, we could define an alternative
         name for show in the TextWindow class:

           MESSAGE show IN BorderedWindow     // as above
           MESSAGE showText IS show IN TextWindow

         This provides a convenient way to access both methods. Of course,
         we could have achieved something similar the other way around:

           MESSAGE showBorder IS show IN BorderedWindow

         This relies on the fact the default show would be inherited from
         TextWindow, and defines an alternative name (showBorder) for the
         other show method, inherited from BorderedWindow.

 Warning

         In the first part of the example above, the show message was
         defined so that it would invoke the show method defined in
         BorderedWindow, instead of the one defined in TextWindow. However,
         this can lead to a problem if the show message is used in any of
         the methods defined in TextWindow, or other ancestor classes. When
         these methods are invoked via an instance of BordTextWindow,
         sending the show message will invoke the method defined in
         BorderedWindow. This is unlikely to be the behavior expected by
         the methods in TextWindow, and usually leads to serious problems.

         For this reason, it is best to use the MESSAGE...[IS...] IN
         command to redefine messages which are not used in the class in
         which they are originally defined. See Multiple Inheritance
         for more information.

 See Also

         Multiple Inheritance