| Re: keyword inheritance and object inheritance [message #29897 is a reply to message #29896] |
Sat, 23 March 2002 09:44  |
Ted Cary
Messages: 53
Registered: October 2001
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Hi Ben,
You should use _REF_EXTRA when you have output keywords. It should
therefore not be necessary in an object's SETPROPERTY method, but is almost
indispensable in any subclass object's GETPROPERTY method.
Output keywords are possible exactly because IDL functions are normally
passed keyword arguments by reference, so that changing the value of a
keyword argument inside a function will change the value of the argument
outside the function as well. When keywords are stored and passed along in
the _EXTRA structures created in function declarations, however, they are
stored in fields by value and the reference information is lost. This is
fine if you only want to USE the value, but not if you want to CHANGE the
value stored in the argument. In order to change the value in some other
function, _REF_EXTRA allows you to pass along the output keywords by
reference.
Use _REF_EXTRA instead of _EXTRA in GETPROPERTY function declarations of
subclass 'child' objects (Mother/Father, MyObj/YourObj in your example) if
you want to override the GETPROPERTY methods of their superclass 'parent'
objects. In the call to a superclass (parent's) GETPROPERTY, pass the
structure stored by _REF_EXTRA via the _EXTRA keyword. Your
MOTHER::GETPROPERTY is written correctly.
To correct your code rewrite all subclass GETPROPERTY methods so that they
are like your MOTHER::GETPROPERTY. You can remove the _REF_EXTRAs from all
the SETPROPERTYs.
Another option which avoids function overriding and _REF_EXTRAs is to use
the EXTRACT function on David Fanning's website. It takes advantage of the
fact that self fields of subclass objects inherit the fields of their
superclasses. This will work if your self fields ARE the properties you
want to extract, as in your example. For more complicated objects, however,
GETPROPERTY is better. HTH
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