| Re: iTools ambiguity [message #64769 is a reply to message #64693] |
Wed, 14 January 2009 07:46  |
Michael Galloy
Messages: 1114
Registered: April 2006
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Senior Member |
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On Jan 13, 10:48 pm, Robbie <ret...@iinet.net.au> wrote:
> The documentation for IDLitDataContainer says that its superclasses
> are IDLitContainer and IDLitData however I attempt the following
>
> obj = obj_new('IDLitDataContainer')
> obj -> IDLitContainer::Add, obj_new('IDLitComponent')
>
> and I get an error. Instead I must do
>
> obj -> _IDLitContainer::Add, obj_new('IDLitComponent')
>
> Evidently the IDLitContainer behaviour has been implemented using
> _IDLitContainer (an abstract class).
>
> This is a somewhat loaded question, but which superclass is correct?
> Is it _IDLitContainer or is it IDLitContainer?
Looking at the source code it appears that _IDLitContainer is the real
superclass that implements the "add" method. IDLitContainer is just a
thin wrapper around _IDLitContainer (and also subclassing
IDLitComponent).
The reason for all those _ classes is multiple inheritance: classes
can't inherit from IDLitComponent twice (the dreaded diamond of death
in the inheritance hierarchy). In this case, IDLitDataContainer
inherits from IDLitData (a subclass of IDLitComponent) and
_IDLitDataContainer. If it inherited from IDLitDataContainer directly
then it would be an error, so an _IDLitContainer class is created
which is all the functionality needed in IDLitContainer, but that
doesn't inherit from IDLitComponent.
So the documentation is telling a "white lie" here, IDLitDataContainer
doesn't inherit from IDLitContainer, but it has all the functionality
as if it did. Of course, depending on what you are doing, this might
really throw you off since it is not true.
There are many of the _ classes, I think generally they implement all
the functionality and then are inherited by a class without the _ that
also inherits from IDLitComponent.
This is what you get when you use multiple inheritance.
Mike
--
www.michaelgalloy.com
Tech-X Corporation
Associate Research Scientist
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