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Re: Re: Re: Faking new() method in high-level package

by jreades (Friar)
on May 08, 2002 at 23:13 UTC ( #165209=note: print w/ replies, xml ) Need Help??


in reply to Re: Re: Faking new() method in high-level package
in thread Faking new() method in high-level package

The problem that you're running into is that you're trying to run inheritance the wrong way (and possibly for the wrong reasons).

The short answer is that you can't have a high-level class be aware of a low-level class' functions without embedding those kind of ugly stubs in your code (at least, not that I know of).

The longer answer is this:

If all subclasses of class Utils will have a method called do_something(), then having a method stub (meaning a method that does nothing) is appropriate.

sub do_something() { return 1; }

This would allow you to pass around objects belonging to any of Util's subclasses without worrying about their specific type. You would know that all Util subclasses could be made to perform some action simply by calling do_something(), whether or not the something to be done is the same for each subclass.

Looked at another way, if Util needs to be aware of a subclass' implementation of method then you've got the either the class hierarchy, or the class in which this method is declared, wrong. If calling Util::method() relies on Util::LowLevel::method(), then method() doesn't belong in the Util class since it doesn't work at that level of abstraction.

Does that make sense?

The other thing that is sounds like you are running into is the conflict between inheritance and composition.

You're trying to build a toolset to be offered by Util while relying on a LowLevel toolset supplied by a class that inherits from it. This sounds like a case for composition, not inheritance -- HASA vs. ISA. If your Util class needs a LowLevel toolset, then you might want to tackle it this way:

package Utils; sub new { my $class = shift; my @args = @_; my $self = {}; $self->{toolset} = new Toolset; return bless $self, $class } sub read() { my $self = shift; $self->{low_level_stuff} = new Functions::LowLevel; } sub doSomeOperation() { return $self-{low_level_stuff}->read(); } package Toolset sub new {} sub read {} package Toolset::LowLevel @Toolset::LowLevel::ISA = qw( Toolset ); sub new {} sub read { my $self = shift; return $self->readLine(); }

This approach probably only makes sense if you plan on having more than one toolset subclass (or you're doing a lot of OO work for little return). But now you have isolated Utils from the LowLevel Toolset class successfully and you could, for instance, make Toolset smart enough to instantiate itself differently (and return different low-level toolsets) based on the platform or something. This is one way to get a balance of abstraction and functionality, although there are, undoubtedly, many more


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Re: Re: Re: Re: Faking new() method in high-level package
by belden (Friar) on May 09, 2002 at 00:05 UTC
    The problem that you're running into is that you're trying to run inheritance the wrong way (and possibly for the wrong reasons).

    :) theDamian's "Object Oriented Perl" arrived yesterday from Amazon; I haven't had a chance to look at it yet. I expect I'll understand this more fully after a few chapters.

    Looked at another way, if Util needs to be aware of a subclass' implementation of method then you've got the either the class hierarchy, or the class in which this method is declared, wrong... Does that make sense?

    Yes! Though I did have to re-read your whole post a few times before I understood.

    I'd ++ you more than once if I could on both of your replies; thank you for your clear and understandable explanations. I'm printing this page for my Perl binder.

    blyman
    setenv EXINIT 'set noai ts=2'

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