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•Re: Re^2: It's a dog, but what kind? (polymorphism , in Perl OO)

by merlyn (Sage)
on Mar 23, 2004 at 20:58 UTC ( #339185=note: print w/replies, xml ) Need Help??

in reply to Re^2: It's a dog, but what kind? (polymorphism , in Perl OO)
in thread It's a dog, but what kind? (polymorphism , in Perl OO)

From the article you can't see yet:
But here's the problem. When I survey experienced object-oriented programmers, and ask them what they expect new means when called on an instance (without looking at the implementation), the result usually divides rather equally into three camps: those that go "huh, why would you do that" and think it should throw an error, those that say that it would clone the object, and those that say it would copy the object's class but not the contents.

So, no matter what you intend if you make your new do one of those three things, two thirds of the people who look at it will be wrong. It's not intuitive. So, don't write code like that, and especially don't just cargo-cult that from the manpage into your code. If you want an object like another object, use ref explicitly, as shown above. If you want a clone, put cloning code into your package, and call clone, as we saw earlier.

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

  • Comment on •Re: Re^2: It's a dog, but what kind? (polymorphism , in Perl OO)

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Re^5: It's a dog, but what kind? (polymorphism , in Perl OO) (!abstract)
by tye (Sage) on Mar 23, 2004 at 23:06 UTC

    Yes, as I said, I understand the objection. I note that you don't quote what your alternative is nor acknowledge the problem that I was bringing up.

    Did you give a concrete class when you asked your question? As I've said, I agree that there is a problem in theory, and that sometimes there is a problem in practice. But I've also found that, in practice, exactly what it means to get a 'new' object from an object of a *specific* class is often quite clear (and I don't think it usually means what you appear to be referring to as "clone" nor "copy").

    You appear to be using "clone" to mean something close to "copy all of the object's attributes" and using "copy" (based on emphasis) to mean something close to "copy none of the object's attributes".

    I don't think I've ever seen people talking about classes as having two types of attributes (that I'll define shortly). So I'm not surprised that asking questions in the abstract fail to get people to think about splitting attributes into two types. In practice, for many Perl classes, I think this split happens quite naturally.

    I'll call the two types 'basis' attributes and 'convenience' attributes. The basis attributes are items that must be passed in to new(). The convenience attributes have more to do with the personal preferences of the user of the module.

    $obj->new( $basis1, $basis2, ... ) creates a new object based on the passed-in basis attributes but copying the convenience attributes from $obj.

    Now, some classes have attributes that don't clearly fall into one of these categories, and I suspect in that such cases the meaning of 'instance new' would not be as clear.

    And I suspect that people who think $obj->new( ... ) should be "clone" are either thinking of convenience attributes or aren't thinking of the "..." part while people who think $obj->new( ... ) should do ref($obj)->new( ... ) aren't thinking of convenience attributes (that perhaps are more common in Perl OO than in other flavors of OO).

    - tye        

      The alternative is that if you want a copy or clone operation, they should be seperate methods from the constructor.

      : () { :|:& };:

      Note: All code is untested, unless otherwise stated

        That is so far off from what I was talking about as far as what I meant by "alternative" and completely misses the point of why the obvious alternative is unacceptable to me that I suspect you did not bother to read Re^2: A few Perl OOP questions. (disparaging).

        Normally I'd try to steer back toward that but I'm just not coming up with anything. In fact, I'm not sure you even read and understood all of what I wrote in this thread.

        - tye        

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