in reply to •Re: Copying Objects
in thread Copying Objects

Thanks - I hadn't thought of the aspect of reuse thru object hierarchies. It's true what I'm really looking for is potentially a few operations (not just copy/clone) that would be applicable to every object.

For this particular app (its a multi-language code generator for process control modeling) I don't really have a lot of hierachies unfortunately. But, about the third time I cut and paste my generic copy() function I figured there must be a better way.

I like your copy verion - it is much more compact. I think next time around I would abstract everything from a 'mother of all objects' class.

It's still too bad the language doesn't build something in.

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•Re: Re: •Re: Copying Objects
by merlyn (Sage) on Jul 17, 2003 at 15:09 UTC
    It's still too bad the language doesn't build something in.
    The problem is that the "right thing" to do is very different depending on the object. Sometimes you want no copy (for a singleton object, for example). Sometimes you want a shallow copy. Sometimes you want a deep copy. Only the author of a class can really know that.

    For example, suppose you had an object that had a link to the database from which it came. When you "copied" that object, you surely wouldn't want an entire clone of the database!

    Smalltalk solves this by having a "copy" method that "does the normal thing" for each class, which defaults to a deep copy, and a "shallowCopy" and "deepCopy" method that defaults to "copy". If a need to have something other than a deepcopy arises for the default (like the link to a database example), then the class author changes copy to be shallowCopy or some shallowCopy plus a copy of some of the deeper memebers. It works rather nicely, but does require some thinking for each class.

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

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