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
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