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G.8. The Gory Details

For sake of clarity of explanation, I had to oversimplify some of the facts about objects. Here's a few of the gorier details:

  • Every example I gave of a constructor was a class method. But object methods can be constructors, too, if the class was written to work that way: $new = $old->copy, $node_y = $node_x->new_subnode, or the like.

  • I've given the impression that there's two kinds of methods: object methods and class methods. In fact, the same method can be both, because it's not the kind of method it is, but the kind of calls it's written to accept—calls that pass an object, or calls that pass a class name.

  • The term "object value" isn't something you'll find used much anywhere else. It's just my shorthand for what would properly be called an "object reference" or "reference to a blessed item." In fact, people usually say "object" when they properly mean a reference to that object.

  • I mentioned creating objects with constructors, but I didn't mention destroying them with destructor—a destructor is a kind of method that you call to tidy up the object once you're done with it, and want it to neatly go away (close connections, delete temporary files, free up memory, etc.). But because of the way Perl handles memory, most modules won't require the user to know about destructors.

  • I said that class method syntax has to have the class name, as in $session = Net::FTP->new($host). Actually, you can instead use any expression that returns a class name: $ftp_class = 'Net::FTP'; $session = $ftp_class->new($host). Moreover, instead of the method name for object- or class-method calls, you can use a scalar holding the method name: $foo->$method($host). But, in practice, these syntaxes are rarely useful.

And finally, to learn about objects from the perspective of writing your own classes, see the perltoot documentation, or Damian Conway's exhaustive and clear book Object Oriented Perl (Manning Publications, 1999).

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