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Would someone mind helping me understand this Perl OO code?

by romandas (Pilgrim)
on Dec 22, 2009 at 15:48 UTC ( #813906=perlquestion: print w/replies, xml ) Need Help??
romandas has asked for the wisdom of the Perl Monks concerning the following question:

Note that I do not own this code. I believe I pulled it from CPAN, and that it rightfully belongs to James MacFarlane. If this is incorrect, let me know and I'll attribute it accordingly. I couldn't find where I originally pulled it from.

Here are the relevant snippets:

#----------------------------------------------------------------- package Node; #----------------------------------------------------------------- sub new { my ($pkg, $ip, $mac, $hostname) = @_; my $obj = {}; bless $obj, $pkg; $obj->IP_Address($ip); $obj->MAC_Address($mac); $obj->Hostname($hostname); return $obj; }

So, from what I see above, Node->new() should be called using 4 arguments. However, when the original author used Node->new(), he did this:

$nodeobj = Node->new($ip, $mac_address, $machine); push(@nodes, $nodeobj);

This only uses three arguments. And if you omit one, then it appears the values would suffer a sort of 'one off' error. But.. the code appears to work fine. What gives?

Update: Thanks all!

Replies are listed 'Best First'.
Re: Would someone mind helping me understand this Perl OO code?
by ikegami (Pope) on Dec 22, 2009 at 15:52 UTC
    If you assign a list of 3 values to a list of 4 scalars, the 4th scalar will be undefined.

    ( True, but irrelevant to the parent post. oops! )

      Right. Which is what I meant by the 'one off' error - since $pkg is the first variable in the @_ argument list, it would mistakenly get $main::ip assigned to $Node::pkg, right? So is the code wrong? Like I said, it seems to work though.

        Node->new(...) implicitly passes the package name (Node) as the first argument to the constructor...  (so you have 4 args in the right slots, and everything is fine)

        Oops, ignore my earlier post.
        Node->new($ip, $mac_address, $machine);
        is basically the same as
        Node::new('Node', $ip, $mac_address, $machine);
        The main difference is that it checks the parent classes for new if sub Node::new doesn't exist.

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