Would someone mind helping me understand this Perl OO code?

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;
}
[download]

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);
[download]

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!

Comment on Would someone mind helping me understand this Perl OO code? Select or Download Code

Replies are listed 'Best First'.
Re: Would someone mind helping me understand this Perl OO code? by ikegami (Patriarch) 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! )	[reply]
Re^2: Would someone mind helping me understand this Perl OO code? by romandas (Pilgrim) on Dec 22, 2009 at 15:59 UTC
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 $Node::pkg, right? So is the code wrong? Like I said, it seems to work though.	[reply]
Re^3: Would someone mind helping me understand this Perl OO code? by almut (Canon) on Dec 22, 2009 at 16:04 UTC
`Node->new(...)` implicitly passes the package name (Node) as the first argument to the constructor... (so you have 4 args in the right slots, and everything is fine)	[reply] [d/l]
Re^4: Would someone mind helping me understand this Perl OO code? by romandas (Pilgrim) on Dec 22, 2009 at 16:11 UTC
Re^5: Would someone mind helping me understand this Perl OO code? by almut (Canon) on Dec 22, 2009 at 16:16 UTC
Re^5: Would someone mind helping me understand this Perl OO code? by kennethk (Abbot) on Dec 22, 2009 at 16:18 UTC
Re^5: Would someone mind helping me understand this Perl OO code? by ikegami (Patriarch) on Dec 22, 2009 at 16:18 UTC
Re^3: Would someone mind helping me understand this Perl OO code? by ikegami (Patriarch) on Dec 22, 2009 at 16:10 UTC
Oops, ignore my earlier post. `Node->new($ip, $mac_address, $machine);` [download] is basically the same as `Node::new('Node', $ip, $mac_address, $machine);` [download] The main difference is that it checks the parent classes for `new` if sub `Node::new` doesn't exist.	[reply] [d/l] [select]


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