Could you provide a simple example of how one would use a class variable in Perl?

Sure!

#! perl
use strict;
use warnings;

package Widget
{
    my $count = 0;

    sub new
    {
        my ($class, @args) = @_;
        my  %self = @args;
        ++$count;
        return bless \%self, $class;
    }

    sub DESTROY   {      --$count; }
    sub get_count { return $count; }
}

package main;

my $w1 = Widget->new(name => 'foo', value => 42);
my $w2 = Widget->new(name => 'bar', value =>  7);
my $w3 = Widget->new(name => 'baz', value => 12);
printf "Now there are %d Widgets\n", Widget::get_count;
undef $w1;
printf "Now there are %d Widgets\n", Widget::get_count;

[download]

Output:

15:52 >perl 856_SoPW.pl
Now there are 3 Widgets
Now there are 2 Widgets

15:52 >

[download]
Is there any other way, using the code I wrote, to access the value of an instance variable with a method that did not receive the variable as a parameter?

Not that I can think of. Anyway, that’s not how it’s supposed to work in Perl.

a class variable could be defined outside the constructor but still as a lexically scoped, 'my' variable? What about using 'our'?

Using our creates a package variable, which can then be exported to other packages, or accessed directly by other packages using the $Widget::count syntax. Using my creates a lexical variable, which is scoped to the package. The rule is: use a lexical (my) variable unless you have a good reason to make it a package/global (our) variable. Update (Jan 30, 2014): See Ovid’s classic post 'our' is not 'my'.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

In reply to Re^3: Object oriented Perl and Java: A brief examination of design. by Athanasius
in thread Object oriented Perl and Java: A brief examination of design. by Anonymous Monk

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