Question

So, i’m a bit of a perl newb. Although I had something much more complicated going, i all of a sudden hit a roadblock and cannot figure out wtf is wrong with the code. I’ve simplified it so greatly that it’s only a very small fragment of code.

Test.pl

package Test;

sub new {
  my ($class) = shift;
  my $self = {
    _attr => "asdfa"
  };
  bless $self, $class;
  return $self;
}
sub log {
  print "\nAccessed via class: ".$self->{_attr};
}

process.pl

#!/usr/bin/perl
do "Test.pl";
use strict;
use warnings;
use diagnostics;

my($test) = new Test();
$test->log;
print "\nAccessed via main: ".$test->{_attr};

I run process.pl and I get the following output

Accessed via class:
Accessed via main: asdfa

I also get the warning

Use of uninitialized value in
concatenation (.) or string at Test.pl
line 12 (#1)
(W uninitialized) An undefined value was used as if it were already
defined. It was interpreted as a “” or a 0, but maybe it was a mistake.
To suppress this warning assign a defined value to your variables.

So the problem is that $self is actually undefined. Why, I have no idea. Is this not the way to initialize an object?

Answer 1

The object instance is passed as the first argument to the method. The usual thing to do is to store it in a variable called $self, but perl doesn’t set this up for you, you must do it yourself:

sub log {
  my $self = shift;
  print "\nAccessed via class: ".$self->{_attr};
}

Note that although you have defined strict and warnings in your main code, you haven’t in test.pl which means that $self was silently created with an undefined value rather than throwing a compile error for using an undeclared variable.

Furthermore, I would advise you to put package Test into a file called Test.pm, add a true statement 1; on its own at the end of the file, and call it by saying use Test; instead of do "test.pl". It’s a much cleaner way to write modular code in Perl.