Zen Seeker has asked for the wisdom of the Perl Monks concerning the following question:

I read the responses regarding using @ISA with strict in How do I use @ISA and use strict at the same time? but still don't see an answer to the question. I am trying to build inherited objects within a single source module. The following source fails (Perl 5.8.0) as noted 
use strict;
use warnings;

# Can't locate object method "new" via package "Test1"
my $test1 = new Test1;

package Test;

sub new {
        my $proto = shift;
        my $class = ref($proto) || shift;
        my $self = {};
        return bless $self, $class;
}

package Test1;
@Test1::ISA = qw(Test);

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There is no change if Test1 is 
package Test1;
use vars qw(@ISA);
@ISA = qw(Test);

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20041101 Edit by ysth: [id://] instead of <a> tag

Replies are listed 'Best First'.
Re: Revisiting strict and @ISA
by tilly (Archbishop) on Oct 29, 2004 at 21:35 UTC
    When you attempt to call the method, @ISA has not yet been assigned. You could put that line in a BEGIN{}, or have it in a separate file.

    Incidentally you messed up the my $class = ref($proto) || $proto meme. Which there are some good reasons not to use, it is better to make the method start with my $class = shift;

    A random style note. As is pointed out in Code Complete, while many different indentation/brace styles are pretty much equivalent (as long as you are consistent), there is experimental evidence indicating that comprehension is best with indents in the 2-4 range. An indent of 6 or 8 may look more attractive, but is harder to work with. Therefore I'd strongly suggest reducing how much you indent.

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      Thanks for the BEGIN solution; that did the trick. Actual code uses something like 
      my $case = new Case1(...); 
... 
print $case->xml;
Package Guts;
sub new { ... }
sub xml { return '...'; }
Package Case1;
BEGIN { @Case1::ISA = qw(Guts) }
sub xml {
  my $self = shift;
  return '<case1>' . $self->SUPER::xml . '</case1>';
}
Package Case2;
BEGIN { @Case2::ISA = qw(Guts) }
sub xml {
  my $self = shift;
  return '<case2>' . $self->SUPER::xml . '</case2>';
}

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Re: Revisiting strict and @ISA
by chromatic (Archbishop) on Oct 29, 2004 at 21:35 UTC

    When you try to create your new Test1 object, the array assignment later in the file hasn't happened yet.

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Re: Revisiting strict and @ISA
by fglock (Vicar) on Oct 29, 2004 at 21:35 UTC

    A little reordering of module declarations fixes it:

    use strict;
use warnings;

package Test;

sub new {
        my $proto = shift;
        my $class = ref($proto) || $proto;
        my $self = {};
        return bless $self, $class;
}

package Test1;
@Test1::ISA = qw(Test);

package Main;

my $test1 = new Test1;
print "isa ",ref($test1),"\n"

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Re: Revisiting strict and @ISA
by diotalevi (Canon) on Oct 29, 2004 at 21:50 UTC
    Other people have given you the right answer to your question. I'd like to note that the package name 'Test' is really, really bad. There is a Test package already in the core perl distribution and you risk mixing the two, especially if another module uses the built-in Test module. You should nearly always name your modules in two or more words like 'My::Test'.
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Re: Revisiting strict and @ISA
by etcshadow (Priest) on Oct 29, 2004 at 21:39 UTC
    Something that basically no-one does but practically everyone *should* do is wrap their @ISA assignments in BEGIN blocks.

    Change that to: 

    package Test1;
use vars qw(@ISA);
BEGIN {
   @ISA = qw(Test);
}

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    And you should be just fine. 
    ------------
:Wq
Not an editor command: Wq

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      This is also written as use base 'Parent::Class'.
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      Something that basically no-one does but practically everyone *should* do is wrap their @ISA assignments in BEGIN blocks.

      I don't think that that's a valuable practice. If you want you can try to convince me. Before doing so I'd suggest reading the thread at (tye)Re: Supersplit for a fuller understanding of my views about BEGIN. (And to see that not everyone necessarily agrees with me on this.)

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