really simple OO inheritance question

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

Been using perl on and off for a while but on a rainy afternoon started looking at OO. Normally with I can figure out all problems by reading other people's posts but I don't seem to be getting anywhere with this one, despite it being REALLY BASIC! Your forgiveness in advance is required.

#!/usr/bin/perl

use strict;

Cow->speak;

{ package Animal;
  sub speak {
    my $class = shift;
    print "a $class goes ", $class->sound, "!\n";
  }
}

{ package Cow;
  our @ISA = qw(Animal);
  sub sound { "moooo" }
}
[download]

I copied this directly from http://perldoc.perl.org/perlboot.html. When I run it I get

Can't locate object method "speak" via package "Cow" 
at ./testOO1.pl line 6.
[download]

In fact I can't seem to get inheritance to work at all. What extremely silly thing am I (not) doing? Thanks

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Re: really simple OO inheritance question by jdporter (Paladin) on Jun 27, 2010 at 12:15 UTC
You're calling the method before all the class initialization is complete. Normally I'd put all the class definitions before any "executable code". It turns out, only one statement in your code above is not getting executed in time for your `Cow->speak` call, and that's the `our @ISA = qw(Animal);` [download] statement. That is (unfortunately) "normal" code, not definitional. You can make it happen earlier by putting it in a begin block: `BEGIN { our @ISA = qw(Animal); }` [download] but it's considered better practice (by some, anyway) to use a real pragmatic statement to define your inheritance relations: `use base 'Animal';` [download] What is the sound of Windows? Is it not the sound of a wall upon which people have smashed their heads... all the way through?	[reply] [d/l] [select]
Re^2: really simple OO inheritance question by tomgracey (Scribe) on Jun 27, 2010 at 13:19 UTC
Ah yes, it works now! That'll teach me for treating classes as subroutines... Looks like I've some way to go. Thanks greatly for putting me out of my misery!	[reply]
Re: really simple OO inheritance question by moritz (Cardinal) on Jun 27, 2010 at 12:07 UTC
It works if you put the `Cow->speak` below the class definitions. The reason is that the assignment to @ISA happens at run time, so Cow only inherits from Animal for code that comes after the package/class definition. Perl 6 - links to (nearly) everything that is Perl 6.	[reply] [d/l]