mmmm... dragonchild also thought this was a goofy design and yet I always do this - a habit tatooed into me when I was a mere Perl love-child.

So I went and checked with some venerated Object Ogres, UML Gods, demi-gods, and Snippet Slayers...

the concerted response was that as a generalisation, an inherited constructor should ALWAYS ensure that the parent constructor has been executed before doing any localised specialisation.

So I have cast around for a trivial example that shows why you MUST call the parent class constructor...

This is the output from the code below - $enterprise works, but $voyager blows up!

Starting Enterprise...
StarShip initialised...
Enterprise starship initialised...
Enterprise crew count: 0

Starting Voyager...
Voyager starship initialised...
Can't use an undefined value as an ARRAY reference 
  at ./trek.pl line 39.
[download]

The difference is that Enterprise correctly calls the StarShip constructor, whereas Voyager decides to do its own thing, not realising that they are saving up a warp breach!

#!/usr/local/bin/perl -w
use strict;

# This example shows why child class should always
# execute their parent constructor:
# - Enterprise class gets it right
# - Voyager class blows up

print "Starting Enterprise...\n";

my $enterprise = Enterprise->new( 
  captain => 'pickard', 
  compliment => 3000 );

print "Enterprise crew count: ", 
      $enterprise->getCrewCount(),"\n\n";

print "Starting Voyager...\n";
my $voyager = Voyager->new( 
  captain => 'janeway', 
  compliment => 300 );
print "Voyager crew count: ", 
  $voyager->getCrewCount(),"\n\n";

#---------------------------------------------------
package StarShip;
sub new {
  my $class = shift;
  my $self = {};
  bless $self,$class;
  $self->_ssInit(@_);
  return $self;
}
sub _ssInit {
  my $self = shift;
  my %param = @_;
  for my $property qw( captain compliment ) {
    die "StarShip: missing '$property'"
      unless $param{$property};
    $self->{$property} = $param{$property};
  }
  $self->{crew} = [];
  print "StarShip initialised...\n";
}
sub getCrewCount {
  my $self = shift;
  return scalar @{$self->{crew}};
}

#---------------------------------------------------
package Enterprise;
use base qw(StarShip);
sub new {
  my $class = shift;
  # We perform our parents constructor
  # to ensure that our object is in a consistent
  # state from the parents perspective
  my $self = $class->SUPER::new(@_);
  $self->_entInit(@_);
  return $self;
}
sub _entInit {
  my $self = shift;
  $self->{class}  = 'constellation';
  $self->{id}     = 'NCC1701D';
  print "Enterprise starship initialised...\n";
}

#---------------------------------------------------
package Voyager;
use base qw(StarShip);
sub new {
  my $class = shift;
  # Whoops - we didnt call StarShip constructor!
  my $self = {};
  bless $self,$class;
  $self->_voyagerInit(@_);
  return $self;
}
sub _voyagerInit {
  my $self = shift;
  $self->{class}  = 'smaller';
  $self->{id}     = 'VOYAGER1';
  $self->jumpTo('gamma quadrant');
  print "Voyager starship initialised...\n";
}
sub jumpTo {
  my $self = shift;
  $self->{x} = rand(-1)*1000;
  $self->{y} = rand(-1)*1000;
  $self->{z} = rand(-1)*1000;
}
[download]

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