comment on

Monks, I've got an oo design question and couldn't think of a succinct title. Bear with me if the requirements sound a little strange.

I've got a package Dragon.pm whose constructor calls an _init() method. All well and good. _init() calls another method called awaken() to print an message when an object is constructed:

sub new() {
    my $class = shift;          # get class so constructor can be inhe
+rited
    my $self = {};
    bless ($self,$class);
    $self->_init( @_ );         # call _init here or in subclass
    return $self;
}

sub _init {
    my $self = shift;
    $self->{NAME}  = shift || undef;
    $self->{AGE}   = undef;
    $self->{COLOR} = undef;
    $self->awaken;
}
sub awaken {
    my $self = shift;
    ($self->get_name eq "nameless") ? print "A " : 0;
    print $self->get_name .", ";
    ($self->get_name ne "nameless") ? print "an " : 0;
    print $self->get_age .", ";
    print $self->get_color ." dragon awakens from his slumber!\n";
}
[download]

This all works perfectly.

I also have a subclass of Dragon.pm called Trogdor.pm with its own _init() that calls the parent _init() and then does its own initialization.

sub _init {
    my $self = shift;
    if ($self->SUPER::_init( @_ )) {
        $self->{NAME} = "Trogdor";
        $self->{AGE}  = "one year old";
        $self->{COLOR}= "green";
        $self->burninate;
    }
    print "Trogdor::_init got called\n";
}
[download]

My problem is that SUPER::_init sets the object fields with some default (undef) values, then calls the awaken() method, which prints its message before Trogdor's _init can reset the fields with correct values.

I understand why this is happening. My question is, how would you design these two packages so that awaken() would be called after initialization, regardless of whether the object was a Dragon or a subclass of Dragon?

Here's the full code, plus a file dragons.pl to test it.

Dragon.pm

package Dragon;
use strict;

sub new() {
    my $class = shift;          # get class so constructor can be inhe
+rited
    my $self = {};
    bless ($self,$class);
    $self->_init( @_ );         # call _init here or in subclass
    return $self;
}

sub _init {
    my $self = shift;
    $self->{NAME}  = shift || undef;
    $self->{AGE}   = undef;
    $self->{COLOR} = undef;
    $self->awaken;
}

sub get_name {
    my $self = shift;
    return $self->{NAME} || "nameless";
}

sub get_age {
    my $self = shift;
    return $self->{AGE} || "ageless";
}

sub get_color {
    my $self = shift;
    return $self->{COLOR} || "obscure";
}

sub awaken {
    my $self = shift;
    ($self->get_name eq "nameless") ? print "A " : 0;
    print $self->get_name .", ";
    ($self->get_name ne "nameless") ? print "an " : 0;
    print $self->get_age .", ";
    print $self->get_color ." dragon awakens from his slumber!\n";
}
1;
[download]

Trogdor.pm

package Trogdor;
use strict;
use base qw(Dragon);

sub _init {
    my $self = shift;
    if ($self->SUPER::_init( @_ )) {
        $self->{NAME} = "Trogdor";
        $self->{AGE}  = "one year old";
        $self->{COLOR}= "green";
        $self->burninate;
    }
    print "Trogdor::_init got called\n";
}

sub awaken {
    my $self = shift;
    ($self->get_name eq "nameless") ? print "A " : 0;
    print $self->get_name .", ";
    ($self->get_name ne "nameless") ? print "an " : 0;
    print $self->get_age .", ";
    print $self->get_color ." dragon awakens from his slumber!(from Tr
+ogdor)\n";
}

sub burninate {
    my $self = shift;
    return $self->get_name ." burninates the land!\n";
}
1;
[download]

dragons.pl

#!/usr/bin/perl
use strict;
use Trogdor;

#my $d = new Dragon();          # create a nameless dragon
#my $d = new Dragon("Smaug");   # create a dragon named Smaug
my $d = new Trogdor();          # create Trogdor
print $d->get_color;            # confirm Trogdor's color is reset
[download]

Thanks,

----------

Using perl 5.8.1-RC3 unless otherwise noted.
Apache/1.3.33 (Darwin) unless otherwise noted.
Mac OS X 10.3.9 unless otherwise noted.
[download]

In reply to oo design q: SUPER::_init by alienhuman

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