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Programming Perl, 3rd Edition
book

Programming Perl, 3rd Edition

by Larry Wall, Tom Christiansen, Jon Orwant
July 2000
Intermediate to advanced content levelIntermediate to advanced
1104 pages
35h 1m
English
O'Reilly Media, Inc.
Content preview from Programming Perl, 3rd Edition

use fields

In the Pet module:

package Pet;
use strict;
use fields qw(name weight _Pet_pid);
my $PID = 0;
sub new {
    my Pet $self = shift;
    unless (ref $self) {
        $self = fields::new($self);
        $self->{_Pet_pid} = "this is Pet's secret ID";
    }
    $self->{name} = "Hey, you!";
    $self->{weight} = 20;
    return $self;
}
1;

In a separate program, demopet:

use Pet;
my Pet $rock = new Pet;           # typed lexical

$rock->{name}     = "quartz";           
$rock->{weight}   = "2kg";
$rock->{_Pet_pid} = 1233;         # private attribute

$rock->{color}    = "blue";       # generates compile-time error

In the Dog module:

package Dog;
use strict;
use base 'Pet';                   # inherit fields and methods from Pet
use fields qw(name pedigree);     # override Pet name attribute,
                                  # add new pedigree attribute
use fields qw(wag _Dog_private);  # not shared with Pet
sub new {
    my $class = shift;
    my $self = fields::new($class);
    $self->SUPER::new();          # init base fields
    $self->{pedigree} = "none";   # init own fields
    return $self;
}

In a separate program, demodog:

use Dog;my Dog $spot = new Dog;           # typed lexical
$spot->{name}     = "Theloneus";  # not inherited
$spot->{weight}   = "30lbs";      # inherited
$spot->{pedigree} = "mutt";       # not inherited
$spot->{color}    = "brown";      # generates compile-time error
$spot->{_Pet_pid} = 3324;         # generates compile-time error

The fields pragma provides a method of declaring class fields that can be type checked at compile time. This relies on a feature known as pseudohashes: if a typed lexical variable (my Pet $rock) is holding a reference (the Pet object) and is used ...

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Publisher Resources

ISBN: 0596000278Supplemental ContentErrata