What's the difference between an object and a class in Perl?

Question

I'm having a little trouble getting my head around the conceptual difference between an object and a class. I don't really understand the distinction between the two in any programming language, but currently I'm working with Perl, and Moose, so I'd prefer an explanation using those things.

Cheers

Answer 1

There are lots of "a class is a blueprint, an object is something built from that blueprint", but since you've asked for a specific example using Moose and Perl, I thought I'd provide one.

In this following example, we're going have a class named 'Hacker'. The class (like a blueprint) describes what hackers are (their attributes) and what they can do (their methods):

package Hacker;       # Perl 5 spells 'class' as 'package'

use Moose;            # Also enables strict and warnings;

# Attributes in Moose are declared with 'has'.  So a hacker
# 'has' a given_name, a surname, a login name (which they can't change)
# and a list of languages they know.

has 'given_name'       => (is => 'rw', isa => 'Str');
has 'surname'          => (is => 'rw', isa => 'Str');
has 'login'            => (is => 'ro', isa => 'Str');
has 'languages'        => (is => 'rw', isa => 'ArrayRef[Str]');

# Methods are what a hacker can *do*, and are declared in basic Moose
# with subroutine declarations.

# As a simple method, hackers can return their full name when asked.

sub full_name {
    my ($self) = @_;   # $self is my specific hacker.

    # Attributes in Moose are automatically given 'accessor' methods, so
    # it's easy to query what they are for a specific ($self) hacker.

    return join(" ", $self->given_name, $self->surname);
}

# Hackers can also say hello.

sub say_hello {
    my ($self) = @_;

    print "Hello, my name is ", $self->full_name, "\n";

    return;
}

# Hackers can say which languages they like best.

sub praise_languages {
    my ($self) = @_;

    my $languages = $self->languages;

    print "I enjoy programming in: @$languages\n";

    return;
}

1;   # Perl likes files to end in a true value for historical reasons.

Now that we've got our Hacker class, we can start making Hacker objects:

#!/usr/bin/perl
use strict;
use warnings;
use autodie;

use Hacker;    # Assuming the above is in Hacker.pm

# $pjf is a Hacker object

my $pjf = Hacker->new(
    given_name => "Paul",
    surname    => "Fenwick",
    login      => "pjf",
    languages  => [ qw( Perl C JavaScript) ],
);

# So is $jarich

my $jarich = Hacker->new(
    given_name => "Jacinta",
    surname    => "Richardson",
    login      => "jarich",
    languages  => [ qw( Perl C Haskell ) ],
);

# $pjf can introduce themselves.

$pjf->say_hello;
$pjf->praise_languages;

print "\n----\n\n";

# So can $jarich

$jarich->say_hello;
$jarich->praise_languages;

This results in the following output:

Hello, my name is Paul Fenwick
I enjoy programming in: Perl C JavaScript

----

Hello, my name is Jacinta Richardson
I enjoy programming in: Perl C Haskell

If I want I can have as many Hacker objects as I like, but there's still only one Hacker class that describes how all of these work.

All the best,

Paul