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