This is the tenth set of slightly updated slides from a Perl programming course that I held some years ago. I want to share it with everyone looking for intransitive Perl-knowledge. A table of content for all presentations can be found at i-can.eu. The source code for the examples and the presentations in ODP format are on https://github.com/kberov/PerlProgrammingCourse

1. Perl Programming
Course
Packages, modules, classes
and objects
Krassimir Berov
I-can.eu

2. Contents
1. What is a module?
2. What is a package?
3. What is a class?
4. What is an object?
5. Working with objects
• bless and ref
• tie and tied
• use and require
6. Using Modules
7. Creating a module

3. What is a module?
• A module is just a set of related functions and
variables in a library file
• ...a Perl package with the same name as the file.
• It is specifically designed to be reusable by other
modules or programs
• May provide mechanism for exporting some of its
symbols into the symbol table of any package
using it
• May function as a class definition and make its
semantics available implicitly through method calls
on the class and its objects
• See Exporter and perlmodlib

4. What is a package?
• package NAMESPACE
• declares the compilation unit as being in the given
namespace
• The scope of the package declaration is from the
declaration itself through the end of the enclosing
block, file, or eval (the same as the my operator)
• Refer to things in other packages by prefixing the
identifier with the package name and a double colon:
$Package::Variable
• See perlfunc/package, perlmod/Packages
package Human;
our $legs = 2;
our $hands = 2;

5. What is a class?
• There is no special class syntax in Perl
• A package acts as a class if it provides subroutines
to act as methods
• A class may be thought as a user-defined type.
• use base to both load the base classes (packages)
and inherit from them
• A class should provide one or more ways to
generate objects
package Dog;
use base qw(Mammal);
sub new {
my $class = shift;
my $self = {};
bless $self, $class;
}

6. What is an object?
• An Object is Simply a Reference
with user-defined type
• Perl doesn't provide any special syntax for constructors
• A Method is simply a Subroutine
• A method expects its first argument to be the object
(reference) or package (string) it is being invoked on
• A constructor is a subroutine that returns a reference to
something "blessed" into a class
• Constructors are often class methods
• You could think of the method call as just another form
of dereferencing

7. Working with objects
• bless REF,CLASSNAME
bless REF
• tells the thingy referenced by REF that it is
now an object in the CLASSNAME package
and returns the reference
• If CLASSNAME is omitted, the current
package is used
• NOTE: Always use the two-argument version
to enable inheritance
Make sure that CLASSNAME is a true value

8. Working with objects
• bless - Example:
#bless.pl
{
package Dog;
sub new {
my $class = shift;
my $self = {
name =>'Puffy',
nationality =>'bg_BG',
};
bless $self, $class;
}
sub name {
return $_[0]->{name}
}
#...
}

9. Working with objects
• ref EXPR
ref
Returns a non-empty string if EXPR is a reference, the
empty string otherwise.
• If EXPR is not specified, $_ will be used.
• The value returned depends on the type of thing the
reference is a reference to.
• Builtin types include:
SCALAR, ARRAY, HASH, CODE, REF, GLOB, LVALUE,
FORMAT, IO, Regexp
• If the referenced object has been blessed into a package,
then that package name is returned instead.
• You can think of ref as a typeof operator.
• See ref.pl for MORE.

10. Working with objects
• tie VARIABLE,CLASSNAME,LIST
• binds a variable to a package class that will provide the
implementation for the variable
• VARIABLE is the name of the variable to be tied
• CLASSNAME is the name of a class implementing objects
of correct type
• Any additional arguments are passed to the new method
of the class (meaning TIESCALAR , TIEHANDLE ,
TIEARRAY , or TIEHASH )
• See perlfunc/tied, perltie, Tie::Hash, Tie::Array,
Tie::Scalar, and Tie::Handle
• See DB_File or the Config module for interesting tie
implementations. See also tie.pl for EXAMPLE

11. Working with objects
• tied VARIABLE
• Returns a reference to the object underlying
VARIABLE
• The same value was originally returned by the tie
call that bound the variable to a package.
• Returns the undefined value if VARIABLE isn't
tied to a package.
• See tie.pl and tied.pl for examples

12. Working with objects
• use Module VERSION LIST
use Module
use VERSION
• It is exactly equivalent to
BEGIN { require Module; Module->import( LIST ); }
• The BEGIN forces the require and import to
happen at compile time.
• import imports the list of features into the current
package
• If no import method can be found, the call is skipped
• In use VERSION form VERSION has to be a numeric
argument such as 5.016, which will be compared to
$]

13. Working with objects
• require VERSION
require EXPR
require
• demands that a library file be included if it hasn't
already been included
• The file is included via the do-FILE mechanism,
• Lexical variables in the invoking script will be
invisible to the included code.
• The file must return true as the last statement to
indicate successful execution of any initialization
code
• If EXPR is a bareword, the require assumes a ".pm"
extension and replaces "::" with "/" in the filename

14. Using Modules
• See If you have what you need in CORE
modules or get the module you need
using ppm
berov@berov:~> ppm

15. Using Modules
• Example
#using_modules.pl
use strict; use warnings; use utf8;
use FindBin;
BEGIN {
$ENV{APP_ROOT} = $FindBin::Bin .'/..';
}
use lib($ENV{APP_ROOT}.'/lib');
use Data::Table;#patched... See TODO in module
use Data::Dumper;
...

16. Packages, modules, classes
and objects
• Ressources
• Beginning Perl
(Chapter 11 – Object-Oriented Perl)
• perlboot - Beginner's Object-Oriented Tutorial
• perlobj - Perl objects
• perltoot - Tom's object-oriented tutorial for perl
• perltooc - Tom's OO Tutorial for Class Data in
Perl
• perlbot - Bag'o Object Tricks (the BOT)

17. Packages, modules, classes
and objects
Questions?