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Packages
Perl provides a mechanism for alternate namespaces to protect packages
from stomping on each others variables.
By default, a perl script starts compiling into the package known as
"main".
By use of the package declaration, you can switch namespaces.
Example:
<![CDATA[
#!/usr/bin/perl -w
package my_pkg;
sub my_print {
print "oh me!\n";
}
package my_other_pkg;
sub my_print {
print "oH! That's also me!\n";
}
package main;
&my_pkg::my_print;
&my_other_pkg::my_print;
]]>
Perl Modules
A module is a package that is defined in a (library) file of the same name
and with the extension ".pm" .
Perl modules are included by saying:
use Module;
or
use Module LIST;
This is exactly equivalent to
BEGIN { require "Module.pm"; import Module; }
or
BEGIN { require "Module.pm"; import Module LIST; }
Modules and packages are searched for in the path defined
in the @ISA array.
Perl Classes
Because a class in Perl is really just a package, using package
variables to hold class attributes
and package subroutines that function as methods.
Example "Person class" stored in Person.pm:
<![CDATA[
package Person;
use strict;
##################################################
## the object constructor (simplistic version) ##
##################################################
sub new {
my $self = {};
$self->{NAME} = undef;
$self->{AGE} = undef;
$self->{PEERS} = [];
bless($self); # but see below
return $self;
}
##############################################
## methods to access per-object data ##
## ##
## With args, they set the value. Without ##
## any, they only retrieve it/them. ##
##############################################
sub name {
my $self = shift;
if (@_) { $self->{NAME} = shift }
return $self->{NAME};
}
sub age {
my $self = shift;
if (@_) { $self->{AGE} = shift }
return $self->{AGE};
}
sub peers {
my $self = shift;
if (@_) { @{ $self->{PEERS} } = @_ }
return @{ $self->{PEERS} };
}
1; # so the require or use succeeds
]]>
In order to manufacture objects, a class needs to have a constructor method.
A constructor gives you back not just a regular data type, but a brand-new
object in that class.
This magic is taken care of by the bless() function, whose sole
purpose is to enable its referent to be used as an object.
While a constructor may be named anything you'd like, most Perl
programmers seem to like to call theirs new(). However, new() is not
a reserved word, and a class is under no obligation to supply such.
Some programmers have also been known to use a function with the same
name as the class as the constructor.
...Example "using the person class in a script":
<![CDATA[
use Person;
$him = Person->new();
$him->name("Jason");
$him->age(23);
$him->peers( "Norbert", "Rhys", "Phineas" );
push @All_Recs, $him; # save object in array for later
printf "%s is %d years old.\n", $him->name, $him->age;
print "His peers are: ", join(", ", $him->peers), "\n";
printf "Last rec's name is %s\n", $All_Recs[-1]->name;
]]>