> Reputation: 0

> Hi guys,

> I was just reading about closures on perl.com and read that Tom
> Christiansen said that closures can be used to achieve data hiding in
> OO perl. I thought about it and came up with an example to do that:

> A module written without closures:
> package Dinner;
> use strict;
> use warnings;

> my $self = {};

> sub new {
>    my ($class) = @_;
>    bless $self, $class;
>    return $self;
> }

> sub washhands {
>    my ($obj) = @_;
>    $obj->{'handsclean'} = 1;
>    return 1;
> }

> sub eatfood {
>    my ($obj) = @_;
>    if (exists $obj->{'handsclean'}) {
>        print "\nYou washed your hands - eat all you want\n";
>    } else {
>        print "\nYou filthy animal!! Go wash your hands\n";
>    }
>    return 1;
> }

> 1;

> Its corresponding use:

> #!/usr/bin/perl -w
> use strict;
> use Dinner;
> my $dirtyeater = Dinner->new();
> # I don't want to wash hands, I will just set the flag directly
> # $dirtyeater->washhands;
> $dirtyeater->{'handsclean'} = 1;
> $dirtyeater->eatfood;

> A Data::Dumper dump of $dirtyeater gives

> $VAR1 = bless( { 'handsclean' => 1 }, 'Dinner' );

> $dirtyeater can eat food if he knows what to change in the objects
> data structure. I know that if you are playing with the object's ds
> you do it at your own risk but I want to prevent the end user from
> changing any object ds specific variables. So I write another module
> called Dinnerclosure.pm:

> package Dinnerclosure;
> use strict;
> use warnings;

> sub new {
>    my ($class) = @_;
>    my $self = {};
>    my $selfmirage = {};
>    $selfmirage->{'washhands'} = sub {
>                                   washhands($self);
>                                 };
>    $selfmirage->{'eatfood'} = sub {
>                                   eatfood($self);
>                               };
>    bless $selfmirage, $class;
>    return $selfmirage;
> }

> sub washhands {
>    my ($obj) = @_;
>    $obj->{'handsclean'} = 1;
>    return 1;
> }

> sub eatfood {
>    my ($obj) = @_;
>    if (exists $obj->{'handsclean'}) {
>        print "\nYou washed your hands - eat all you want\n";
>    } else {
>        print "\nYou filthy animal!! Go wash your hands\n";
>    }
>    return 1;
> }

> 1;

> The testcode:

> #!/usr/bin/perl -w
> use strict;
> use Dinnerclosure;
> my $cleaneater = Dinnerclosure->new();
> # comment this line and uncomment the next, but it won't wash hands
> $cleaneater->{'washhands'}->();
> #$cleaneater->{'handsclean'} = 1;
> $cleaneater->{'eatfood'}->();
> print Dumper $cleaneater;

> $dirtyeater cannot change the 'cleanhands' flag. If he does, he will
> be changing the $selfmirage hashref which does not decide cleanliness,
> $self does. And $dirtyeater cannot access $self because the object is:

> $VAR1 = bless( {
>                 'eatfood' => sub { "DUMMY" },
>                 'washhands' => sub { "DUMMY" }
>               }, 'Dinnerclosure' );

> Advantages:

>   1. You have to wash hands before eating. :) - to generalize - you
> can only call subs to manipulate data.There is no other choice.
>   2. $self and $selfmirage are lexically scoped within the new sub.
> They don't even have to be declared at top of package before any subs
> in case of Dinner.pm - better lexical scoping
>   3. The end user does not see any object innards in the ds

> WYDSIWYDG - What you don't see is what you don't get :)

> -- Saurabh