PERLBOT

TITLE

Bag'o Object Tricks For Perl5 (the BOT)

INTRODUCTION

The following collection of tricks and hints is intended to whet curious appetites about such things as the use of instance variables and the mechanics of object and class relationships. The reader is encouraged to consult relevant textbooks for discussion of Object Oriented definitions and methodology. This is not intended as a comprehensive guide to Perl5's object oriented features, nor should it be construed as a style guide.

The Perl motto still holds: There's more than one way to do it.

INSTANCE VARIABLES

An anonymous array or anonymous hash can be used to hold instance variables. Named parameters are also demonstrated.

            package Foo;

            sub new {
                    my $type = shift;
                    my %params = @_;
                    my $self = {};
                    $self->{'High'} = $params{'High'};
                    $self->{'Low'}  = $params{'Low'};
                    bless $self;
            }

            package Bar;

            sub new {
                    my $type = shift;
                    my %params = @_;
                    my $self = [];
                    $self->[0] = $params{'Left'};
                    $self->[1] = $params{'Right'};
                    bless $self;
            }

            package main;

            $a = new Foo ( 'High' => 42, 'Low' => 11 );
            print "High=$a->{'High'}\n";
            print "Low=$a->{'Low'}\n";

            $b = new Bar ( 'Left' => 78, 'Right' => 40 );
            print "Left=$b->[0]\n";
            print "Right=$b->[1]\n";

SCALAR INSTANCE VARIABLES

An anonymous scalar can be used when only one instance variable is needed.

            package Foo;

            sub new {
                    my $type = shift;
                    my $self;
                    $self = shift;
                    bless \$self;
            }

            package main;

            $a = new Foo 42;
            print "a=$$a\n";

INSTANCE VARIABLE INHERITANCE

This example demonstrates how one might inherit instance variables from a superclass for inclusion in the new class. This requires calling the superclass's constructor and adding one's own instance variables to the new object.

            package Bar;

            sub new {
                    my $self = {};
                    $self->{'buz'} = 42;
                    bless $self;
            }

            package Foo;
            @ISA = qw( Bar );

            sub new {
                    my $self = new Bar;
                    $self->{'biz'} = 11;
                    bless $self;
            }

            package main;

            $a = new Foo;
            print "buz = ", $a->{'buz'}, "\n";
            print "biz = ", $a->{'biz'}, "\n";

OBJECT RELATIONSHIPS

The following demonstrates how one might implement "containing" and "using" relationships between objects.

            package Bar;

            sub new {
                    my $self = {};
                    $self->{'buz'} = 42;
                    bless $self;
            }

            package Foo;

            sub new {
                    my $self = {};
                    $self->{'Bar'} = new Bar ();
                    $self->{'biz'} = 11;
                    bless $self;
            }

            package main;

            $a = new Foo;
            print "buz = ", $a->{'Bar'}->{'buz'}, "\n";
            print "biz = ", $a->{'biz'}, "\n";

OVERRIDING SUPERCLASS METHODS

The following example demonstrates how one might override a superclass method and then call the method after it has been overridden. The Foo::Inherit class allows the programmer to call an overridden superclass method without actually knowing where that method is defined.

            package Buz;
            sub goo { print "here's the goo\n" }

            package Bar; @ISA = qw( Buz );
            sub google { print "google here\n" }

            package Baz;
            sub mumble { print "mumbling\n" }

            package Foo;
            @ISA = qw( Bar Baz );
            @Foo::Inherit::ISA = @ISA;  # Access to overridden methods.

            sub new { bless [] }
            sub grr { print "grumble\n" }
            sub goo {
                    my $self = shift;
                    $self->Foo::Inherit::goo();
            }
            sub mumble {
                    my $self = shift;
                    $self->Foo::Inherit::mumble();
            }
            sub google {
                    my $self = shift;
                    $self->Foo::Inherit::google();
            }

            package main;

            $foo = new Foo;
            $foo->mumble;
            $foo->grr;
            $foo->goo;
            $foo->google;

USING RELATIONSHIP WITH SDBM

This example demonstrates an interface for the SDBM class. This creates a "using" relationship between the SDBM class and the new class Mydbm.

            use SDBM_File;
            use POSIX;

            package Mydbm;

            sub TIEHASH {
                my $self = shift;
                my $ref  = SDBM_File->new(@_);
                bless {'dbm' => $ref};
            }
            sub FETCH {
                my $self = shift;
                my $ref  = $self->{'dbm'};
                $ref->FETCH(@_);
            }
            sub STORE {
                my $self = shift; 
                if (defined $_[0]){
                    my $ref = $self->{'dbm'};
                    $ref->STORE(@_);
                } else {
                    die "Cannot STORE an undefined key in Mydbm\n";
                }
            }

            package main;

            tie %foo, Mydbm, "Sdbm", O_RDWR|O_CREAT, 0640;
            $foo{'bar'} = 123;
            print "foo-bar = $foo{'bar'}\n";

            tie %bar, Mydbm, "Sdbm2", O_RDWR|O_CREAT, 0640;
            $bar{'Cathy'} = 456;
            print "bar-Cathy = $bar{'Cathy'}\n";

THINKING OF CODE REUSE

One strength of Object-Oriented languages is the ease with which old code can use new code. The following examples will demonstrate first how one can hinder code reuse and then how one can promote code reuse.

This first example illustrates a class which uses a fully-qualified method call to access the "private" method BAZ(). The second example will show that it is impossible to override the BAZ() method.

            package FOO;

            sub new { bless {} }
            sub bar {
                    my $self = shift;
                    $self->FOO::private::BAZ;
            }

            package FOO::private;

            sub BAZ {
                    print "in BAZ\n";
            }

            package main;

            $a = FOO->new;
            $a->bar;

Now we try to override the BAZ() method. We would like FOO::bar() to call GOOP::BAZ(), but this cannot happen since FOO::bar() explicitly calls FOO::private::BAZ().

            package FOO;

            sub new { bless {} }
            sub bar {
                    my $self = shift;
                    $self->FOO::private::BAZ;
            }

            package FOO::private;

            sub BAZ {
                    print "in BAZ\n";
            }

            package GOOP;
            @ISA = qw( FOO );
            sub new { bless {} }

            sub BAZ {
                    print "in GOOP::BAZ\n";
            }

            package main;

            $a = GOOP->new;
            $a->bar;

To create reusable code we must modify class FOO, flattening class FOO::private. The next example shows a reusable class FOO which allows the method GOOP::BAZ() to be used in place of FOO::BAZ().

            package FOO;

            sub new { bless {} }
            sub bar {
                    my $self = shift;
                    $self->BAZ;
            }

            sub BAZ {
                    print "in BAZ\n";
            }

            package GOOP;
            @ISA = qw( FOO );

            sub new { bless {} }
            sub BAZ {
                    print "in GOOP::BAZ\n";
            }

            package main;

            $a = GOOP->new;
            $a->bar;