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NAME
     perlbot - 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 Mydbm0;
                 }
             }

             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 BAZ0;
             }

             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 BAZ0;
             }

             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;


CLASS CONTEXT AND THE OBJECT
     Use the object to solve package and class context  problems.
     Everything a method needs should be available via the object
     or should be passed as a parameter to the method.

     A class will sometimes have static or global data to be used
     by the methods.  A subclass may want to override that data
     and replace it with new data.  When this happens the
     superclass may not know how to find the new copy of the
     data.

     This problem can be solved by using the object to define the
     context of the method.  Let the method look in the object
     for a reference to the data.  The alternative is to force
     the method to go hunting for the data ("Is it in my class,
     or in a subclass?  Which subclass?"), and this can be
     inconvenient and will lead to hackery.  It is better to just
     let the object tell the method where that data is located.

             package Bar;

             %fizzle = ( 'Password' => 'XYZZY' );

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

             sub enter {
                     my $self = shift;

                     #  Don't try to guess if we should use %Bar::fizzle
                     # or %Foo::fizzle.  The object already knows which
                     # we should use, so just ask it.
                     #
                     my $fizzle = $self->{'fizzle'};

                     print "The word is ", $fizzle->{'Password'},"\n";
             }

             package Foo;
             @ISA = qw( Bar );

             %fizzle = ( 'Password' => 'Rumple' );

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

             package main;

             $a = Bar->new;
             $b = Foo->new;
             $a->enter;
             $b->enter;


 

 

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