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RedHat 9 (Linux i386) - man page for perlbot (redhat section 1)

PERLBOT(1)			 Perl Programmers Reference Guide		       PERLBOT(1)

NAME
       perlbot - Bag'o Object Tricks (the BOT)

DESCRIPTION
       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 rela-
       tionships.  The reader is encouraged to consult relevant textbooks for discussion of
       Object Oriented definitions and methodology.  This is not intended as a tutorial for
       object-oriented programming or as a comprehensive guide to Perl's object oriented fea-
       tures, nor should it be construed as a style guide.

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

OO SCALING TIPS
       1    Do not attempt to verify the type of $self.  That'll break if the class is inherited,
	    when the type of $self is valid but its package isn't what you expect.  See rule 5.

       2    If an object-oriented (OO) or indirect-object (IO) syntax was used, then the object
	    is probably the correct type and there's no need to become paranoid about it.  Perl
	    isn't a paranoid language anyway.  If people subvert the OO or IO syntax then they
	    probably know what they're doing and you should let them do it.  See rule 1.

       3    Use the two-argument form of bless().  Let a subclass use your constructor.  See
	    "INHERITING A CONSTRUCTOR".

       4    The subclass is allowed to know things about its immediate superclass, the superclass
	    is allowed to know nothing about a subclass.

       5    Don't be trigger happy with inheritance.  A "using", "containing", or "delegation"
	    relationship (some sort of aggregation, at least) is often more appropriate.  See
	    "OBJECT RELATIONSHIPS", "USING RELATIONSHIP WITH SDBM", and "DELEGATION".

       6    The object is the namespace.  Make package globals accessible via the object.  This
	    will remove the guess work about the symbol's home package.  See "CLASS CONTEXT AND
	    THE OBJECT".

       7    IO syntax is certainly less noisy, but it is also prone to ambiguities that can cause
	    difficult-to-find bugs.  Allow people to use the sure-thing OO syntax, even if you
	    don't like it.

       8    Do not use function-call syntax on a method.  You're going to be bitten someday.
	    Someone might move that method into a superclass and your code will be broken.  On
	    top of that you're feeding the paranoia in rule 2.

       9    Don't assume you know the home package of a method.  You're making it difficult for
	    someone to override that method.  See "THINKING OF CODE REUSE".

INSTANCE VARIABLES
       An anonymous array or anonymous hash can be used to hold instance variables.  Named param-
       eters are also demonstrated.

	       package Foo;

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

	       package Bar;

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

	       package main;

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

	       $b = Bar->new( 'Left' => 78, 'Right' => 40 );
	       print "Left=$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, $type;
	       }

	       package main;

	       $a = Foo->new( 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 $type = shift;
		       my $self = {};
		       $self->{'buz'} = 42;
		       bless $self, $type;
	       }

	       package Foo;
	       @ISA = qw( Bar );

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

	       package main;

	       $a = Foo->new;
	       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 $type = shift;
		       my $self = {};
		       $self->{'buz'} = 42;
		       bless $self, $type;
	       }

	       package Foo;

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

	       package main;

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

OVERRIDING SUPERCLASS METHODS
       The following example demonstrates how to override a superclass method and then call the
       overridden method.  The SUPER pseudo-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 );

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

	       package main;

	       $foo = Foo->new;
	       $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" rela-
       tionship between the SDBM class and the new class Mydbm.

	       package Mydbm;

	       require SDBM_File;
	       require Tie::Hash;
	       @ISA = qw( Tie::Hash );

	       sub TIEHASH {
		   my $type = shift;
		   my $ref  = SDBM_File->new(@_);
		   bless {'dbm' => $ref}, $type;
	       }
	       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;
	       use Fcntl qw( O_RDWR O_CREAT );

	       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 over-
       ride the BAZ() method.

	       package FOO;

	       sub new {
		       my $type = shift;
		       bless {}, $type;
	       }
	       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 because FOO::bar() explicitly calls FOO::private::BAZ().

	       package FOO;

	       sub new {
		       my $type = shift;
		       bless {}, $type;
	       }
	       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 {
		       my $type = shift;
		       bless {}, $type;
	       }

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

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

	       package GOOP;
	       @ISA = qw( FOO );

	       sub new {
		       my $type = shift;
		       bless {}, $type;
	       }
	       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 super-
       class 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 sub-
       class?"), and this can be inconvenient and will lead to hackery.  It is better just to let
       the object tell the method where that data is located.

	       package Bar;

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

	       sub new {
		       my $type = shift;
		       my $self = {};
		       $self->{'fizzle'} = \%fizzle;
		       bless $self, $type;
	       }

	       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 $type = shift;
		       my $self = Bar->new;
		       $self->{'fizzle'} = \%fizzle;
		       bless $self, $type;
	       }

	       package main;

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

INHERITING A CONSTRUCTOR
       An inheritable constructor should use the second form of bless() which allows blessing
       directly into a specified class.  Notice in this example that the object will be a BAR not
       a FOO, even though the constructor is in class FOO.

	       package FOO;

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

	       sub baz {
		       print "in FOO::baz()\n";
	       }

	       package BAR;
	       @ISA = qw(FOO);

	       sub baz {
		       print "in BAR::baz()\n";
	       }

	       package main;

	       $a = BAR->new;
	       $a->baz;

DELEGATION
       Some classes, such as SDBM_File, cannot be effectively subclassed because they create for-
       eign objects.  Such a class can be extended with some sort of aggregation technique such
       as the "using" relationship mentioned earlier or by delegation.

       The following example demonstrates delegation using an AUTOLOAD() function to perform mes-
       sage-forwarding.  This will allow the Mydbm object to behave exactly like an SDBM_File
       object.	The Mydbm class could now extend the behavior by adding custom FETCH() and
       STORE() methods, if this is desired.

	       package Mydbm;

	       require SDBM_File;
	       require Tie::Hash;
	       @ISA = qw(Tie::Hash);

	       sub TIEHASH {
		       my $type = shift;
		       my $ref = SDBM_File->new(@_);
		       bless {'delegate' => $ref};
	       }

	       sub AUTOLOAD {
		       my $self = shift;

		       # The Perl interpreter places the name of the
		       # message in a variable called $AUTOLOAD.

		       # DESTROY messages should never be propagated.
		       return if $AUTOLOAD =~ /::DESTROY$/;

		       # Remove the package name.
		       $AUTOLOAD =~ s/^Mydbm:://;

		       # Pass the message to the delegate.
		       $self->{'delegate'}->$AUTOLOAD(@_);
	       }

	       package main;
	       use Fcntl qw( O_RDWR O_CREAT );

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

perl v5.8.0				    2003-02-18				       PERLBOT(1)


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