AmigActive 2/AACD 2.iso/AACD/Programming/perlman/man/perlbot.txt

Text File | 1999-09-09 | 16.4 KB | 514 lines

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
       relationships.  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 features,
       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 the section on 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 the section on OBJECT RELATIONSHIPS, the section
            on USING RELATIONSHIP WITH SDBM, and the section on
            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 the
            section on CLASS CONTEXT AND THE OBJECT.

       7    IO syntax is certainly less noisy, but it is also
            prone to ambiguities which 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 the section on THINKING OF CODE
            REUSE.

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, $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" relationship 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 override 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
       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 $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 foreign 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 message-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";