-=head1 NAME
-
-perlbot - Bag'o Object Tricks (the BOT)
-
-=head1 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 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.
-
-=head1 OO SCALING TIPS
-
-=over 5
-
-=item 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.
-
-=item 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.
-
-=item 3
-
-Use the two-argument form of bless(). Let a subclass use your constructor.
-See L<INHERITING A CONSTRUCTOR>.
-
-=item 4
-
-The subclass is allowed to know things about its immediate superclass, the
-superclass is allowed to know nothing about a subclass.
-
-=item 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 L<OBJECT RELATIONSHIPS>, L<USING RELATIONSHIP WITH SDBM>,
-and L<"DELEGATION">.
-
-=item 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 L<CLASS CONTEXT AND THE OBJECT>.
-
-=item 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.
-
-=item 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.
-
-=item 9
-
-Don't assume you know the home package of a method. You're making it
-difficult for someone to override that method. See L<THINKING OF CODE REUSE>.
-
-=back
-
-=head1 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";
-
-=head1 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";
-
-
-=head1 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";
-
-
-
-=head1 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";
-
-
-
-=head1 OVERRIDING SUPERCLASS METHODS
-
-The following example demonstrates how to override a superclass method and
-then call the overridden method. The B<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 );
+=encoding utf8
- 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;
-
-
-=head1 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 TieHash;
- @ISA = qw( TieHash );
-
- 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";
-
-=head1 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;
-
-=head1 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;
-
-=head1 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;
-
-=head1 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 TieHash;
- @ISA = qw(TieHash);
-
- 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.
+=head1 NAME
- # DESTROY messages should never be propagated.
- return if $AUTOLOAD =~ /::DESTROY$/;
+perlbot - Links to information on object-oriented programming in Perl
- # Remove the package name.
- $AUTOLOAD =~ s/^Mydbm:://;
+=head1 DESCRIPTION
- # Pass the message to the delegate.
- $self->{'delegate'}->$AUTOLOAD(@_);
- }
+For information on OO programming with Perl, please see L<perlootut>
+and L<perlobj>.
- package main;
- use Fcntl qw( O_RDWR O_CREAT );
+(The above documents supersede the collection of tricks that was formerly here
+in perlbot.)
- tie %foo, Mydbm, "adbm", O_RDWR|O_CREAT, 0640;
- $foo{'bar'} = 123;
- print "foo-bar = $foo{'bar'}\n";
+=cut
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