528 lines
11 KiB
Plaintext
528 lines
11 KiB
Plaintext
=head1 NAME
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perlbot - Bag'o Object Tricks (the BOT)
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=head1 DESCRIPTION
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The following collection of tricks and hints is intended to whet curious
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appetites about such things as the use of instance variables and the
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mechanics of object and class relationships. The reader is encouraged to
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consult relevant textbooks for discussion of Object Oriented definitions and
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methodology. This is not intended as a tutorial for object-oriented
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programming or as a comprehensive guide to Perl's object oriented features,
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nor should it be construed as a style guide.
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The Perl motto still holds: There's more than one way to do it.
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=head1 OO SCALING TIPS
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=over 5
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=item 1
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Do not attempt to verify the type of $self. That'll break if the class is
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inherited, when the type of $self is valid but its package isn't what you
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expect. See rule 5.
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=item 2
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If an object-oriented (OO) or indirect-object (IO) syntax was used, then the
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object is probably the correct type and there's no need to become paranoid
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about it. Perl isn't a paranoid language anyway. If people subvert the OO
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or IO syntax then they probably know what they're doing and you should let
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them do it. See rule 1.
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=item 3
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Use the two-argument form of bless(). Let a subclass use your constructor.
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See L<INHERITING A CONSTRUCTOR>.
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=item 4
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The subclass is allowed to know things about its immediate superclass, the
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superclass is allowed to know nothing about a subclass.
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=item 5
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Don't be trigger happy with inheritance. A "using", "containing", or
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"delegation" relationship (some sort of aggregation, at least) is often more
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appropriate. See L<OBJECT RELATIONSHIPS>, L<USING RELATIONSHIP WITH SDBM>,
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and L<"DELEGATION">.
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=item 6
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The object is the namespace. Make package globals accessible via the
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object. This will remove the guess work about the symbol's home package.
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See L<CLASS CONTEXT AND THE OBJECT>.
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=item 7
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IO syntax is certainly less noisy, but it is also prone to ambiguities that
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can cause difficult-to-find bugs. Allow people to use the sure-thing OO
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syntax, even if you don't like it.
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=item 8
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Do not use function-call syntax on a method. You're going to be bitten
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someday. Someone might move that method into a superclass and your code
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will be broken. On top of that you're feeding the paranoia in rule 2.
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=item 9
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Don't assume you know the home package of a method. You're making it
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difficult for someone to override that method. See L<THINKING OF CODE REUSE>.
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=back
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=head1 INSTANCE VARIABLES
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An anonymous array or anonymous hash can be used to hold instance
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variables. Named parameters are also demonstrated.
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package Foo;
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sub new {
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my $type = shift;
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my %params = @_;
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my $self = {};
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$self->{'High'} = $params{'High'};
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$self->{'Low'} = $params{'Low'};
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bless $self, $type;
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}
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package Bar;
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sub new {
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my $type = shift;
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my %params = @_;
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my $self = [];
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$self->[0] = $params{'Left'};
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$self->[1] = $params{'Right'};
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bless $self, $type;
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}
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package main;
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$a = Foo->new( 'High' => 42, 'Low' => 11 );
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print "High=$a->{'High'}\n";
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print "Low=$a->{'Low'}\n";
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$b = Bar->new( 'Left' => 78, 'Right' => 40 );
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print "Left=$b->[0]\n";
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print "Right=$b->[1]\n";
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=head1 SCALAR INSTANCE VARIABLES
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An anonymous scalar can be used when only one instance variable is needed.
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package Foo;
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sub new {
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my $type = shift;
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my $self;
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$self = shift;
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bless \$self, $type;
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}
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package main;
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$a = Foo->new( 42 );
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print "a=$$a\n";
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=head1 INSTANCE VARIABLE INHERITANCE
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This example demonstrates how one might inherit instance variables from a
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superclass for inclusion in the new class. This requires calling the
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superclass's constructor and adding one's own instance variables to the new
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object.
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package Bar;
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sub new {
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my $type = shift;
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my $self = {};
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$self->{'buz'} = 42;
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bless $self, $type;
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}
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package Foo;
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@ISA = qw( Bar );
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sub new {
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my $type = shift;
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my $self = Bar->new;
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$self->{'biz'} = 11;
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bless $self, $type;
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}
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package main;
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$a = Foo->new;
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print "buz = ", $a->{'buz'}, "\n";
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print "biz = ", $a->{'biz'}, "\n";
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=head1 OBJECT RELATIONSHIPS
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The following demonstrates how one might implement "containing" and "using"
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relationships between objects.
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package Bar;
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sub new {
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my $type = shift;
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my $self = {};
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$self->{'buz'} = 42;
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bless $self, $type;
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}
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package Foo;
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sub new {
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my $type = shift;
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my $self = {};
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$self->{'Bar'} = Bar->new;
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$self->{'biz'} = 11;
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bless $self, $type;
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}
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package main;
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$a = Foo->new;
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print "buz = ", $a->{'Bar'}->{'buz'}, "\n";
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print "biz = ", $a->{'biz'}, "\n";
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=head1 OVERRIDING SUPERCLASS METHODS
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The following example demonstrates how to override a superclass method and
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then call the overridden method. The B<SUPER> pseudo-class allows the
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programmer to call an overridden superclass method without actually knowing
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where that method is defined.
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package Buz;
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sub goo { print "here's the goo\n" }
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package Bar; @ISA = qw( Buz );
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sub google { print "google here\n" }
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package Baz;
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sub mumble { print "mumbling\n" }
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package Foo;
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@ISA = qw( Bar Baz );
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sub new {
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my $type = shift;
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bless [], $type;
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}
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sub grr { print "grumble\n" }
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sub goo {
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my $self = shift;
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$self->SUPER::goo();
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}
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sub mumble {
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my $self = shift;
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$self->SUPER::mumble();
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}
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sub google {
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my $self = shift;
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$self->SUPER::google();
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}
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package main;
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$foo = Foo->new;
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$foo->mumble;
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$foo->grr;
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$foo->goo;
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$foo->google;
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=head1 USING RELATIONSHIP WITH SDBM
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This example demonstrates an interface for the SDBM class. This creates a
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"using" relationship between the SDBM class and the new class Mydbm.
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package Mydbm;
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require SDBM_File;
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require Tie::Hash;
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@ISA = qw( Tie::Hash );
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sub TIEHASH {
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my $type = shift;
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my $ref = SDBM_File->new(@_);
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bless {'dbm' => $ref}, $type;
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}
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sub FETCH {
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my $self = shift;
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my $ref = $self->{'dbm'};
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$ref->FETCH(@_);
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}
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sub STORE {
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my $self = shift;
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if (defined $_[0]){
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my $ref = $self->{'dbm'};
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$ref->STORE(@_);
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} else {
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die "Cannot STORE an undefined key in Mydbm\n";
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}
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}
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package main;
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use Fcntl qw( O_RDWR O_CREAT );
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tie %foo, "Mydbm", "Sdbm", O_RDWR|O_CREAT, 0640;
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$foo{'bar'} = 123;
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print "foo-bar = $foo{'bar'}\n";
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tie %bar, "Mydbm", "Sdbm2", O_RDWR|O_CREAT, 0640;
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$bar{'Cathy'} = 456;
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print "bar-Cathy = $bar{'Cathy'}\n";
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=head1 THINKING OF CODE REUSE
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One strength of Object-Oriented languages is the ease with which old code
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can use new code. The following examples will demonstrate first how one can
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hinder code reuse and then how one can promote code reuse.
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This first example illustrates a class which uses a fully-qualified method
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call to access the "private" method BAZ(). The second example will show
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that it is impossible to override the BAZ() method.
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package FOO;
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sub new {
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my $type = shift;
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bless {}, $type;
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}
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sub bar {
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my $self = shift;
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$self->FOO::private::BAZ;
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}
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package FOO::private;
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sub BAZ {
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print "in BAZ\n";
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}
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package main;
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$a = FOO->new;
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$a->bar;
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Now we try to override the BAZ() method. We would like FOO::bar() to call
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GOOP::BAZ(), but this cannot happen because FOO::bar() explicitly calls
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FOO::private::BAZ().
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package FOO;
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sub new {
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my $type = shift;
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bless {}, $type;
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}
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sub bar {
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my $self = shift;
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$self->FOO::private::BAZ;
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}
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package FOO::private;
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sub BAZ {
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print "in BAZ\n";
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}
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package GOOP;
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@ISA = qw( FOO );
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sub new {
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my $type = shift;
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bless {}, $type;
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}
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sub BAZ {
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print "in GOOP::BAZ\n";
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}
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package main;
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$a = GOOP->new;
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$a->bar;
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To create reusable code we must modify class FOO, flattening class
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FOO::private. The next example shows a reusable class FOO which allows the
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method GOOP::BAZ() to be used in place of FOO::BAZ().
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package FOO;
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sub new {
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my $type = shift;
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bless {}, $type;
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}
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sub bar {
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my $self = shift;
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$self->BAZ;
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}
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sub BAZ {
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print "in BAZ\n";
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}
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package GOOP;
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@ISA = qw( FOO );
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sub new {
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my $type = shift;
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bless {}, $type;
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}
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sub BAZ {
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print "in GOOP::BAZ\n";
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}
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package main;
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$a = GOOP->new;
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$a->bar;
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=head1 CLASS CONTEXT AND THE OBJECT
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Use the object to solve package and class context problems. Everything a
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method needs should be available via the object or should be passed as a
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parameter to the method.
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A class will sometimes have static or global data to be used by the
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methods. A subclass may want to override that data and replace it with new
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data. When this happens the superclass may not know how to find the new
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copy of the data.
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This problem can be solved by using the object to define the context of the
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method. Let the method look in the object for a reference to the data. The
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alternative is to force the method to go hunting for the data ("Is it in my
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class, or in a subclass? Which subclass?"), and this can be inconvenient
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and will lead to hackery. It is better just to let the object tell the
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method where that data is located.
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package Bar;
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%fizzle = ( 'Password' => 'XYZZY' );
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sub new {
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my $type = shift;
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my $self = {};
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$self->{'fizzle'} = \%fizzle;
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bless $self, $type;
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}
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sub enter {
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my $self = shift;
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# Don't try to guess if we should use %Bar::fizzle
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# or %Foo::fizzle. The object already knows which
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# we should use, so just ask it.
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#
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my $fizzle = $self->{'fizzle'};
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print "The word is ", $fizzle->{'Password'}, "\n";
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}
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package Foo;
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@ISA = qw( Bar );
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%fizzle = ( 'Password' => 'Rumple' );
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sub new {
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my $type = shift;
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my $self = Bar->new;
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$self->{'fizzle'} = \%fizzle;
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bless $self, $type;
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}
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package main;
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$a = Bar->new;
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$b = Foo->new;
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$a->enter;
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$b->enter;
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=head1 INHERITING A CONSTRUCTOR
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An inheritable constructor should use the second form of bless() which allows
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blessing directly into a specified class. Notice in this example that the
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object will be a BAR not a FOO, even though the constructor is in class FOO.
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package FOO;
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sub new {
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my $type = shift;
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my $self = {};
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bless $self, $type;
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}
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sub baz {
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print "in FOO::baz()\n";
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}
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package BAR;
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@ISA = qw(FOO);
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sub baz {
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print "in BAR::baz()\n";
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}
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package main;
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$a = BAR->new;
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$a->baz;
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=head1 DELEGATION
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Some classes, such as SDBM_File, cannot be effectively subclassed because
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they create foreign objects. Such a class can be extended with some sort of
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aggregation technique such as the "using" relationship mentioned earlier or
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by delegation.
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The following example demonstrates delegation using an AUTOLOAD() function to
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perform message-forwarding. This will allow the Mydbm object to behave
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exactly like an SDBM_File object. The Mydbm class could now extend the
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behavior by adding custom FETCH() and STORE() methods, if this is desired.
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package Mydbm;
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require SDBM_File;
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require Tie::Hash;
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@ISA = qw(Tie::Hash);
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sub TIEHASH {
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my $type = shift;
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my $ref = SDBM_File->new(@_);
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bless {'delegate' => $ref};
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}
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sub AUTOLOAD {
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my $self = shift;
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# The Perl interpreter places the name of the
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# message in a variable called $AUTOLOAD.
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# DESTROY messages should never be propagated.
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return if $AUTOLOAD =~ /::DESTROY$/;
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# Remove the package name.
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$AUTOLOAD =~ s/^Mydbm:://;
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# Pass the message to the delegate.
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$self->{'delegate'}->$AUTOLOAD(@_);
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}
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package main;
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use Fcntl qw( O_RDWR O_CREAT );
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tie %foo, "Mydbm", "adbm", O_RDWR|O_CREAT, 0640;
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$foo{'bar'} = 123;
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print "foo-bar = $foo{'bar'}\n";
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