PERLBOT(1)
NNAAMMEE
perlbot - Bag'o Object Tricks (the BOT)
DDEESSCCRRIIPPTTIIOONN
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.
OOOO SSCCAALLIINNGG TTIIPPSS
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 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 the section on THINKING OF CODE
REUSE.
IINNSSTTAANNCCEE VVAARRIIAABBLLEESS
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";
SSCCAALLAARR IINNSSTTAANNCCEE VVAARRIIAABBLLEESS
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";
IINNSSTTAANNCCEE VVAARRIIAABBLLEE IINNHHEERRIITTAANNCCEE
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";
OOBBJJEECCTT RREELLAATTIIOONNSSHHIIPPSS
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";
OOVVEERRRRIIDDIINNGG SSUUPPEERRCCLLAASSSS MMEETTHHOODDSS
The following example demonstrates how to override a
superclass method and then call the overridden method.
The SSUUPPEERR 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;
UUSSIINNGG RREELLAATTIIOONNSSHHIIPP WWIITTHH SSDDBBMM
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";
TTHHIINNKKIINNGG OOFF CCOODDEE RREEUUSSEE
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;
CCLLAASSSS CCOONNTTEEXXTT AANNDD TTHHEE OOBBJJEECCTT
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
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;
IINNHHEERRIITTIINNGG AA CCOONNSSTTRRUUCCTTOORR
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;
DDEELLEEGGAATTIIOONN
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";