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
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. If you're looking for tutorials, be sure
to read perlboot, perltoot, and perltooc.
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) syn-
tax 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 sub-
vert 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 CON-
TEXT AND THE OBJECT".
7 IO syntax is certainly less noisy, but it is also prone
to ambiguities that can cause difficult-to-find bugs.
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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 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 vari-
able is needed.
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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 "contain-
ing" and "using" relationships between objects.
package Bar;
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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 super-
class 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 );
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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;
Note that "SUPER" refers to the superclasses of the current
package ("Foo"), not to the superclasses of $self.
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 );
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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;
}
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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().
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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 subclass?"), and this can be incon-
venient and will lead to hackery. It is better just to let
the object tell the method where that data is located.
package Bar;
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%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;
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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 sub-
classed 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 delega-
tion.
The following example demonstrates delegation using an AUTO-
LOAD() 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;
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# 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";
SEE ALSO
perlboot, perltoot, perltooc.
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