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) 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 "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 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
"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;
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 );
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 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;
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";
SEE ALSO¶
perlboot, perltoot, perltooc.
