perlbot - Bag'o Object Tricks (the BOT)
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.
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".
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'}0;
print "Low=$a->{'Low'}0;
$b = Bar->new( 'Left' => 78, 'Right' => 40 );
print "Left=$b->[0]0;
print "Right=$b->[1]0;
SCALAR INSTANCE VARIABLES [Toc] [Back] 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 elf, $type;
}
package main;
$a = Foo->new( 42 );
print "a=$$a0;
INSTANCE VARIABLE INHERITANCE [Toc] [Back] 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'}, "0;
print "biz = ", $a->{'biz'}, "0;
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'}, "0;
print "biz = ", $a->{'biz'}, "0;
OVERRIDING SUPERCLASS METHODS [Toc] [Back] 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 goo0 }
package Bar; @ISA = qw( Buz );
sub google { print "google here0 }
package Baz;
sub mumble { print "mumbling0 }
package Foo;
@ISA = qw( Bar Baz );
sub new {
my $type = shift;
bless [], $type;
}
sub grr { print "grumble0 }
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 [Toc] [Back] 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 Mydbm0;
}
}
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'}0;
tie %bar, "Mydbm", "Sdbm2", O_RDWR|O_CREAT, 0640;
$bar{'Cathy'} = 456;
print "bar-Cathy = $bar{'Cathy'}0;
THINKING OF CODE REUSE [Toc] [Back] 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 fullyqualified
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 BAZ0;
}
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 BAZ0;
}
package GOOP;
@ISA = qw( FOO );
sub new {
my $type = shift;
bless {}, $type;
}
sub BAZ {
print "in GOOP::BAZ0;
}
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 BAZ0;
}
package GOOP;
@ISA = qw( FOO );
sub new {
my $type = shift;
bless {}, $type;
}
sub BAZ {
print "in GOOP::BAZ0;
}
package main;
$a = GOOP->new;
$a->bar;
CLASS CONTEXT AND THE OBJECT [Toc] [Back] 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'}, "0;
}
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 [Toc] [Back] 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()0;
}
package BAR;
@ISA = qw(FOO);
sub baz {
print "in BAR::baz()0;
}
package main;
$a = BAR->new;
$a->baz;
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'}0;
perlboot, perltoot, perltooc.
perl v5.8.5 2002-11-06 11 [ Back ] |
