3 perlbot - Bag'o Object Tricks (the BOT)
7 The following collection of tricks and hints is intended to whet curious
8 appetites about such things as the use of instance variables and the
9 mechanics of object and class relationships. The reader is encouraged to
10 consult relevant textbooks for discussion of Object Oriented definitions and
11 methodology. This is not intended as a tutorial for object-oriented
12 programming or as a comprehensive guide to Perl's object oriented features,
13 nor should it be construed as a style guide.
15 The Perl motto still holds: There's more than one way to do it.
17 =head1 OO SCALING TIPS
23 Do not attempt to verify the type of $self. That'll break if the class is
24 inherited, when the type of $self is valid but its package isn't what you
29 If an object-oriented (OO) or indirect-object (IO) syntax was used, then the
30 object is probably the correct type and there's no need to become paranoid
31 about it. Perl isn't a paranoid language anyway. If people subvert the OO
32 or IO syntax then they probably know what they're doing and you should let
33 them do it. See rule 1.
37 Use the two-argument form of bless(). Let a subclass use your constructor.
38 See L<INHERITING A CONSTRUCTOR>.
42 The subclass is allowed to know things about its immediate superclass, the
43 superclass is allowed to know nothing about a subclass.
47 Don't be trigger happy with inheritance. A "using", "containing", or
48 "delegation" relationship (some sort of aggregation, at least) is often more
49 appropriate. See L<OBJECT RELATIONSHIPS>, L<USING RELATIONSHIP WITH SDBM>,
54 The object is the namespace. Make package globals accessible via the
55 object. This will remove the guess work about the symbol's home package.
56 See L<CLASS CONTEXT AND THE OBJECT>.
60 IO syntax is certainly less noisy, but it is also prone to ambiguities which
61 can cause difficult-to-find bugs. Allow people to use the sure-thing OO
62 syntax, even if you don't like it.
66 Do not use function-call syntax on a method. You're going to be bitten
67 someday. Someone might move that method into a superclass and your code
68 will be broken. On top of that you're feeding the paranoia in rule 2.
72 Don't assume you know the home package of a method. You're making it
73 difficult for someone to override that method. See L<THINKING OF CODE REUSE>.
77 =head1 INSTANCE VARIABLES
79 An anonymous array or anonymous hash can be used to hold instance
80 variables. Named parameters are also demonstrated.
88 $self->{'High'} = $params{'High'};
89 $self->{'Low'} = $params{'Low'};
100 $self->[0] = $params{'Left'};
101 $self->[1] = $params{'Right'};
107 $a = Foo->new( 'High' => 42, 'Low' => 11 );
108 print "High=$a->{'High'}\n";
109 print "Low=$a->{'Low'}\n";
111 $b = Bar->new( 'Left' => 78, 'Right' => 40 );
112 print "Left=$b->[0]\n";
113 print "Right=$b->[1]\n";
115 =head1 SCALAR INSTANCE VARIABLES
117 An anonymous scalar can be used when only one instance variable is needed.
134 =head1 INSTANCE VARIABLE INHERITANCE
136 This example demonstrates how one might inherit instance variables from a
137 superclass for inclusion in the new class. This requires calling the
138 superclass's constructor and adding one's own instance variables to the new
163 print "buz = ", $a->{'buz'}, "\n";
164 print "biz = ", $a->{'biz'}, "\n";
168 =head1 OBJECT RELATIONSHIPS
170 The following demonstrates how one might implement "containing" and "using"
171 relationships between objects.
187 $self->{'Bar'} = Bar->new;
195 print "buz = ", $a->{'Bar'}->{'buz'}, "\n";
196 print "biz = ", $a->{'biz'}, "\n";
200 =head1 OVERRIDING SUPERCLASS METHODS
202 The following example demonstrates how one might override a superclass
203 method and then call the method after it has been overridden. The
204 Foo::Inherit class allows the programmer to call an overridden superclass
205 method without actually knowing where that method is defined.
209 sub goo { print "here's the goo\n" }
211 package Bar; @ISA = qw( Buz );
212 sub google { print "google here\n" }
215 sub mumble { print "mumbling\n" }
218 @ISA = qw( Bar Baz );
219 @Foo::Inherit::ISA = @ISA; # Access to overridden methods.
225 sub grr { print "grumble\n" }
228 $self->Foo::Inherit::goo();
232 $self->Foo::Inherit::mumble();
236 $self->Foo::Inherit::google();
248 =head1 USING RELATIONSHIP WITH SDBM
250 This example demonstrates an interface for the SDBM class. This creates a
251 "using" relationship between the SDBM class and the new class Mydbm.
257 @ISA = qw( TieHash );
261 my $ref = SDBM_File->new(@_);
262 bless {'dbm' => $ref}, $type;
266 my $ref = $self->{'dbm'};
272 my $ref = $self->{'dbm'};
275 die "Cannot STORE an undefined key in Mydbm\n";
280 use Fcntl qw( O_RDWR O_CREAT );
282 tie %foo, Mydbm, "Sdbm", O_RDWR|O_CREAT, 0640;
284 print "foo-bar = $foo{'bar'}\n";
286 tie %bar, Mydbm, "Sdbm2", O_RDWR|O_CREAT, 0640;
288 print "bar-Cathy = $bar{'Cathy'}\n";
290 =head1 THINKING OF CODE REUSE
292 One strength of Object-Oriented languages is the ease with which old code
293 can use new code. The following examples will demonstrate first how one can
294 hinder code reuse and then how one can promote code reuse.
296 This first example illustrates a class which uses a fully-qualified method
297 call to access the "private" method BAZ(). The second example will show
298 that it is impossible to override the BAZ() method.
308 $self->FOO::private::BAZ;
311 package FOO::private;
322 Now we try to override the BAZ() method. We would like FOO::bar() to call
323 GOOP::BAZ(), but this cannot happen because FOO::bar() explicitly calls
334 $self->FOO::private::BAZ;
337 package FOO::private;
351 print "in GOOP::BAZ\n";
359 To create reusable code we must modify class FOO, flattening class
360 FOO::private. The next example shows a reusable class FOO which allows the
361 method GOOP::BAZ() to be used in place of FOO::BAZ().
386 print "in GOOP::BAZ\n";
394 =head1 CLASS CONTEXT AND THE OBJECT
396 Use the object to solve package and class context problems. Everything a
397 method needs should be available via the object or should be passed as a
398 parameter to the method.
400 A class will sometimes have static or global data to be used by the
401 methods. A subclass may want to override that data and replace it with new
402 data. When this happens the superclass may not know how to find the new
405 This problem can be solved by using the object to define the context of the
406 method. Let the method look in the object for a reference to the data. The
407 alternative is to force the method to go hunting for the data ("Is it in my
408 class, or in a subclass? Which subclass?"), and this can be inconvenient
409 and will lead to hackery. It is better to just let the object tell the
410 method where that data is located.
414 %fizzle = ( 'Password' => 'XYZZY' );
419 $self->{'fizzle'} = \%fizzle;
426 # Don't try to guess if we should use %Bar::fizzle
427 # or %Foo::fizzle. The object already knows which
428 # we should use, so just ask it.
430 my $fizzle = $self->{'fizzle'};
432 print "The word is ", $fizzle->{'Password'}, "\n";
438 %fizzle = ( 'Password' => 'Rumple' );
443 $self->{'fizzle'} = \%fizzle;
454 =head1 INHERITING A CONSTRUCTOR
456 An inheritable constructor should use the second form of bless() which allows
457 blessing directly into a specified class. Notice in this example that the
458 object will be a BAR not a FOO, even though the constructor is in class FOO.
469 print "in FOO::baz()\n";
476 print "in BAR::baz()\n";
486 Some classes, such as SDBM_File, cannot be effectively subclassed because
487 they create foreign objects. Such a class can be extended with some sort of
488 aggregation technique such as the "using" relationship mentioned earlier or
491 The following example demonstrates delegation using an AUTOLOAD() function to
492 perform message-forwarding. This will allow the Mydbm object to behave
493 exactly like an SDBM_File object. The Mydbm class could now extend the
494 behavior by adding custom FETCH() and STORE() methods, if this is desired.
504 my $ref = SDBM_File->new(@_);
505 bless {'delegate' => $ref};
511 # The Perl interpreter places the name of the
512 # message in a variable called $AUTOLOAD.
514 # DESTROY messages should never be propagated.
515 return if $AUTOLOAD =~ /::DESTROY$/;
517 # Remove the package name.
518 $AUTOLOAD =~ s/^Mydbm:://;
520 # Pass the message to the delegate.
521 $self->{'delegate'}->$AUTOLOAD(@_);
525 use Fcntl qw( O_RDWR O_CREAT );
527 tie %foo, Mydbm, "adbm", O_RDWR|O_CREAT, 0640;
529 print "foo-bar = $foo{'bar'}\n";