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 that
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.
89 $self->{High} = $params{High};
90 $self->{Low} = $params{Low};
103 $self->[0] = $params{Left};
104 $self->[1] = $params{Right};
111 my $a = Foo->new( High => 42, Low => 11 );
112 print "High = $a->{High}\n";
113 print "Low = $a->{Low}\n";
115 my $b = Bar->new( Left => 78, Right => 40 );
116 print "Left = $b->[0]\n";
117 print "Right = $b->[1]\n";
119 =head1 SCALAR INSTANCE VARIABLES
121 An anonymous scalar can be used when only one instance variable is needed.
134 my $a = Foo->new( 42 );
138 =head1 INSTANCE VARIABLE INHERITANCE
140 This example demonstrates how one might inherit instance variables from a
141 superclass for inclusion in the new class. This requires calling the
142 superclass's constructor and adding one's own instance variables to the new
157 our @ISA = qw( Bar );
171 print "buz = $a->{buz}\n";
172 print "biz = $a->{biz}\n";
176 =head1 OBJECT RELATIONSHIPS
178 The following demonstrates how one might implement "containing" and "using"
179 relationships between objects.
198 $self->{Bar} = Bar->new;
207 print "buz = $a->{Bar}->{buz}\n";
208 print "biz = $a->{biz}\n";
212 =head1 OVERRIDING SUPERCLASS METHODS
214 The following example demonstrates how to override a superclass method and
215 then call the overridden method. The B<SUPER> pseudo-class allows the
216 programmer to call an overridden superclass method without actually knowing
217 where that method is defined.
220 sub goo { print "here's the goo\n" }
224 our @ISA = qw( Buz );
225 sub google { print "google here\n" }
229 sub mumble { print "mumbling\n" }
232 our @ISA = qw( Bar Baz );
238 sub grr { print "grumble\n" }
245 $self->SUPER::mumble();
249 $self->SUPER::google();
261 =head1 USING RELATIONSHIP WITH SDBM
263 This example demonstrates an interface for the SDBM class. This creates a
264 "using" relationship between the SDBM class and the new class Mydbm.
271 our @ISA = qw( Tie::Hash );
275 my $ref = SDBM_File->new(@_);
276 bless { dbm => $ref }, $type;
281 my $ref = $self->{dbm};
288 if ( defined $_[0] ) {
289 my $ref = $self->{dbm};
292 die "Cannot STORE an undefined key in Mydbm\n";
297 use Fcntl qw( O_RDWR O_CREAT );
299 tie my %foo, 'Mydbm', 'Sdbm', O_RDWR|O_CREAT, 0640;
301 print "foo-bar = $foo{bar}\n";
303 tie my %bar, 'Mydbm', 'Sdbm2', O_RDWR|O_CREAT, 0640;
305 print "bar-Cathy = $bar{Cathy}\n";
307 =head1 THINKING OF CODE REUSE
309 One strength of Object-Oriented languages is the ease with which old code
310 can use new code. The following examples will demonstrate first how one can
311 hinder code reuse and then how one can promote code reuse.
313 This first example illustrates a class which uses a fully-qualified method
314 call to access the "private" method BAZ(). The second example will show
315 that it is impossible to override the BAZ() method.
326 $self->FOO::private::BAZ;
329 package FOO::private;
340 Now we try to override the BAZ() method. We would like FOO::bar() to call
341 GOOP::BAZ(), but this cannot happen because FOO::bar() explicitly calls
353 $self->FOO::private::BAZ;
356 package FOO::private;
364 our @ISA = qw( FOO );
372 print "in GOOP::BAZ\n";
380 To create reusable code we must modify class FOO, flattening class
381 FOO::private. The next example shows a reusable class FOO which allows the
382 method GOOP::BAZ() to be used in place of FOO::BAZ().
402 our @ISA = qw( FOO );
410 print "in GOOP::BAZ\n";
418 =head1 CLASS CONTEXT AND THE OBJECT
420 Use the object to solve package and class context problems. Everything a
421 method needs should be available via the object or should be passed as a
422 parameter to the method.
424 A class will sometimes have static or global data to be used by the
425 methods. A subclass may want to override that data and replace it with new
426 data. When this happens the superclass may not know how to find the new
429 This problem can be solved by using the object to define the context of the
430 method. Let the method look in the object for a reference to the data. The
431 alternative is to force the method to go hunting for the data ("Is it in my
432 class, or in a subclass? Which subclass?"), and this can be inconvenient
433 and will lead to hackery. It is better just to let the object tell the
434 method where that data is located.
438 my %fizzle = ( Password => 'XYZZY' );
443 $self->{fizzle} = \%fizzle;
450 # Don't try to guess if we should use %Bar::fizzle
451 # or %Foo::fizzle. The object already knows which
452 # we should use, so just ask it.
454 my $fizzle = $self->{fizzle};
456 print "The word is $fizzle->{Password}\n";
461 our @ISA = qw( Bar );
463 my %fizzle = ( Password => 'Rumple' );
468 $self->{fizzle} = \%fizzle;
480 =head1 INHERITING A CONSTRUCTOR
482 An inheritable constructor should use the second form of bless() which allows
483 blessing directly into a specified class. Notice in this example that the
484 object will be a BAR not a FOO, even though the constructor is in class FOO.
495 print "in FOO::baz()\n";
503 print "in BAR::baz()\n";
513 Some classes, such as SDBM_File, cannot be effectively subclassed because
514 they create foreign objects. Such a class can be extended with some sort of
515 aggregation technique such as the "using" relationship mentioned earlier or
518 The following example demonstrates delegation using an AUTOLOAD() function to
519 perform message-forwarding. This will allow the Mydbm object to behave
520 exactly like an SDBM_File object. The Mydbm class could now extend the
521 behavior by adding custom FETCH() and STORE() methods, if this is desired.
528 our @ISA = qw( Tie::Hash );
533 my $ref = SDBM_File->new(@_);
534 bless { delegate => $ref };
540 # The Perl interpreter places the name of the
541 # message in a variable called $AUTOLOAD.
543 # DESTROY messages should never be propagated.
544 return if $AUTOLOAD =~ /::DESTROY$/;
546 # Remove the package name.
547 $AUTOLOAD =~ s/^Mydbm:://;
549 # Pass the message to the delegate.
550 $self->{delegate}->$AUTOLOAD(@_);
554 use Fcntl qw( O_RDWR O_CREAT );
556 tie my %foo, 'Mydbm', 'adbm', O_RDWR|O_CREAT, 0640;
558 print "foo-bar = $foo{bar}\n";