[p5sagit/p5-mst-13.2.git] / pod / perlbot.pod

=head1 NAME

perlbot - Bag'o Object Tricks (the BOT)

=head1 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.

The Perl motto still holds:  There's more than one way to do it.

=head1 OO SCALING TIPS

=over 5

=item 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.

=item 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.

=item 3

Use the two-argument form of bless().  Let a subclass use your constructor.
See L<INHERITING A CONSTRUCTOR>.

=item 4

The subclass is allowed to know things about its immediate superclass, the
superclass is allowed to know nothing about a subclass.

=item 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 L<OBJECT RELATIONSHIPS>, L<USING RELATIONSHIP WITH SDBM>,
and L<"DELEGATION">.

=item 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 L<CLASS CONTEXT AND THE OBJECT>.

=item 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.

=item 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.

=item 9

Don't assume you know the home package of a method.  You're making it
difficult for someone to override that method.  See L<THINKING OF CODE REUSE>.

=back

=head1 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";

=head1 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";


=head1 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";


=head1 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";


=head1 OVERRIDING SUPERCLASS METHODS

The following example demonstrates how to override a superclass method and
then call the overridden method.  The B<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;


=head1 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";

=head1 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;

=head1 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;

=head1 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;

=head1 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";
Commit	Line	Data
a0d0e21e	1	=head1 NAME
a0d0e21e	2
c2960299	3	perlbot - Bag'o Object Tricks (the BOT)
a0d0e21e	4
cb1a09d0	5	=head1 DESCRIPTION
a0d0e21e	6
	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
c2960299	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.
a0d0e21e	14
	15	The Perl motto still holds: There's more than one way to do it.
	16
c2960299	17	=head1 OO SCALING TIPS
	18
	19	=over 5
	20
	21	=item 1
	22
	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
	25	expect. See rule 5.
	26
	27	=item 2
	28
	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.
	34
	35	=item 3
	36
	37	Use the two-argument form of bless(). Let a subclass use your constructor.
	38	See L<INHERITING A CONSTRUCTOR>.
	39
	40	=item 4
	41
	42	The subclass is allowed to know things about its immediate superclass, the
	43	superclass is allowed to know nothing about a subclass.
	44
	45	=item 5
	46
	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>,
	50	and L<"DELEGATION">.
	51
	52	=item 6
	53
	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>.
	57
	58	=item 7
	59
5f05dabc	60	IO syntax is certainly less noisy, but it is also prone to ambiguities that
c2960299	61	can cause difficult-to-find bugs. Allow people to use the sure-thing OO
	62	syntax, even if you don't like it.
	63
	64	=item 8
	65
	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.
	69
	70	=item 9
	71
	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>.
	74
	75	=back
	76
a0d0e21e	77	=head1 INSTANCE VARIABLES
	78
	79	An anonymous array or anonymous hash can be used to hold instance
	80	variables. Named parameters are also demonstrated.
	81
	82	package Foo;
	83
	84	sub new {
	85	my $type = shift;
	86	my %params = @_;
	87	my $self = {};
	88	$self->{'High'} = $params{'High'};
	89	$self->{'Low'} = $params{'Low'};
c2960299	90	bless $self, $type;
a0d0e21e	91	}
	92
	93
	94	package Bar;
	95
	96	sub new {
	97	my $type = shift;
	98	my %params = @_;
	99	my $self = [];
	100	$self->[0] = $params{'Left'};
	101	$self->[1] = $params{'Right'};
c2960299	102	bless $self, $type;
a0d0e21e	103	}
	104
	105	package main;
	106
c2960299	107	$a = Foo->new( 'High' => 42, 'Low' => 11 );
a0d0e21e	108	print "High=$a->{'High'}\n";
	109	print "Low=$a->{'Low'}\n";
	110
c2960299	111	$b = Bar->new( 'Left' => 78, 'Right' => 40 );
a0d0e21e	112	print "Left=$b->[0]\n";
	113	print "Right=$b->[1]\n";
	114
a0d0e21e	115	=head1 SCALAR INSTANCE VARIABLES
	116
	117	An anonymous scalar can be used when only one instance variable is needed.
	118
	119	package Foo;
	120
	121	sub new {
	122	my $type = shift;
	123	my $self;
	124	$self = shift;
c2960299	125	bless \$self, $type;
a0d0e21e	126	}
	127
	128	package main;
	129
c2960299	130	$a = Foo->new( 42 );
a0d0e21e	131	print "a=$$a\n";
	132
	133
	134	=head1 INSTANCE VARIABLE INHERITANCE
	135
	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
	139	object.
	140
	141	package Bar;
	142
	143	sub new {
c2960299	144	my $type = shift;
a0d0e21e	145	my $self = {};
a0d0e21e	146	$self->{'buz'} = 42;
c2960299	147	bless $self, $type;
a0d0e21e	148	}
	149
	150	package Foo;
	151	@ISA = qw( Bar );
	152
	153	sub new {
c2960299	154	my $type = shift;
c2960299	155	my $self = Bar->new;
a0d0e21e	156	$self->{'biz'} = 11;
c2960299	157	bless $self, $type;
a0d0e21e	158	}
	159
	160	package main;
	161
c2960299	162	$a = Foo->new;
a0d0e21e	163	print "buz = ", $a->{'buz'}, "\n";
	164	print "biz = ", $a->{'biz'}, "\n";
	165
	166
	167
	168	=head1 OBJECT RELATIONSHIPS
	169
	170	The following demonstrates how one might implement "containing" and "using"
	171	relationships between objects.
	172
	173	package Bar;
	174
	175	sub new {
c2960299	176	my $type = shift;
a0d0e21e	177	my $self = {};
a0d0e21e	178	$self->{'buz'} = 42;
c2960299	179	bless $self, $type;
a0d0e21e	180	}
	181
	182	package Foo;
	183
	184	sub new {
c2960299	185	my $type = shift;
a0d0e21e	186	my $self = {};
c2960299	187	$self->{'Bar'} = Bar->new;
a0d0e21e	188	$self->{'biz'} = 11;
c2960299	189	bless $self, $type;
a0d0e21e	190	}
	191
	192	package main;
	193
c2960299	194	$a = Foo->new;
a0d0e21e	195	print "buz = ", $a->{'Bar'}->{'buz'}, "\n";
	196	print "biz = ", $a->{'biz'}, "\n";
	197
	198
	199
	200	=head1 OVERRIDING SUPERCLASS METHODS
	201
4633a7c4	202	The following example demonstrates how to override a superclass method and
	203	then call the overridden method. The B<SUPER> pseudo-class allows the
	204	programmer to call an overridden superclass method without actually knowing
	205	where that method is defined.
a0d0e21e	206
	207	package Buz;
	208	sub goo { print "here's the goo\n" }
	209
	210	package Bar; @ISA = qw( Buz );
	211	sub google { print "google here\n" }
	212
	213	package Baz;
	214	sub mumble { print "mumbling\n" }
	215
	216	package Foo;
	217	@ISA = qw( Bar Baz );
a0d0e21e	218
c2960299	219	sub new {
	220	my $type = shift;
	221	bless [], $type;
	222	}
a0d0e21e	223	sub grr { print "grumble\n" }
	224	sub goo {
	225	my $self = shift;
4633a7c4	226	$self->SUPER::goo();
a0d0e21e	227	}
	228	sub mumble {
	229	my $self = shift;
4633a7c4	230	$self->SUPER::mumble();
a0d0e21e	231	}
	232	sub google {
	233	my $self = shift;
4633a7c4	234	$self->SUPER::google();
a0d0e21e	235	}
	236
	237	package main;
	238
c2960299	239	$foo = Foo->new;
a0d0e21e	240	$foo->mumble;
	241	$foo->grr;
	242	$foo->goo;
	243	$foo->google;
	244
	245
c2960299	246	=head1 USING RELATIONSHIP WITH SDBM
a0d0e21e	247
	248	This example demonstrates an interface for the SDBM class. This creates a
	249	"using" relationship between the SDBM class and the new class Mydbm.
	250
a0d0e21e	251	package Mydbm;
a0d0e21e	252
c2960299	253	require SDBM_File;
af131e48	254	require Tie::Hash;
af131e48	255	@ISA = qw( Tie::Hash );
c2960299	256
a0d0e21e	257	sub TIEHASH {
c2960299	258	my $type = shift;
a0d0e21e	259	my $ref = SDBM_File->new(@_);
c2960299	260	bless {'dbm' => $ref}, $type;
a0d0e21e	261	}
	262	sub FETCH {
	263	my $self = shift;
	264	my $ref = $self->{'dbm'};
	265	$ref->FETCH(@_);
	266	}
	267	sub STORE {
54310121	268	my $self = shift;
a0d0e21e	269	if (defined $_[0]){
	270	my $ref = $self->{'dbm'};
	271	$ref->STORE(@_);
	272	} else {
	273	die "Cannot STORE an undefined key in Mydbm\n";
	274	}
	275	}
	276
	277	package main;
c2960299	278	use Fcntl qw( O_RDWR O_CREAT );
a0d0e21e	279
c954a603	280	tie %foo, "Mydbm", "Sdbm", O_RDWR\|O_CREAT, 0640;
a0d0e21e	281	$foo{'bar'} = 123;
	282	print "foo-bar = $foo{'bar'}\n";
	283
c954a603	284	tie %bar, "Mydbm", "Sdbm2", O_RDWR\|O_CREAT, 0640;
a0d0e21e	285	$bar{'Cathy'} = 456;
	286	print "bar-Cathy = $bar{'Cathy'}\n";
	287
	288	=head1 THINKING OF CODE REUSE
	289
	290	One strength of Object-Oriented languages is the ease with which old code
	291	can use new code. The following examples will demonstrate first how one can
	292	hinder code reuse and then how one can promote code reuse.
	293
	294	This first example illustrates a class which uses a fully-qualified method
	295	call to access the "private" method BAZ(). The second example will show
	296	that it is impossible to override the BAZ() method.
	297
	298	package FOO;
	299
c2960299	300	sub new {
	301	my $type = shift;
	302	bless {}, $type;
	303	}
a0d0e21e	304	sub bar {
	305	my $self = shift;
	306	$self->FOO::private::BAZ;
	307	}
	308
	309	package FOO::private;
	310
	311	sub BAZ {
	312	print "in BAZ\n";
	313	}
	314
	315	package main;
	316
	317	$a = FOO->new;
	318	$a->bar;
	319
	320	Now we try to override the BAZ() method. We would like FOO::bar() to call
d1b91892	321	GOOP::BAZ(), but this cannot happen because FOO::bar() explicitly calls
a0d0e21e	322	FOO::private::BAZ().
	323
	324	package FOO;
	325
c2960299	326	sub new {
	327	my $type = shift;
	328	bless {}, $type;
	329	}
a0d0e21e	330	sub bar {
	331	my $self = shift;
	332	$self->FOO::private::BAZ;
	333	}
	334
	335	package FOO::private;
	336
	337	sub BAZ {
	338	print "in BAZ\n";
	339	}
	340
	341	package GOOP;
	342	@ISA = qw( FOO );
c2960299	343	sub new {
	344	my $type = shift;
	345	bless {}, $type;
	346	}
a0d0e21e	347
	348	sub BAZ {
	349	print "in GOOP::BAZ\n";
	350	}
	351
	352	package main;
	353
	354	$a = GOOP->new;
	355	$a->bar;
	356
	357	To create reusable code we must modify class FOO, flattening class
	358	FOO::private. The next example shows a reusable class FOO which allows the
	359	method GOOP::BAZ() to be used in place of FOO::BAZ().
	360
	361	package FOO;
	362
c2960299	363	sub new {
	364	my $type = shift;
	365	bless {}, $type;
	366	}
a0d0e21e	367	sub bar {
	368	my $self = shift;
	369	$self->BAZ;
	370	}
	371
	372	sub BAZ {
	373	print "in BAZ\n";
	374	}
	375
	376	package GOOP;
	377	@ISA = qw( FOO );
	378
c2960299	379	sub new {
	380	my $type = shift;
	381	bless {}, $type;
	382	}
a0d0e21e	383	sub BAZ {
	384	print "in GOOP::BAZ\n";
	385	}
	386
	387	package main;
	388
	389	$a = GOOP->new;
	390	$a->bar;
	391
	392	=head1 CLASS CONTEXT AND THE OBJECT
	393
	394	Use the object to solve package and class context problems. Everything a
	395	method needs should be available via the object or should be passed as a
	396	parameter to the method.
	397
	398	A class will sometimes have static or global data to be used by the
	399	methods. A subclass may want to override that data and replace it with new
	400	data. When this happens the superclass may not know how to find the new
	401	copy of the data.
	402
	403	This problem can be solved by using the object to define the context of the
	404	method. Let the method look in the object for a reference to the data. The
	405	alternative is to force the method to go hunting for the data ("Is it in my
	406	class, or in a subclass? Which subclass?"), and this can be inconvenient
5f05dabc	407	and will lead to hackery. It is better just to let the object tell the
a0d0e21e	408	method where that data is located.
	409
	410	package Bar;
	411
	412	%fizzle = ( 'Password' => 'XYZZY' );
	413
	414	sub new {
c2960299	415	my $type = shift;
a0d0e21e	416	my $self = {};
a0d0e21e	417	$self->{'fizzle'} = \%fizzle;
c2960299	418	bless $self, $type;
a0d0e21e	419	}
	420
	421	sub enter {
	422	my $self = shift;
54310121	423
a0d0e21e	424	# Don't try to guess if we should use %Bar::fizzle
	425	# or %Foo::fizzle. The object already knows which
	426	# we should use, so just ask it.
	427	#
	428	my $fizzle = $self->{'fizzle'};
	429
	430	print "The word is ", $fizzle->{'Password'}, "\n";
	431	}
	432
	433	package Foo;
	434	@ISA = qw( Bar );
	435
	436	%fizzle = ( 'Password' => 'Rumple' );
	437
	438	sub new {
c2960299	439	my $type = shift;
a0d0e21e	440	my $self = Bar->new;
a0d0e21e	441	$self->{'fizzle'} = \%fizzle;
c2960299	442	bless $self, $type;
a0d0e21e	443	}
	444
	445	package main;
	446
	447	$a = Bar->new;
	448	$b = Foo->new;
	449	$a->enter;
	450	$b->enter;
	451
d1b91892	452	=head1 INHERITING A CONSTRUCTOR
	453
	454	An inheritable constructor should use the second form of bless() which allows
	455	blessing directly into a specified class. Notice in this example that the
	456	object will be a BAR not a FOO, even though the constructor is in class FOO.
	457
	458	package FOO;
	459
	460	sub new {
	461	my $type = shift;
	462	my $self = {};
	463	bless $self, $type;
	464	}
	465
	466	sub baz {
	467	print "in FOO::baz()\n";
	468	}
	469
	470	package BAR;
	471	@ISA = qw(FOO);
	472
	473	sub baz {
	474	print "in BAR::baz()\n";
	475	}
	476
	477	package main;
	478
	479	$a = BAR->new;
	480	$a->baz;
	481
	482	=head1 DELEGATION
	483
	484	Some classes, such as SDBM_File, cannot be effectively subclassed because
	485	they create foreign objects. Such a class can be extended with some sort of
	486	aggregation technique such as the "using" relationship mentioned earlier or
	487	by delegation.
	488
	489	The following example demonstrates delegation using an AUTOLOAD() function to
	490	perform message-forwarding. This will allow the Mydbm object to behave
	491	exactly like an SDBM_File object. The Mydbm class could now extend the
	492	behavior by adding custom FETCH() and STORE() methods, if this is desired.
	493
	494	package Mydbm;
	495
	496	require SDBM_File;
af131e48	497	require Tie::Hash;
af131e48	498	@ISA = qw(Tie::Hash);
d1b91892	499
	500	sub TIEHASH {
	501	my $type = shift;
	502	my $ref = SDBM_File->new(@_);
	503	bless {'delegate' => $ref};
	504	}
	505
	506	sub AUTOLOAD {
	507	my $self = shift;
	508
	509	# The Perl interpreter places the name of the
	510	# message in a variable called $AUTOLOAD.
	511
	512	# DESTROY messages should never be propagated.
	513	return if $AUTOLOAD =~ /::DESTROY$/;
	514
	515	# Remove the package name.
	516	$AUTOLOAD =~ s/^Mydbm:://;
	517
	518	# Pass the message to the delegate.
	519	$self->{'delegate'}->$AUTOLOAD(@_);
	520	}
	521
	522	package main;
	523	use Fcntl qw( O_RDWR O_CREAT );
	524
c954a603	525	tie %foo, "Mydbm", "adbm", O_RDWR\|O_CREAT, 0640;
d1b91892	526	$foo{'bar'} = 123;
d1b91892	527	print "foo-bar = $foo{'bar'}\n";