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Under GNU General Public License
2012-05-26 01:27 @38.107.179.238 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)
PERLBOT(1) Perl Programmers Reference Guide PERLBOT(1)
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.
perl v5.10.0 2011-06-28 PERLBOT(1)
Generated by $Id: phpMan.php,v 4.49 2006/02/26 13:18:18 chedong Exp $ Author: Che Dong
On Apache
Under GNU General Public License
2012-05-26 01:27 @38.107.179.238 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)