perlbot
(1)
名前
perlbot - Bag'o Object Tricks (the BOT)
形式
Please see following description for synopsis
説明
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.
perl v5.12.5 Last change: 2012-11-03 1
Perl Programmers Reference Guide PERLBOT(1)
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.
perl v5.12.5 Last change: 2012-11-03 2
Perl Programmers Reference Guide PERLBOT(1)
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 {
perl v5.12.5 Last change: 2012-11-03 3
Perl Programmers Reference Guide PERLBOT(1)
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;
perl v5.12.5 Last change: 2012-11-03 4
Perl Programmers Reference Guide PERLBOT(1)
$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 );
perl v5.12.5 Last change: 2012-11-03 5
Perl Programmers Reference Guide PERLBOT(1)
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 {
perl v5.12.5 Last change: 2012-11-03 6
Perl Programmers Reference Guide PERLBOT(1)
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;
perl v5.12.5 Last change: 2012-11-03 7
Perl Programmers Reference Guide PERLBOT(1)
}
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";
}
perl v5.12.5 Last change: 2012-11-03 8
Perl Programmers Reference Guide PERLBOT(1)
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
perl v5.12.5 Last change: 2012-11-03 9
Perl Programmers Reference Guide PERLBOT(1)
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";
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
perl v5.12.5 Last change: 2012-11-03 10
Perl Programmers Reference Guide PERLBOT(1)
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
perlboot, perltoot, perltooc.
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
perl v5.12.5 Last change: 2012-11-03 11