PERLTOOT(1) Perl Programmers Reference Guide PERLTOOT(1)NAMEperltoot - Tom's object-oriented tutorial for perl
DESCRIPTION
Object-oriented programming is a big seller these days.
Some managers would rather have objects than sliced bread.
Why is that? What's so special about an object? Just what
is an object anyway?
An object is nothing but a way of tucking away complex
behaviours into a neat little easy-to-use bundle. (This is
what professors call abstraction.) Smart people who have
nothing to do but sit around for weeks on end figuring out
really hard problems make these nifty objects that even reg-
ular people can use. (This is what professors call software
reuse.) Users (well, programmers) can play with this little
bundle all they want, but they aren't to open it up and mess
with the insides. Just like an expensive piece of hardware,
the contract says that you void the warranty if you muck
with the cover. So don't do that.
The heart of objects is the class, a protected little
private namespace full of data and functions. A class is a
set of related routines that addresses some problem area.
You can think of it as a user-defined type. The Perl package
mechanism, also used for more traditional modules, is used
for class modules as well. Objects "live" in a class, mean-
ing that they belong to some package.
More often than not, the class provides the user with little
bundles. These bundles are objects. They know whose class
they belong to, and how to behave. Users ask the class to
do something, like "give me an object." Or they can ask one
of these objects to do something. Asking a class to do some-
thing for you is calling a class method. Asking an object to
do something for you is calling an object method. Asking
either a class (usually) or an object (sometimes) to give
you back an object is calling a constructor, which is just a
kind of method.
That's all well and good, but how is an object different
from any other Perl data type? Just what is an object
really; that is, what's its fundamental type? The answer to
the first question is easy. An object is different from any
other data type in Perl in one and only one way: you may
dereference it using not merely string or numeric subscripts
as with simple arrays and hashes, but with named subroutine
calls. In a word, with methods.
The answer to the second question is that it's a reference,
and not just any reference, mind you, but one whose referent
has been bless()ed into a particular class (read: package).
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What kind of reference? Well, the answer to that one is a
bit less concrete. That's because in Perl the designer of
the class can employ any sort of reference they'd like as
the underlying intrinsic data type. It could be a scalar,
an array, or a hash reference. It could even be a code
reference. But because of its inherent flexibility, an
object is usually a hash reference.
Creating a Class
Before you create a class, you need to decide what to name
it. That's because the class (package) name governs the
name of the file used to house it, just as with regular
modules. Then, that class (package) should provide one or
more ways to generate objects. Finally, it should provide
mechanisms to allow users of its objects to indirectly mani-
pulate these objects from a distance.
For example, let's make a simple Person class module. It
gets stored in the file Person.pm. If it were called a
Happy::Person class, it would be stored in the file
Happy/Person.pm, and its package would become Happy::Person
instead of just Person. (On a personal computer not running
Unix or Plan 9, but something like Mac OS or VMS, the direc-
tory separator may be different, but the principle is the
same.) Do not assume any formal relationship between
modules based on their directory names. This is merely a
grouping convenience, and has no effect on inheritance,
variable accessibility, or anything else.
For this module we aren't going to use Exporter, because
we're a well-behaved class module that doesn't export any-
thing at all. In order to manufacture objects, a class needs
to have a constructor method. A constructor gives you back
not just a regular data type, but a brand-new object in that
class. This magic is taken care of by the bless() function,
whose sole purpose is to enable its referent to be used as
an object. Remember: being an object really means nothing
more than that methods may now be called against it.
While a constructor may be named anything you'd like, most
Perl programmers seem to like to call theirs new(). How-
ever, new() is not a reserved word, and a class is under no
obligation to supply such. Some programmers have also been
known to use a function with the same name as the class as
the constructor.
Object Representation
By far the most common mechanism used in Perl to represent a
Pascal record, a C struct, or a C++ class is an anonymous
hash. That's because a hash has an arbitrary number of data
fields, each conveniently accessed by an arbitrary name of
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your own devising.
If you were just doing a simple struct-like emulation, you
would likely go about it something like this:
$rec = {
name => "Jason",
age => 23,
peers => [ "Norbert", "Rhys", "Phineas"],
};
If you felt like it, you could add a bit of visual distinc-
tion by up-casing the hash keys:
$rec = {
NAME => "Jason",
AGE => 23,
PEERS => [ "Norbert", "Rhys", "Phineas"],
};
And so you could get at "$rec->{NAME}" to find "Jason", or
"@{ $rec->{PEERS} }" to get at "Norbert", "Rhys", and
"Phineas". (Have you ever noticed how many 23-year-old pro-
grammers seem to be named "Jason" these days? :-)
This same model is often used for classes, although it is
not considered the pinnacle of programming propriety for
folks from outside the class to come waltzing into an
object, brazenly accessing its data members directly. Gen-
erally speaking, an object should be considered an opaque
cookie that you use object methods to access. Visually,
methods look like you're dereffing a reference using a func-
tion name instead of brackets or braces.
Class Interface
Some languages provide a formal syntactic interface to a
class's methods, but Perl does not. It relies on you to
read the documentation of each class. If you try to call an
undefined method on an object, Perl won't complain, but the
program will trigger an exception while it's running. Like-
wise, if you call a method expecting a prime number as its
argument with a non-prime one instead, you can't expect the
compiler to catch this. (Well, you can expect it all you
like, but it's not going to happen.)
Let's suppose you have a well-educated user of your Person
class, someone who has read the docs that explain the
prescribed interface. Here's how they might use the Person
class:
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use Person;
$him = Person->new();
$him->name("Jason");
$him->age(23);
$him->peers( "Norbert", "Rhys", "Phineas" );
push @All_Recs, $him; # save object in array for later
printf "%s is %d years old.\n", $him->name, $him->age;
print "His peers are: ", join(", ", $him->peers), "\n";
printf "Last rec's name is %s\n", $All_Recs[-1]->name;
As you can see, the user of the class doesn't know (or at
least, has no business paying attention to the fact) that
the object has one particular implementation or another.
The interface to the class and its objects is exclusively
via methods, and that's all the user of the class should
ever play with.
Constructors and Instance Methods
Still, someone has to know what's in the object. And that
someone is the class. It implements methods that the pro-
grammer uses to access the object. Here's how to implement
the Person class using the standard hash-ref-as-an-object
idiom. We'll make a class method called new() to act as the
constructor, and three object methods called name(), age(),
and peers() to get at per-object data hidden away in our
anonymous hash.
package Person;
use strict;
##################################################
## the object constructor (simplistic version) ##
##################################################
sub new {
my $self = {};
$self->{NAME} = undef;
$self->{AGE} = undef;
$self->{PEERS} = [];
bless($self); # but see below
return $self;
}
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##############################################
## methods to access per-object data ##
## ##
## With args, they set the value. Without ##
## any, they only retrieve it/them. ##
##############################################
sub name {
my $self = shift;
if (@_) { $self->{NAME} = shift }
return $self->{NAME};
}
sub age {
my $self = shift;
if (@_) { $self->{AGE} = shift }
return $self->{AGE};
}
sub peers {
my $self = shift;
if (@_) { @{ $self->{PEERS} } = @_ }
return @{ $self->{PEERS} };
}
1; # so the require or use succeeds
We've created three methods to access an object's data,
name(), age(), and peers(). These are all substantially
similar. If called with an argument, they set the appropri-
ate field; otherwise they return the value held by that
field, meaning the value of that hash key.
Planning for the Future: Better Constructors
Even though at this point you may not even know what it
means, someday you're going to worry about inheritance.
(You can safely ignore this for now and worry about it later
if you'd like.) To ensure that this all works out smoothly,
you must use the double-argument form of bless(). The second
argument is the class into which the referent will be
blessed. By not assuming our own class as the default second
argument and instead using the class passed into us, we make
our constructor inheritable.
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sub new {
my $class = shift;
my $self = {};
$self->{NAME} = undef;
$self->{AGE} = undef;
$self->{PEERS} = [];
bless ($self, $class);
return $self;
}
That's about all there is for constructors. These methods
bring objects to life, returning neat little opaque bundles
to the user to be used in subsequent method calls.
Destructors
Every story has a beginning and an end. The beginning of
the object's story is its constructor, explicitly called
when the object comes into existence. But the ending of its
story is the destructor, a method implicitly called when an
object leaves this life. Any per-object clean-up code is
placed in the destructor, which must (in Perl) be called
DESTROY.
If constructors can have arbitrary names, then why not des-
tructors? Because while a constructor is explicitly called,
a destructor is not. Destruction happens automatically via
Perl's garbage collection (GC) system, which is a quick but
somewhat lazy reference-based GC system. To know what to
call, Perl insists that the destructor be named DESTROY.
Perl's notion of the right time to call a destructor is not
well-defined currently, which is why your destructors should
not rely on when they are called.
Why is DESTROY in all caps? Perl on occasion uses purely
uppercase function names as a convention to indicate that
the function will be automatically called by Perl in some
way. Others that are called implicitly include BEGIN, END,
AUTOLOAD, plus all methods used by tied objects, described
in perltie.
In really good object-oriented programming languages, the
user doesn't care when the destructor is called. It just
happens when it's supposed to. In low-level languages
without any GC at all, there's no way to depend on this hap-
pening at the right time, so the programmer must explicitly
call the destructor to clean up memory and state, crossing
their fingers that it's the right time to do so. Unlike
C++, an object destructor is nearly never needed in Perl,
and even when it is, explicit invocation is uncalled for.
In the case of our Person class, we don't need a destructor
because Perl takes care of simple matters like memory
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deallocation.
The only situation where Perl's reference-based GC won't
work is when there's a circularity in the data structure,
such as:
$this->{WHATEVER} = $this;
In that case, you must delete the self-reference manually if
you expect your program not to leak memory. While admit-
tedly error-prone, this is the best we can do right now.
Nonetheless, rest assured that when your program is fin-
ished, its objects' destructors are all duly called. So you
are guaranteed that an object eventually gets properly des-
troyed, except in the unique case of a program that never
exits. (If you're running Perl embedded in another applica-
tion, this full GC pass happens a bit more frequently--
whenever a thread shuts down.)
Other Object Methods
The methods we've talked about so far have either been con-
structors or else simple "data methods", interfaces to data
stored in the object. These are a bit like an object's data
members in the C++ world, except that strangers don't access
them as data. Instead, they should only access the object's
data indirectly via its methods. This is an important rule:
in Perl, access to an object's data should only be made
through methods.
Perl doesn't impose restrictions on who gets to use which
methods. The public-versus-private distinction is by conven-
tion, not syntax. (Well, unless you use the Alias module
described below in "Data Members as Variables".) Occasion-
ally you'll see method names beginning or ending with an
underscore or two. This marking is a convention indicating
that the methods are private to that class alone and some-
times to its closest acquaintances, its immediate subc-
lasses. But this distinction is not enforced by Perl
itself. It's up to the programmer to behave.
There's no reason to limit methods to those that simply
access data. Methods can do anything at all. The key point
is that they're invoked against an object or a class. Let's
say we'd like object methods that do more than fetch or set
one particular field.
sub exclaim {
my $self = shift;
return sprintf "Hi, I'm %s, age %d, working with %s",
$self->{NAME}, $self->{AGE}, join(", ", @{$self->{PEERS}});
}
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Or maybe even one like this:
sub happy_birthday {
my $self = shift;
return ++$self->{AGE};
}
Some might argue that one should go at these this way:
sub exclaim {
my $self = shift;
return sprintf "Hi, I'm %s, age %d, working with %s",
$self->name, $self->age, join(", ", $self->peers);
}
sub happy_birthday {
my $self = shift;
return $self->age( $self->age() + 1 );
}
But since these methods are all executing in the class
itself, this may not be critical. There are tradeoffs to be
made. Using direct hash access is faster (about an order of
magnitude faster, in fact), and it's more convenient when
you want to interpolate in strings. But using methods (the
external interface) internally shields not just the users of
your class but even you yourself from changes in your data
representation.
Class Data
What about "class data", data items common to each object in
a class? What would you want that for? Well, in your Person
class, you might like to keep track of the total people
alive. How do you implement that?
You could make it a global variable called $Person::Census.
But about only reason you'd do that would be if you wanted
people to be able to get at your class data directly. They
could just say $Person::Census and play around with it.
Maybe this is ok in your design scheme. You might even con-
ceivably want to make it an exported variable. To be
exportable, a variable must be a (package) global. If this
were a traditional module rather than an object-oriented
one, you might do that.
While this approach is expected in most traditional modules,
it's generally considered rather poor form in most object
modules. In an object module, you should set up a protec-
tive veil to separate interface from implementation. So
provide a class method to access class data just as you pro-
vide object methods to access object data.
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So, you could still keep $Census as a package global and
rely upon others to honor the contract of the module and
therefore not play around with its implementation. You
could even be supertricky and make $Census a tied object as
described in perltie, thereby intercepting all accesses.
But more often than not, you just want to make your class
data a file-scoped lexical. To do so, simply put this at
the top of the file:
my $Census = 0;
Even though the scope of a my() normally expires when the
block in which it was declared is done (in this case the
whole file being required or used), Perl's deep binding of
lexical variables guarantees that the variable will not be
deallocated, remaining accessible to functions declared
within that scope. This doesn't work with global variables
given temporary values via local(), though.
Irrespective of whether you leave $Census a package global
or make it instead a file-scoped lexical, you should make
these changes to your Person::new() constructor:
sub new {
my $class = shift;
my $self = {};
$Census++;
$self->{NAME} = undef;
$self->{AGE} = undef;
$self->{PEERS} = [];
bless ($self, $class);
return $self;
}
sub population {
return $Census;
}
Now that we've done this, we certainly do need a destructor
so that when Person is destroyed, the $Census goes down.
Here's how this could be done:
sub DESTROY { --$Census }
Notice how there's no memory to deallocate in the destruc-
tor? That's something that Perl takes care of for you all
by itself.
Alternatively, you could use the Class::Data::Inheritable
module from CPAN.
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Accessing Class Data
It turns out that this is not really a good way to go about
handling class data. A good scalable rule is that you must
never reference class data directly from an object method.
Otherwise you aren't building a scalable, inheritable class.
The object must be the rendezvous point for all operations,
especially from an object method. The globals (class data)
would in some sense be in the "wrong" package in your
derived classes. In Perl, methods execute in the context of
the class they were defined in, not that of the object that
triggered them. Therefore, namespace visibility of package
globals in methods is unrelated to inheritance.
Got that? Maybe not. Ok, let's say that some other class
"borrowed" (well, inherited) the DESTROY method as it was
defined above. When those objects are destroyed, the origi-
nal $Census variable will be altered, not the one in the new
class's package namespace. Perhaps this is what you want,
but probably it isn't.
Here's how to fix this. We'll store a reference to the data
in the value accessed by the hash key "_CENSUS". Why the
underscore? Well, mostly because an initial underscore
already conveys strong feelings of magicalness to a C pro-
grammer. It's really just a mnemonic device to remind our-
selves that this field is special and not to be used as a
public data member in the same way that NAME, AGE, and PEERS
are. (Because we've been developing this code under the
strict pragma, prior to perl version 5.004 we'll have to
quote the field name.)
sub new {
my $class = shift;
my $self = {};
$self->{NAME} = undef;
$self->{AGE} = undef;
$self->{PEERS} = [];
# "private" data
$self->{"_CENSUS"} = \$Census;
bless ($self, $class);
++ ${ $self->{"_CENSUS"} };
return $self;
}
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sub population {
my $self = shift;
if (ref $self) {
return ${ $self->{"_CENSUS"} };
} else {
return $Census;
}
}
sub DESTROY {
my $self = shift;
-- ${ $self->{"_CENSUS"} };
}
Debugging Methods
It's common for a class to have a debugging mechanism. For
example, you might want to see when objects are created or
destroyed. To do that, add a debugging variable as a file-
scoped lexical. For this, we'll pull in the standard Carp
module to emit our warnings and fatal messages. That way
messages will come out with the caller's filename and line
number instead of our own; if we wanted them to be from our
own perspective, we'd just use die() and warn() directly
instead of croak() and carp() respectively.
use Carp;
my $Debugging = 0;
Now add a new class method to access the variable.
sub debug {
my $class = shift;
if (ref $class) { confess "Class method called as object method" }
unless (@_ == 1) { confess "usage: CLASSNAME->debug(level)" }
$Debugging = shift;
}
Now fix up DESTROY to murmur a bit as the moribund object
expires:
sub DESTROY {
my $self = shift;
if ($Debugging) { carp "Destroying $self " . $self->name }
-- ${ $self->{"_CENSUS"} };
}
One could conceivably make a per-object debug state. That
way you could call both of these:
Person->debug(1); # entire class
$him->debug(1); # just this object
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To do so, we need our debugging method to be a "bimodal"
one, one that works on both classes and objects. Therefore,
adjust the debug() and DESTROY methods as follows:
sub debug {
my $self = shift;
confess "usage: thing->debug(level)" unless @_ == 1;
my $level = shift;
if (ref($self)) {
$self->{"_DEBUG"} = $level; # just myself
} else {
$Debugging = $level; # whole class
}
}
sub DESTROY {
my $self = shift;
if ($Debugging || $self->{"_DEBUG"}) {
carp "Destroying $self " . $self->name;
}
-- ${ $self->{"_CENSUS"} };
}
What happens if a derived class (which we'll call Employee)
inherits methods from this Person base class? Then
"Employee->debug()", when called as a class method, manipu-
lates $Person::Debugging not $Employee::Debugging.
Class Destructors
The object destructor handles the death of each distinct
object. But sometimes you want a bit of cleanup when the
entire class is shut down, which currently only happens when
the program exits. To make such a class destructor, create
a function in that class's package named END. This works
just like the END function in traditional modules, meaning
that it gets called whenever your program exits unless it
execs or dies of an uncaught signal. For example,
sub END {
if ($Debugging) {
print "All persons are going away now.\n";
}
}
When the program exits, all the class destructors (END func-
tions) are be called in the opposite order that they were
loaded in (LIFO order).
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Documenting the Interface
And there you have it: we've just shown you the implementa-
tion of this Person class. Its interface would be its docu-
mentation. Usually this means putting it in pod ("plain old
documentation") format right there in the same file. In our
Person example, we would place the following docs anywhere
in the Person.pm file. Even though it looks mostly like
code, it's not. It's embedded documentation such as would
be used by the pod2man, pod2html, or pod2text programs. The
Perl compiler ignores pods entirely, just as the translators
ignore code. Here's an example of some pods describing the
informal interface:
=head1 NAME
Person - class to implement people
=head1 SYNOPSIS
use Person;
#################
# class methods #
#################
$ob = Person->new;
$count = Person->population;
#######################
# object data methods #
#######################
### get versions ###
$who = $ob->name;
$years = $ob->age;
@pals = $ob->peers;
### set versions ###
$ob->name("Jason");
$ob->age(23);
$ob->peers( "Norbert", "Rhys", "Phineas" );
########################
# other object methods #
########################
$phrase = $ob->exclaim;
$ob->happy_birthday;
=head1 DESCRIPTION
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The Person class implements dah dee dah dee dah....
That's all there is to the matter of interface versus imple-
mentation. A programmer who opens up the module and plays
around with all the private little shiny bits that were
safely locked up behind the interface contract has voided
the warranty, and you shouldn't worry about their fate.
Aggregation
Suppose you later want to change the class to implement
better names. Perhaps you'd like to support both given names
(called Christian names, irrespective of one's religion) and
family names (called surnames), plus nicknames and titles.
If users of your Person class have been properly accessing
it through its documented interface, then you can easily
change the underlying implementation. If they haven't, then
they lose and it's their fault for breaking the contract and
voiding their warranty.
To do this, we'll make another class, this one called
Fullname. What's the Fullname class look like? To answer
that question, you have to first figure out how you want to
use it. How about we use it this way:
$him = Person->new();
$him->fullname->title("St");
$him->fullname->christian("Thomas");
$him->fullname->surname("Aquinas");
$him->fullname->nickname("Tommy");
printf "His normal name is %s\n", $him->name;
printf "But his real name is %s\n", $him->fullname->as_string;
Ok. To do this, we'll change Person::new() so that it sup-
ports a full name field this way:
sub new {
my $class = shift;
my $self = {};
$self->{FULLNAME} = Fullname->new();
$self->{AGE} = undef;
$self->{PEERS} = [];
$self->{"_CENSUS"} = \$Census;
bless ($self, $class);
++ ${ $self->{"_CENSUS"} };
return $self;
}
sub fullname {
my $self = shift;
return $self->{FULLNAME};
}
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Then to support old code, define Person::name() this way:
sub name {
my $self = shift;
return $self->{FULLNAME}->nickname(@_)
|| $self->{FULLNAME}->christian(@_);
}
Here's the Fullname class. We'll use the same technique of
using a hash reference to hold data fields, and methods by
the appropriate name to access them:
package Fullname;
use strict;
sub new {
my $class = shift;
my $self = {
TITLE => undef,
CHRISTIAN => undef,
SURNAME => undef,
NICK => undef,
};
bless ($self, $class);
return $self;
}
sub christian {
my $self = shift;
if (@_) { $self->{CHRISTIAN} = shift }
return $self->{CHRISTIAN};
}
sub surname {
my $self = shift;
if (@_) { $self->{SURNAME} = shift }
return $self->{SURNAME};
}
sub nickname {
my $self = shift;
if (@_) { $self->{NICK} = shift }
return $self->{NICK};
}
sub title {
my $self = shift;
if (@_) { $self->{TITLE} = shift }
return $self->{TITLE};
}
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sub as_string {
my $self = shift;
my $name = join(" ", @$self{'CHRISTIAN', 'SURNAME'});
if ($self->{TITLE}) {
$name = $self->{TITLE} . " " . $name;
}
return $name;
}
1;
Finally, here's the test program:
#!/usr/bin/perl -w
use strict;
use Person;
sub END { show_census() }
sub show_census () {
printf "Current population: %d\n", Person->population;
}
Person->debug(1);
show_census();
my $him = Person->new();
$him->fullname->christian("Thomas");
$him->fullname->surname("Aquinas");
$him->fullname->nickname("Tommy");
$him->fullname->title("St");
$him->age(1);
printf "%s is really %s.\n", $him->name, $him->fullname->as_string;
printf "%s's age: %d.\n", $him->name, $him->age;
$him->happy_birthday;
printf "%s's age: %d.\n", $him->name, $him->age;
show_census();
Inheritance
Object-oriented programming systems all support some notion
of inheritance. Inheritance means allowing one class to
piggy-back on top of another one so you don't have to write
the same code again and again. It's about software reuse,
and therefore related to Laziness, the principal virtue of a
programmer. (The import/export mechanisms in traditional
modules are also a form of code reuse, but a simpler one
than the true inheritance that you find in object modules.)
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Sometimes the syntax of inheritance is built into the core
of the language, and sometimes it's not. Perl has no spe-
cial syntax for specifying the class (or classes) to inherit
from. Instead, it's all strictly in the semantics. Each
package can have a variable called @ISA, which governs
(method) inheritance. If you try to call a method on an
object or class, and that method is not found in that
object's package, Perl then looks to @ISA for other packages
to go looking through in search of the missing method.
Like the special per-package variables recognized by
Exporter (such as @EXPORT, @EXPORT_OK, @EXPORT_FAIL,
%EXPORT_TAGS, and $VERSION), the @ISA array must be a
package-scoped global and not a file-scoped lexical created
via my(). Most classes have just one item in their @ISA
array. In this case, we have what's called "single inheri-
tance", or SI for short.
Consider this class:
package Employee;
use Person;
@ISA = ("Person");
1;
Not a lot to it, eh? All it's doing so far is loading in
another class and stating that this one will inherit methods
from that other class if need be. We have given it none of
its own methods. We rely upon an Employee to behave just
like a Person.
Setting up an empty class like this is called the "empty
subclass test"; that is, making a derived class that does
nothing but inherit from a base class. If the original base
class has been designed properly, then the new derived class
can be used as a drop-in replacement for the old one. This
means you should be able to write a program like this:
use Employee;
my $empl = Employee->new();
$empl->name("Jason");
$empl->age(23);
printf "%s is age %d.\n", $empl->name, $empl->age;
By proper design, we mean always using the two-argument form
of bless(), avoiding direct access of global data, and not
exporting anything. If you look back at the Person::new()
function we defined above, we were careful to do that.
There's a bit of package data used in the constructor, but
the reference to this is stored on the object itself and all
other methods access package data via that reference, so we
should be ok.
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What do we mean by the Person::new() function -- isn't that
actually a method? Well, in principle, yes. A method is
just a function that expects as its first argument a class
name (package) or object (blessed reference).
Person::new() is the function that both the "Person->new()"
method and the "Employee->new()" method end up calling.
Understand that while a method call looks a lot like a func-
tion call, they aren't really quite the same, and if you
treat them as the same, you'll very soon be left with noth-
ing but broken programs. First, the actual underlying cal-
ling conventions are different: method calls get an extra
argument. Second, function calls don't do inheritance, but
methods do.
Method Call Resulting Function Call
-----------------------------------
Person->new() Person::new("Person")
Employee->new() Person::new("Employee")
So don't use function calls when you mean to call a method.
If an employee is just a Person, that's not all too very
interesting. So let's add some other methods. We'll give
our employee data fields to access their salary, their
employee ID, and their start date.
If you're getting a little tired of creating all these
nearly identical methods just to get at the object's data,
do not despair. Later, we'll describe several different
convenience mechanisms for shortening this up. Meanwhile,
here's the straight-forward way:
sub salary {
my $self = shift;
if (@_) { $self->{SALARY} = shift }
return $self->{SALARY};
}
sub id_number {
my $self = shift;
if (@_) { $self->{ID} = shift }
return $self->{ID};
}
sub start_date {
my $self = shift;
if (@_) { $self->{START_DATE} = shift }
return $self->{START_DATE};
}
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Overridden Methods
What happens when both a derived class and its base class
have the same method defined? Well, then you get the
derived class's version of that method. For example, let's
say that we want the peers() method called on an employee to
act a bit differently. Instead of just returning the list
of peer names, let's return slightly different strings. So
doing this:
$empl->peers("Peter", "Paul", "Mary");
printf "His peers are: %s\n", join(", ", $empl->peers);
will produce:
His peers are: PEON=PETER, PEON=PAUL, PEON=MARY
To do this, merely add this definition into the Employee.pm
file:
sub peers {
my $self = shift;
if (@_) { @{ $self->{PEERS} } = @_ }
return map { "PEON=\U$_" } @{ $self->{PEERS} };
}
There, we've just demonstrated the high-falutin' concept
known in certain circles as polymorphism. We've taken on
the form and behaviour of an existing object, and then we've
altered it to suit our own purposes. This is a form of Lazi-
ness. (Getting polymorphed is also what happens when the
wizard decides you'd look better as a frog.)
Every now and then you'll want to have a method call trigger
both its derived class (also known as "subclass") version as
well as its base class (also known as "superclass") version.
In practice, constructors and destructors are likely to want
to do this, and it probably also makes sense in the debug()
method we showed previously.
To do this, add this to Employee.pm:
use Carp;
my $Debugging = 0;
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sub debug {
my $self = shift;
confess "usage: thing->debug(level)" unless @_ == 1;
my $level = shift;
if (ref($self)) {
$self->{"_DEBUG"} = $level;
} else {
$Debugging = $level; # whole class
}
Person::debug($self, $Debugging); # don't really do this
}
As you see, we turn around and call the Person package's
debug() function. But this is far too fragile for good
design. What if Person doesn't have a debug() function, but
is inheriting its debug() method from elsewhere? It would
have been slightly better to say
Person->debug($Debugging);
But even that's got too much hard-coded. It's somewhat
better to say
$self->Person::debug($Debugging);
Which is a funny way to say to start looking for a debug()
method up in Person. This strategy is more often seen on
overridden object methods than on overridden class methods.
There is still something a bit off here. We've hard-coded
our superclass's name. This in particular is bad if you
change which classes you inherit from, or add others. For-
tunately, the pseudoclass SUPER comes to the rescue here.
$self->SUPER::debug($Debugging);
This way it starts looking in my class's @ISA. This only
makes sense from within a method call, though. Don't try to
access anything in SUPER:: from anywhere else, because it
doesn't exist outside an overridden method call. Note that
"SUPER" refers to the superclass of the current package, not
to the superclass of $self.
Things are getting a bit complicated here. Have we done
anything we shouldn't? As before, one way to test whether
we're designing a decent class is via the empty subclass
test. Since we already have an Employee class that we're
trying to check, we'd better get a new empty subclass that
can derive from Employee. Here's one:
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package Boss;
use Employee; # :-)
@ISA = qw(Employee);
And here's the test program:
#!/usr/bin/perl -w
use strict;
use Boss;
Boss->debug(1);
my $boss = Boss->new();
$boss->fullname->title("Don");
$boss->fullname->surname("Pichon Alvarez");
$boss->fullname->christian("Federico Jesus");
$boss->fullname->nickname("Fred");
$boss->age(47);
$boss->peers("Frank", "Felipe", "Faust");
printf "%s is age %d.\n", $boss->fullname->as_string, $boss->age;
printf "His peers are: %s\n", join(", ", $boss->peers);
Running it, we see that we're still ok. If you'd like to
dump out your object in a nice format, somewhat like the way
the 'x' command works in the debugger, you could use the
Data::Dumper module from CPAN this way:
use Data::Dumper;
print "Here's the boss:\n";
print Dumper($boss);
Which shows us something like this:
Here's the boss:
$VAR1 = bless( {
_CENSUS => \1,
FULLNAME => bless( {
TITLE => 'Don',
SURNAME => 'Pichon Alvarez',
NICK => 'Fred',
CHRISTIAN => 'Federico Jesus'
}, 'Fullname' ),
AGE => 47,
PEERS => [
'Frank',
'Felipe',
'Faust'
]
}, 'Boss' );
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Hm.... something's missing there. What about the salary,
start date, and ID fields? Well, we never set them to any-
thing, even undef, so they don't show up in the hash's keys.
The Employee class has no new() method of its own, and the
new() method in Person doesn't know about Employees. (Nor
should it: proper OO design dictates that a subclass be
allowed to know about its immediate superclass, but never
vice-versa.) So let's fix up Employee::new() this way:
sub new {
my $class = shift;
my $self = $class->SUPER::new();
$self->{SALARY} = undef;
$self->{ID} = undef;
$self->{START_DATE} = undef;
bless ($self, $class); # reconsecrate
return $self;
}
Now if you dump out an Employee or Boss object, you'll find
that new fields show up there now.
Multiple Inheritance
Ok, at the risk of confusing beginners and annoying OO
gurus, it's time to confess that Perl's object system
includes that controversial notion known as multiple inheri-
tance, or MI for short. All this means is that rather than
having just one parent class who in turn might itself have a
parent class, etc., that you can directly inherit from two
or more parents. It's true that some uses of MI can get you
into trouble, although hopefully not quite so much trouble
with Perl as with dubiously-OO languages like C++.
The way it works is actually pretty simple: just put more
than one package name in your @ISA array. When it comes
time for Perl to go finding methods for your object, it
looks at each of these packages in order. Well, kinda. It's
actually a fully recursive, depth-first order. Consider a
bunch of @ISA arrays like this:
@First::ISA = qw( Alpha );
@Second::ISA = qw( Beta );
@Third::ISA = qw( First Second );
If you have an object of class Third:
my $ob = Third->new();
$ob->spin();
How do we find a spin() method (or a new() method for that
matter)? Because the search is depth-first, classes will be
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looked up in the following order: Third, First, Alpha,
Second, and Beta.
In practice, few class modules have been seen that actually
make use of MI. One nearly always chooses simple container-
ship of one class within another over MI. That's why our
Person object contained a Fullname object. That doesn't
mean it was one.
However, there is one particular area where MI in Perl is
rampant: borrowing another class's class methods. This is
rather common, especially with some bundled "objectless"
classes, like Exporter, DynaLoader, AutoLoader, and Sel-
fLoader. These classes do not provide constructors; they
exist only so you may inherit their class methods. (It's
not entirely clear why inheritance was done here rather than
traditional module importation.)
For example, here is the POSIX module's @ISA:
package POSIX;
@ISA = qw(Exporter DynaLoader);
The POSIX module isn't really an object module, but then,
neither are Exporter or DynaLoader. They're just lending
their classes' behaviours to POSIX.
Why don't people use MI for object methods much? One reason
is that it can have complicated side-effects. For one
thing, your inheritance graph (no longer a tree) might con-
verge back to the same base class. Although Perl guards
against recursive inheritance, merely having parents who are
related to each other via a common ancestor, incestuous
though it sounds, is not forbidden. What if in our Third
class shown above we wanted its new() method to also call
both overridden constructors in its two parent classes? The
SUPER notation would only find the first one. Also, what
about if the Alpha and Beta classes both had a common ances-
tor, like Nought? If you kept climbing up the inheritance
tree calling overridden methods, you'd end up calling
Nought::new() twice, which might well be a bad idea.
UNIVERSAL: The Root of All Objects
Wouldn't it be convenient if all objects were rooted at some
ultimate base class? That way you could give every object
common methods without having to go and add it to each and
every @ISA. Well, it turns out that you can. You don't see
it, but Perl tacitly and irrevocably assumes that there's an
extra element at the end of @ISA: the class UNIVERSAL. In
version 5.003, there were no predefined methods there, but
you could put whatever you felt like into it.
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However, as of version 5.004 (or some subversive releases,
like 5.003_08), UNIVERSAL has some methods in it already.
These are builtin to your Perl binary, so they don't take
any extra time to load. Predefined methods include isa(),
can(), and VERSION(). isa() tells you whether an object or
class "is" another one without having to traverse the
hierarchy yourself:
$has_io = $fd->isa("IO::Handle");
$itza_handle = IO::Socket->isa("IO::Handle");
The can() method, called against that object or class,
reports back whether its string argument is a callable
method name in that class. In fact, it gives you back a
function reference to that method:
$his_print_method = $obj->can('as_string');
Finally, the VERSION method checks whether the class (or the
object's class) has a package global called $VERSION that's
high enough, as in:
Some_Module->VERSION(3.0);
$his_vers = $ob->VERSION();
However, we don't usually call VERSION ourselves. (Remember
that an all uppercase function name is a Perl convention
that indicates that the function will be automatically used
by Perl in some way.) In this case, it happens when you say
use Some_Module 3.0;
If you wanted to add version checking to your Person class
explained above, just add this to Person.pm:
our $VERSION = '1.1';
and then in Employee.pm you can say
use Person 1.1;
And it would make sure that you have at least that version
number or higher available. This is not the same as load-
ing in that exact version number. No mechanism currently
exists for concurrent installation of multiple versions of a
module. Lamentably.
Alternate Object Representations
Nothing requires objects to be implemented as hash refer-
ences. An object can be any sort of reference so long as
its referent has been suitably blessed. That means scalar,
array, and code references are also fair game.
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A scalar would work if the object has only one datum to
hold. An array would work for most cases, but makes inheri-
tance a bit dodgy because you have to invent new indices for
the derived classes.
Arrays as Objects
If the user of your class honors the contract and sticks to
the advertised interface, then you can change its underlying
interface if you feel like it. Here's another implementa-
tion that conforms to the same interface specification.
This time we'll use an array reference instead of a hash
reference to represent the object.
package Person;
use strict;
my($NAME, $AGE, $PEERS) = ( 0 .. 2 );
############################################
## the object constructor (array version) ##
############################################
sub new {
my $self = [];
$self->[$NAME] = undef; # this is unnecessary
$self->[$AGE] = undef; # as is this
$self->[$PEERS] = []; # but this isn't, really
bless($self);
return $self;
}
sub name {
my $self = shift;
if (@_) { $self->[$NAME] = shift }
return $self->[$NAME];
}
sub age {
my $self = shift;
if (@_) { $self->[$AGE] = shift }
return $self->[$AGE];
}
sub peers {
my $self = shift;
if (@_) { @{ $self->[$PEERS] } = @_ }
return @{ $self->[$PEERS] };
}
1; # so the require or use succeeds
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You might guess that the array access would be a lot faster
than the hash access, but they're actually comparable. The
array is a little bit faster, but not more than ten or fif-
teen percent, even when you replace the variables above like
$AGE with literal numbers, like 1. A bigger difference
between the two approaches can be found in memory use. A
hash representation takes up more memory than an array
representation because you have to allocate memory for the
keys as well as for the values. However, it really isn't
that bad, especially since as of version 5.004, memory is
only allocated once for a given hash key, no matter how many
hashes have that key. It's expected that sometime in the
future, even these differences will fade into obscurity as
more efficient underlying representations are devised.
Still, the tiny edge in speed (and somewhat larger one in
memory) is enough to make some programmers choose an array
representation for simple classes. There's still a little
problem with scalability, though, because later in life when
you feel like creating subclasses, you'll find that hashes
just work out better.
Closures as Objects
Using a code reference to represent an object offers some
fascinating possibilities. We can create a new anonymous
function (closure) who alone in all the world can see the
object's data. This is because we put the data into an
anonymous hash that's lexically visible only to the closure
we create, bless, and return as the object. This object's
methods turn around and call the closure as a regular sub-
routine call, passing it the field we want to affect. (Yes,
the double-function call is slow, but if you wanted fast,
you wouldn't be using objects at all, eh? :-)
Use would be similar to before:
use Person;
$him = Person->new();
$him->name("Jason");
$him->age(23);
$him->peers( [ "Norbert", "Rhys", "Phineas" ] );
printf "%s is %d years old.\n", $him->name, $him->age;
print "His peers are: ", join(", ", @{$him->peers}), "\n";
but the implementation would be radically, perhaps even sub-
limely different:
package Person;
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sub new {
my $class = shift;
my $self = {
NAME => undef,
AGE => undef,
PEERS => [],
};
my $closure = sub {
my $field = shift;
if (@_) { $self->{$field} = shift }
return $self->{$field};
};
bless($closure, $class);
return $closure;
}
sub name { &{ $_[0] }("NAME", @_[ 1 .. $#_ ] ) }
sub age { &{ $_[0] }("AGE", @_[ 1 .. $#_ ] ) }
sub peers { &{ $_[0] }("PEERS", @_[ 1 .. $#_ ] ) }
1;
Because this object is hidden behind a code reference, it's
probably a bit mysterious to those whose background is more
firmly rooted in standard procedural or object-based pro-
gramming languages than in functional programming languages
whence closures derive. The object created and returned by
the new() method is itself not a data reference as we've
seen before. It's an anonymous code reference that has
within it access to a specific version (lexical binding and
instantiation) of the object's data, which are stored in the
private variable $self. Although this is the same function
each time, it contains a different version of $self.
When a method like "$him->name("Jason")" is called, its
implicit zeroth argument is the invoking object--just as it
is with all method calls. But in this case, it's our code
reference (something like a function pointer in C++, but
with deep binding of lexical variables). There's not a lot
to be done with a code reference beyond calling it, so
that's just what we do when we say "&{$_[0]}". This is just
a regular function call, not a method call. The initial
argument is the string "NAME", and any remaining arguments
are whatever had been passed to the method itself.
Once we're executing inside the closure that had been
created in new(), the $self hash reference suddenly becomes
visible. The closure grabs its first argument ("NAME" in
this case because that's what the name() method passed it),
and uses that string to subscript into the private hash hid-
den in its unique version of $self.
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Nothing under the sun will allow anyone outside the execut-
ing method to be able to get at this hidden data. Well,
nearly nothing. You could single step through the program
using the debugger and find out the pieces while you're in
the method, but everyone else is out of luck.
There, if that doesn't excite the Scheme folks, then I just
don't know what will. Translation of this technique into
C++, Java, or any other braindead-static language is left as
a futile exercise for aficionados of those camps.
You could even add a bit of nosiness via the caller() func-
tion and make the closure refuse to operate unless called
via its own package. This would no doubt satisfy certain
fastidious concerns of programming police and related puri-
tans.
If you were wondering when Hubris, the third principle vir-
tue of a programmer, would come into play, here you have it.
(More seriously, Hubris is just the pride in craftsmanship
that comes from having written a sound bit of well-designed
code.)
AUTOLOAD: Proxy Methods
Autoloading is a way to intercept calls to undefined
methods. An autoload routine may choose to create a new
function on the fly, either loaded from disk or perhaps just
eval()ed right there. This define-on-the-fly strategy is
why it's called autoloading.
But that's only one possible approach. Another one is to
just have the autoloaded method itself directly provide the
requested service. When used in this way, you may think of
autoloaded methods as "proxy" methods.
When Perl tries to call an undefined function in a particu-
lar package and that function is not defined, it looks for a
function in that same package called AUTOLOAD. If one
exists, it's called with the same arguments as the original
function would have had. The fully-qualified name of the
function is stored in that package's global variable $AUTO-
LOAD. Once called, the function can do anything it would
like, including defining a new function by the right name,
and then doing a really fancy kind of "goto" right to it,
erasing itself from the call stack.
What does this have to do with objects? After all, we keep
talking about functions, not methods. Well, since a method
is just a function with an extra argument and some fancier
semantics about where it's found, we can use autoloading for
methods, too. Perl doesn't start looking for an AUTOLOAD
method until it has exhausted the recursive hunt up through
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@ISA, though. Some programmers have even been known to
define a UNIVERSAL::AUTOLOAD method to trap unresolved
method calls to any kind of object.
Autoloaded Data Methods
You probably began to get a little suspicious about the
duplicated code way back earlier when we first showed you
the Person class, and then later the Employee class. Each
method used to access the hash fields looked virtually
identical. This should have tickled that great programming
virtue, Impatience, but for the time, we let Laziness win
out, and so did nothing. Proxy methods can cure this.
Instead of writing a new function every time we want a new
data field, we'll use the autoload mechanism to generate
(actually, mimic) methods on the fly. To verify that we're
accessing a valid member, we will check against an "_permit-
ted" (pronounced "under-permitted") field, which is a refer-
ence to a file-scoped lexical (like a C file static) hash of
permitted fields in this record called %fields. Why the
underscore? For the same reason as the _CENSUS field we
once used: as a marker that means "for internal use only".
Here's what the module initialization code and class con-
structor will look like when taking this approach:
package Person;
use Carp;
our $AUTOLOAD; # it's a package global
my %fields = (
name => undef,
age => undef,
peers => undef,
);
sub new {
my $class = shift;
my $self = {
_permitted => \%fields,
%fields,
};
bless $self, $class;
return $self;
}
If we wanted our record to have default values, we could
fill those in where current we have "undef" in the %fields
hash.
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Notice how we saved a reference to our class data on the
object itself? Remember that it's important to access class
data through the object itself instead of having any method
reference %fields directly, or else you won't have a decent
inheritance.
The real magic, though, is going to reside in our proxy
method, which will handle all calls to undefined methods for
objects of class Person (or subclasses of Person). It has
to be called AUTOLOAD. Again, it's all caps because it's
called for us implicitly by Perl itself, not by a user
directly.
sub AUTOLOAD {
my $self = shift;
my $type = ref($self)
or croak "$self is not an object";
my $name = $AUTOLOAD;
$name =~ s/.*://; # strip fully-qualified portion
unless (exists $self->{_permitted}->{$name} ) {
croak "Can't access `$name' field in class $type";
}
if (@_) {
return $self->{$name} = shift;
} else {
return $self->{$name};
}
}
Pretty nifty, eh? All we have to do to add new data fields
is modify %fields. No new functions need be written.
I could have avoided the "_permitted" field entirely, but I
wanted to demonstrate how to store a reference to class data
on the object so you wouldn't have to access that class data
directly from an object method.
Inherited Autoloaded Data Methods
But what about inheritance? Can we define our Employee
class similarly? Yes, so long as we're careful enough.
Here's how to be careful:
package Employee;
use Person;
use strict;
our @ISA = qw(Person);
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my %fields = (
id => undef,
salary => undef,
);
sub new {
my $class = shift;
my $self = $class->SUPER::new();
my($element);
foreach $element (keys %fields) {
$self->{_permitted}->{$element} = $fields{$element};
}
@{$self}{keys %fields} = values %fields;
return $self;
}
Once we've done this, we don't even need to have an AUTOLOAD
function in the Employee package, because we'll grab
Person's version of that via inheritance, and it will all
work out just fine.
Metaclassical Tools
Even though proxy methods can provide a more convenient
approach to making more struct-like classes than tediously
coding up data methods as functions, it still leaves a bit
to be desired. For one thing, it means you have to handle
bogus calls that you don't mean to trap via your proxy. It
also means you have to be quite careful when dealing with
inheritance, as detailed above.
Perl programmers have responded to this by creating several
different class construction classes. These metaclasses are
classes that create other classes. A couple worth looking
at are Class::Struct and Alias. These and other related
metaclasses can be found in the modules directory on CPAN.
Class::Struct
One of the older ones is Class::Struct. In fact, its syntax
and interface were sketched out long before perl5 even soli-
dified into a real thing. What it does is provide you a way
to "declare" a class as having objects whose fields are of a
specific type. The function that does this is called, not
surprisingly enough, struct(). Because structures or
records are not base types in Perl, each time you want to
create a class to provide a record-like data object, you
yourself have to define a new() method, plus separate data-
access methods for each of that record's fields. You'll
quickly become bored with this process. The
Class::Struct::struct() function alleviates this tedium.
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Here's a simple example of using it:
use Class::Struct qw(struct);
use Jobbie; # user-defined; see below
struct 'Fred' => {
one => '$',
many => '@',
profession => 'Jobbie', # does not call Jobbie->new()
};
$ob = Fred->new(profession => Jobbie->new());
$ob->one("hmmmm");
$ob->many(0, "here");
$ob->many(1, "you");
$ob->many(2, "go");
print "Just set: ", $ob->many(2), "\n";
$ob->profession->salary(10_000);
You can declare types in the struct to be basic Perl types,
or user-defined types (classes). User types will be ini-
tialized by calling that class's new() method.
Take care that the "Jobbie" object is not created automati-
cally by the "Fred" class's new() method, so you should
specify a "Jobbie" object when you create an instance of
"Fred".
Here's a real-world example of using struct generation.
Let's say you wanted to override Perl's idea of gethost-
byname() and gethostbyaddr() so that they would return
objects that acted like C structures. We don't care about
high-falutin' OO gunk. All we want is for these objects to
act like structs in the C sense.
use Socket;
use Net::hostent;
$h = gethostbyname("perl.com"); # object return
printf "perl.com's real name is %s, address %s\n",
$h->name, inet_ntoa($h->addr);
Here's how to do this using the Class::Struct module. The
crux is going to be this call:
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struct 'Net::hostent' => [ # note bracket
name => '$',
aliases => '@',
addrtype => '$',
'length' => '$',
addr_list => '@',
];
Which creates object methods of those names and types. It
even creates a new() method for us.
We could also have implemented our object this way:
struct 'Net::hostent' => { # note brace
name => '$',
aliases => '@',
addrtype => '$',
'length' => '$',
addr_list => '@',
};
and then Class::Struct would have used an anonymous hash as
the object type, instead of an anonymous array. The array
is faster and smaller, but the hash works out better if you
eventually want to do inheritance. Since for this struct-
like object we aren't planning on inheritance, this time
we'll opt for better speed and size over better flexibility.
Here's the whole implementation:
package Net::hostent;
use strict;
BEGIN {
use Exporter ();
our @EXPORT = qw(gethostbyname gethostbyaddr gethost);
our @EXPORT_OK = qw(
$h_name @h_aliases
$h_addrtype $h_length
@h_addr_list $h_addr
);
our %EXPORT_TAGS = ( FIELDS => [ @EXPORT_OK, @EXPORT ] );
}
our @EXPORT_OK;
# Class::Struct forbids use of @ISA
sub import { goto &Exporter::import }
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use Class::Struct qw(struct);
struct 'Net::hostent' => [
name => '$',
aliases => '@',
addrtype => '$',
'length' => '$',
addr_list => '@',
];
sub addr { shift->addr_list->[0] }
sub populate (@) {
return unless @_;
my $hob = new(); # Class::Struct made this!
$h_name = $hob->[0] = $_[0];
@h_aliases = @{ $hob->[1] } = split ' ', $_[1];
$h_addrtype = $hob->[2] = $_[2];
$h_length = $hob->[3] = $_[3];
$h_addr = $_[4];
@h_addr_list = @{ $hob->[4] } = @_[ (4 .. $#_) ];
return $hob;
}
sub gethostbyname ($) { populate(CORE::gethostbyname(shift)) }
sub gethostbyaddr ($;$) {
my ($addr, $addrtype);
$addr = shift;
require Socket unless @_;
$addrtype = @_ ? shift : Socket::AF_INET();
populate(CORE::gethostbyaddr($addr, $addrtype))
}
sub gethost($) {
if ($_[0] =~ /^\d+(?:\.\d+(?:\.\d+(?:\.\d+)?)?)?$/) {
require Socket;
&gethostbyaddr(Socket::inet_aton(shift));
} else {
&gethostbyname;
}
}
1;
We've snuck in quite a fair bit of other concepts besides
just dynamic class creation, like overriding core functions,
import/export bits, function prototyping, short-cut function
call via &whatever, and function replacement with "goto
&whatever". These all mostly make sense from the perspec-
tive of a traditional module, but as you can see, we can
also use them in an object module.
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You can look at other object-based, struct-like overrides of
core functions in the 5.004 release of Perl in File::stat,
Net::hostent, Net::netent, Net::protoent, Net::servent,
Time::gmtime, Time::localtime, User::grent, and User::pwent.
These modules have a final component that's all lowercase,
by convention reserved for compiler pragmas, because they
affect the compilation and change a builtin function. They
also have the type names that a C programmer would most
expect.
Data Members as Variables
If you're used to C++ objects, then you're accustomed to
being able to get at an object's data members as simple
variables from within a method. The Alias module provides
for this, as well as a good bit more, such as the possibil-
ity of private methods that the object can call but folks
outside the class cannot.
Here's an example of creating a Person using the Alias
module. When you update these magical instance variables,
you automatically update value fields in the hash. Con-
venient, eh?
package Person;
# this is the same as before...
sub new {
my $class = shift;
my $self = {
NAME => undef,
AGE => undef,
PEERS => [],
};
bless($self, $class);
return $self;
}
use Alias qw(attr);
our ($NAME, $AGE, $PEERS);
sub name {
my $self = attr shift;
if (@_) { $NAME = shift; }
return $NAME;
}
sub age {
my $self = attr shift;
if (@_) { $AGE = shift; }
return $AGE;
}
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sub peers {
my $self = attr shift;
if (@_) { @PEERS = @_; }
return @PEERS;
}
sub exclaim {
my $self = attr shift;
return sprintf "Hi, I'm %s, age %d, working with %s",
$NAME, $AGE, join(", ", @PEERS);
}
sub happy_birthday {
my $self = attr shift;
return ++$AGE;
}
The need for the "our" declaration is because what Alias
does is play with package globals with the same name as the
fields. To use globals while "use strict" is in effect, you
have to predeclare them. These package variables are local-
ized to the block enclosing the attr() call just as if you'd
used a local() on them. However, that means that they're
still considered global variables with temporary values,
just as with any other local().
It would be nice to combine Alias with something like
Class::Struct or Class::MethodMaker.
NOTES
Object Terminology
In the various OO literature, it seems that a lot of dif-
ferent words are used to describe only a few different con-
cepts. If you're not already an object programmer, then you
don't need to worry about all these fancy words. But if you
are, then you might like to know how to get at the same con-
cepts in Perl.
For example, it's common to call an object an instance of a
class and to call those objects' methods instance methods.
Data fields peculiar to each object are often called
instance data or object attributes, and data fields common
to all members of that class are class data, class attri-
butes, or static data members.
Also, base class, generic class, and superclass all describe
the same notion, whereas derived class, specific class, and
subclass describe the other related one.
C++ programmers have static methods and virtual methods, but
Perl only has class methods and object methods. Actually,
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Perl only has methods. Whether a method gets used as a
class or object method is by usage only. You could acciden-
tally call a class method (one expecting a string argument)
on an object (one expecting a reference), or vice versa.
From the C++ perspective, all methods in Perl are virtual.
This, by the way, is why they are never checked for function
prototypes in the argument list as regular builtin and
user-defined functions can be.
Because a class is itself something of an object, Perl's
classes can be taken as describing both a "class as
meta-object" (also called object factory) philosophy and the
"class as type definition" (declaring behaviour, not defin-
ing mechanism) idea. C++ supports the latter notion, but
not the former.
SEE ALSO
The following manpages will doubtless provide more back-
ground for this one: perlmod, perlref, perlobj, perlbot,
perltie, and overload.
perlboot is a kinder, gentler introduction to object-
oriented programming.
perltooc provides more detail on class data.
Some modules which might prove interesting are
Class::Accessor, Class::Class, Class::Contract,
Class::Data::Inheritable, Class::MethodMaker and
Tie::SecureHash
AUTHOR AND COPYRIGHT
Copyright (c) 1997, 1998 Tom Christiansen All rights
reserved.
This documentation is free; you can redistribute it and/or
modify it under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this
file are hereby placed into the public domain. You are per-
mitted and encouraged to use this code in your own programs
for fun or for profit as you see fit. A simple comment in
the code giving credit would be courteous but is not
required.
COPYRIGHT
Acknowledgments
Thanks to Larry Wall, Roderick Schertler, Gurusamy Sarathy,
Dean Roehrich, Raphael Manfredi, Brent Halsey, Greg Bacon,
Brad Appleton, and many others for their helpful comments.
perl v5.8.8 2006-06-30 37
