perltoot - Tom's object-oriented tutorial for perl
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 regular 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, meaning 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 something 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). 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.
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
manipulate 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 directory 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 anything
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(). However,
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 [Toc] [Back]
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 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 distinction
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
programmers 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.
Generally 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 function name instead of brackets or braces.
Class Interface [Toc] [Back]
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.
Likewise, 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:
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.0, $him->name, $him->age;
print "His peers are: ", join(", ", $him->peers), "0;
printf "Last rec's name is %s0, $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 [Toc] [Back]
Still, someone has to know what's in the object. And that
someone is the class. It implements methods that the programmer
uses to access the object. Here's how to implement
the Person class using the standard hash-ref-as-anobject
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;
}
##############################################
## 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 appropriate
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.
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 [Toc] [Back]
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
destructors? 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
happening 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 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
admittedly error-prone, this is the best we can do right
now. Nonetheless, rest assured that when your program is
finished, its objects' destructors are all duly called.
So you are guaranteed that an object eventually gets properly
destroyed, except in the unique case of a program
that never exits. (If you're running Perl embedded in
another application, this full GC pass happens a bit more
frequently--whenever a thread shuts down.)
Other Object Methods [Toc] [Back]
The methods we've talked about so far have either been
constructors 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 convention,
not syntax. (Well, unless you use the Alias module
described below in "Data Members as Variables".)
Occasionally 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 sometimes to its closest acquaintances, its
immediate subclasses. 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}});
}
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.
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 conceivably 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
protective veil to separate interface from implementation.
So provide a class method to access class data just as you
provide object methods to access object data.
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 destructor?
That's something that Perl takes care of for you all
by itself.
Alternatively, you could use the Class::Data::Inheritable
module from CPAN.
Accessing Class Data [Toc] [Back]
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
original $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 programmer. It's really just a mnemonic device to
remind ourselves 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
$self->{"_CENSUS"} = ensus;
bless ($self, $class);
++ ${ $self->{"_CENSUS"} };
return $self;
}
sub population {
my $self = shift;
if (ref $self) {
return ${ $self->{"_CENSUS"} };
} else {
return $Census;
}
}
sub DESTROY {
my $self = shift;
-- ${ $self->{"_CENSUS"} };
}
Debugging Methods [Toc] [Back]
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
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,
manipulates $Person::Debugging not $Employee::Debugging.
Class Destructors [Toc] [Back]
The object destructor handles the death of each distinct
object. But sometimes you want a bit of cleanup when the
entire class is shut down, which currently only happens
when the program exits. To make such a class destructor,
create a function in that class's package named END. This
works just like the END function in traditional modules,
meaning that it gets called whenever your program exits
unless it execs or dies of an uncaught signal. For example,
sub END {
if ($Debugging) {
print "All persons are going away now.0;
}
}
When the program exits, all the class destructors (END
functions) are be called in the opposite order that they
were loaded in (LIFO order).
Documenting the Interface [Toc] [Back]
And there you have it: we've just shown you the implemen-
tation of this Person class. Its interface would be its
documentation. 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
The Person class implements dah dee dah dee dah....
That's all there is to the matter of interface versus
implementation. 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.
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 %s0, $him->name;
printf "But his real name is %s0, $him->fullname->as_string;
Ok. To do this, we'll change Person::new() so that it
supports a full name field this way:
sub new {
my $class = shift;
my $self = {};
$self->{FULLNAME} = Fullname->new();
$self->{AGE} = undef;
$self->{PEERS} = [];
$self->{"_CENSUS"} = ensus;
bless ($self, $class);
++ ${ $self->{"_CENSUS"} };
return $self;
}
sub fullname {
my $self = shift;
return $self->{FULLNAME};
}
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};
}
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: %d0, 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.0, $him->name, $him->fullname->as_string;
printf "%s's age: %d.0, $him->name, $him->age;
$him->happy_birthday;
printf "%s's age: %d.0, $him->name, $him->age;
show_census();
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.)
Sometimes the syntax of inheritance is built into the core
of the language, and sometimes it's not. Perl has no special
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
inheritance", 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.0, $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 Per-
son::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.
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 function call, they aren't really quite the same,
and if you treat them as the same, you'll very soon be
left with nothing but broken programs. First, the actual
underlying calling 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};
}
Overridden Methods [Toc] [Back]
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: %s0, 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 Laziness. (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;
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.
Fortunately, 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:
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.0, $boss->fullname->as_string,
$boss->age;
printf "His peers are: %s0, join(", ", $boss->peers);
Running it, we see that we're still ok. If you'd like to
dump out your object in a nice format, somewhat like the
way the 'x' command works in the debugger, you could use
the Data::Dumper module from CPAN this way:
use Data::Dumper;
print "Here's the boss:0;
print Dumper($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' );
Hm.... something's missing there. What about the salary,
start date, and ID fields? Well, we never set them to
anything, 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 [Toc] [Back]
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
inheritance, 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 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
containership 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 SelfLoader.
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
converge 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 ancestor, 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.
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 $VERSION = '1.1';
and then in Employee.pm you can say
use Person 1.1;
And it would make sure that you have at least that version
number or higher available. This is not the same as
loading in that exact version number. No mechanism currently
exists for concurrent installation of multiple versions
of a module. Lamentably.
Alternate Object Representations [Toc] [Back] Nothing requires objects to be implemented as hash references.
An object can be any sort of reference so long as
its referent has been suitably blessed. That means
scalar, array, and code references are also fair game.
A scalar would work if the object has only one datum to
hold. An array would work for most cases, but makes
inheritance a bit dodgy because you have to invent new
indices for the derived classes.
Arrays as Objects [Toc] [Back]
If the user of your class honors the contract and sticks
to the advertised interface, then you can change its
underlying interface if you feel like it. Here's another
implementation that conforms to the same interface specification.
This time we'll use an array reference instead
of a hash reference to represent the object.
package Person;
use strict;
my($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
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 fifteen 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 [Toc] [Back]
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 subroutine 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.0, $him->name, $him->age;
print "His peers are: ", join(", ", @{$him->peers}),
"0;
but the implementation would be radically, perhaps even
sublimely different:
package Person;
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 objectbased
programming 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 hidden in its unique version of $self.
Nothing under the sun will allow anyone outside the executing
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()
function 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 puritans.
If you were wondering when Hubris, the third pr
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