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NAME    [Toc]    [Back]

       perltoot - Tom's object-oriented tutorial for perl

DESCRIPTION    [Toc]    [Back]

       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.

Creating a Class    [Toc]    [Back]

       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

           use Person;

           $him = Person->new();
           $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

       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
                   $self->{NAME},   $self->{AGE},   join(",    ",

       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
                   $self->name,    $self->age,     join(",     ",

           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    [Toc]    [Back]

       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

       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  = {};
               $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

           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

           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->peers( "Norbert", "Rhys", "Phineas" );

            # other object methods #

            $phrase = $ob->exclaim;

           =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

Aggregation    [Toc]    [Back]

       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();
           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;


       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;



           my $him = Person->new();

           printf "%s is  really  %s.0,  $him->name,  $him->fullname->as_string;
           printf "%s's age: %d.0, $him->name, $him->age;
           printf "%s's age: %d.0, $him->name, $him->age;


Inheritance    [Toc]    [Back]

       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

       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");

       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();
           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

       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


       But even that's got too much hard-coded.  It's somewhat
       better to say


       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


       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;

           my $boss = Boss->new();

           $boss->fullname->surname("Pichon Alvarez");
           $boss->fullname->christian("Federico Jesus");

           $boss->peers("Frank", "Felipe", "Faust");

           printf  "%s  is  age %d.0, $boss->fullname->as_string,
           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 => [
              }, '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();

       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:

           $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
               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->peers( [ "Norbert", "Rhys", "Phineas" ] );
           printf "%s is %d years old.0, $him->name, $him->age;
           print "His peers are: ", join(",  ",  @{$him->peers}),

       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 .. $#_ ] ) }


       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

       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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