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NAME
       perltoot - Tom's object-oriented tutorial for perl

DESCRIPTION
       Object-oriented programming is a big seller these days.
       Some managers would rather have objects than sliced bread.
       Why is that?  What's so special about an object?  Just
       what is an object anyway?

       An object is nothing but a way of tucking away complex
       behaviours into a neat little easy-to-use bundle.  (This
       is what professors call abstraction.) Smart people who
       have nothing to do but sit around for weeks on end
       figuring out really hard problems make these nifty objects
       that even 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



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       (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
       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
       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



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       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
       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");



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           $him->age(23);
           $him->peers( "Norbert", "Rhys", "Phineas" );

           push @All_Recs, $him;  # save object in array for later

           printf "%s is %d years old.\n", $him->name, $him->age;
           print "His peers are: ", join(", ", $him->peers), "\n";

           printf "Last rec's name is %s\n", $All_Recs[-1]->name;

       As you can see, the user of the class doesn't know (or at
       least, has no business paying attention to the fact) that
       the object has one particular implementation or another.
       The interface to the class and its objects is exclusively
       via methods, and that's all the user of the class should
       ever play with.

   Constructors and Instance Methods
       Still, someone has to know what's in the object.  And that
       someone is the class.  It implements methods that the
       programmer uses to access the object.  Here's how to
       implement the Person class using the standard hash-ref-as-
       an-object idiom.  We'll make a class method called new()
       to act as the constructor, and three object methods called
       name(), age(), and peers() to get at per-object data
       hidden away in our anonymous hash.

           package Person;
           use strict;

           ##################################################
           ## the object constructor (simplistic version)  ##
           ##################################################
           sub new {
               my $self  = {};
               $self->{NAME}   = undef;
               $self->{AGE}    = undef;
               $self->{PEERS}  = [];
               bless($self);           # but see below
               return $self;
           }

           ##############################################
           ## 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};
           }



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           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
       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.



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       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
       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



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       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



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       not just the users of your class but even you yourself
       from changes in your data representation.

Class Data
       What about "class data", data items common to each object
       in a class?  What would you want that for?  Well, in your
       Person class, you might like to keep track of the total
       people alive.  How do you implement that?

       You could make it a global variable called
       $Person::Census.  But about only reason you'd do that
       would be if you wanted people to be able to get at your
       class data directly.  They could just say $Person::Census
       and play around with it.  Maybe this is ok in your design
       scheme.  You might even 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:






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           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
       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



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       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
               $self->{"_CENSUS"} = \$Census;
               bless ($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
       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.








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           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
       The object destructor handles the death of each distinct
       object.  But sometimes you want a bit of cleanup when the



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       entire class is shut down, which currently only happens
       when the program exits.  To make such a class destructor,
       create a function in that class's package named END.  This
       works just like the END function in traditional modules,
       meaning that it gets called whenever your program exits
       unless it execs or dies of an uncaught signal.  For
       example,

           sub END {
               if ($Debugging) {
                   print "All persons are going away now.\n";
               }
           }

       When the program exits, all the class destructors (END
       functions) are be called in the opposite order that they
       were loaded in (LIFO order).

   Documenting the Interface
       And there you have it: we've just shown you the
       implementation 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;



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            ### 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.

Aggregation
       Suppose you later want to change the class to implement
       better names.  Perhaps you'd like to support both given
       names (called Christian names, irrespective of one's
       religion) and family names (called surnames), plus
       nicknames and titles.  If users of your Person class have
       been properly accessing it through its documented
       interface, then you can easily change the underlying
       implementation.  If they haven't, then they lose and it's
       their fault for breaking the contract and voiding their
       warranty.

       To do this, we'll make another class, this one called
       Fullname.  What's the Fullname class look like?  To answer
       that question, you have to first figure out how you want
       to use it.  How about we use it this way:

           $him = Person->new();
           $him->fullname->title("St");
           $him->fullname->christian("Thomas");
           $him->fullname->surname("Aquinas");
           $him->fullname->nickname("Tommy");
           printf "His normal name is %s\n", $him->name;
           printf "But his real name is %s\n", $him->fullname->as_string;

       Ok.  To do this, we'll change Person::new() so that it
       supports a full name field this way:






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           sub new {
               my $class = shift;
               my $self  = {};
               $self->{FULLNAME} = Fullname->new();
               $self->{AGE}      = undef;
               $self->{PEERS}    = [];
               $self->{"_CENSUS"} = \$Census;
               bless ($self, $class);
               ++ ${ $self->{"_CENSUS"} };
               return $self;
           }

           sub fullname {
               my $self = shift;
               return $self->{FULLNAME};
           }

       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};



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           }

           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: %d\n", Person->population;
           }

           Person->debug(1);

           show_census();

           my $him = Person->new();

           $him->fullname->christian("Thomas");
           $him->fullname->surname("Aquinas");
           $him->fullname->nickname("Tommy");
           $him->fullname->title("St");
           $him->age(1);

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

           show_census();



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Inheritance
       Object-oriented programming systems all support some
       notion of inheritance.  Inheritance means allowing one
       class to piggy-back on top of another one so you don't
       have to write the same code again and again.  It's about
       software reuse, and therefore related to Laziness, the
       principal virtue of a programmer.  (The import/export
       mechanisms in traditional modules are also a form of code
       reuse, but a simpler one than the true inheritance that
       you find in object modules.)

       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:







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           use Employee;
           my $empl = Employee->new();
           $empl->name("Jason");
           $empl->age(23);
           printf "%s is age %d.\n", $empl->name, $empl->age;

       By proper design, we mean always using the two-argument
       form of bless(), avoiding direct access of global data,
       and not exporting anything.  If you look back at the
       Person::new() function we defined above, we were careful
       to do that.  There's a bit of package data used in the
       constructor, but the reference to this is stored on the
       object itself and all other methods access package data
       via that reference, so we should be ok.

       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};
           }




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           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
       What happens when both a derived class and its base class
       have the same method defined?  Well, then you get the
       derived class's version of that method.  For example,
       let's say that we want the peers() method called on an
       employee to act a bit differently.  Instead of just
       returning the list of peer names, let's return slightly
       different strings.  So doing this:

           $empl->peers("Peter", "Paul", "Mary");
           printf "His peers are: %s\n", join(", ", $empl->peers);

       will produce:

           His peers are: PEON=PETER, PEON=PAUL, PEON=MARY

       To do this, merely add this definition into the
       Employee.pm file:

           sub peers {
               my $self = shift;
               if (@_) { @{ $self->{PEERS} } = @_ }
               return map { "PEON=\U$_" } @{ $self->{PEERS} };
           }

       There, we've just demonstrated the high-falutin' concept
       known in certain circles as polymorphism.  We've taken on
       the form and behaviour of an existing object, and then
       we've altered it to suit our own purposes.  This is a form
       of 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:




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           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:



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           package Boss;
           use Employee;        # :-)
           @ISA = qw(Employee);

       And here's the test program:

           #!/usr/bin/perl -w
           use strict;
           use Boss;
           Boss->debug(1);

           my $boss = Boss->new();

           $boss->fullname->title("Don");
           $boss->fullname->surname("Pichon Alvarez");
           $boss->fullname->christian("Federico Jesus");
           $boss->fullname->nickname("Fred");

           $boss->age(47);
           $boss->peers("Frank", "Felipe", "Faust");

           printf "%s is age %d.\n", $boss->fullname->as_string, $boss->age;
           printf "His peers are: %s\n", join(", ", $boss->peers);

       Running it, we see that we're still ok.  If you'd like to
       dump out your object in a nice format, somewhat like the
       way the 'x' command works in the debugger, you could use
       the Data::Dumper module from CPAN this way:

           use Data::Dumper;
           print "Here's the boss:\n";
           print Dumper($boss);

       Which shows us something like this:

           Here's the boss:
           $VAR1 = bless( {
                _CENSUS => \1,
                FULLNAME => bless( {
                                     TITLE => 'Don',
                                     SURNAME => 'Pichon Alvarez',
                                     NICK => 'Fred',
                                     CHRISTIAN => 'Federico Jesus'
                                   }, 'Fullname' ),
                AGE => 47,
                PEERS => [
                           'Frank',
                           'Felipe',
                           'Faust'
                         ]
              }, 'Boss' );

       Hm.... something's missing there.  What about the salary,
       start date, and ID fields?  Well, we never set them to



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       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
       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 by
       default (see mro for alternate method resolution orders).
       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.



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       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



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       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.

   Deeper UNIVERSAL details
       It is also valid (though perhaps unwise in most cases) to
       put other packages' names in @UNIVERSAL::ISA.  These
       packages will also be implicitly inherited by all classes,
       just as UNIVERSAL itself is.  However, neither UNIVERSAL
       nor any of its parents from the @ISA tree are explicit
       base classes of all objects.  To clarify, given the
       following:




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           @UNIVERSAL::ISA = ('REALLYUNIVERSAL');

           package REALLYUNIVERSAL;
           sub special_method { return "123" }

           package Foo;
           sub normal_method { return "321" }

       Calling Foo->special_method() will return "123", but
       calling Foo->isa('REALLYUNIVERSAL') or
       Foo->isa('UNIVERSAL') will return false.

       If your class is using an alternate mro like C3 (see mro),
       method resolution within UNIVERSAL / @UNIVERSAL::ISA will
       still occur in the default depth-first left-to-right
       manner, after the class's C3 mro is exhausted.

       All of the above is made more intuitive by realizing what
       really happens during method lookup, which is roughly like
       this ugly pseudo-code:

           get_mro(class) {
               # recurses down the @ISA's starting at class,
               # builds a single linear array of all
               # classes to search in the appropriate order.
               # The method resolution order (mro) to use
               # for the ordering is whichever mro "class"
               # has set on it (either default (depth first
               # l-to-r) or C3 ordering).
               # The first entry in the list is the class
               # itself.
           }

           find_method(class, methname) {
               foreach $class (get_mro(class)) {
                   if($class->has_method(methname)) {
                       return ref_to($class->$methname);
                   }
               }
               foreach $class (get_mro(UNIVERSAL)) {
                   if($class->has_method(methname)) {
                       return ref_to($class->$methname);
                   }
               }
               return undef;
           }

       However the code that implements UNIVERSAL::isa does not
       search in UNIVERSAL itself, only in the package's actual
       @ISA.

Alternate Object Representations
       Nothing requires objects to be implemented as hash
       references.  An object can be any sort of reference so



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       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
       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




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       You might guess that the array access would be a lot
       faster than the hash access, but they're actually
       comparable.  The array is a little bit faster, but not
       more than ten or 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
       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.\n", $him->name, $him->age;
           print "His peers are: ", join(", ", @{$him->peers}), "\n";

       but the implementation would be radically, perhaps even
       sublimely different:

           package Person;

           sub new {
                my $class  = shift;
                my $self = {



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                   NAME  => undef,
                   AGE   => undef,
                   PEERS => [],
                };
                my $closure = sub {
                   my $field = shift;
                   if (@_) { $self->{$field} = shift }
                   return    $self->{$field};
               };
               bless($closure, $class);
               return $closure;
           }

           sub name   { &{ $_[0] }("NAME",  @_[ 1 .. $#_ ] ) }
           sub age    { &{ $_[0] }("AGE",   @_[ 1 .. $#_ ] ) }
           sub peers  { &{ $_[0] }("PEERS", @_[ 1 .. $#_ ] ) }

           1;

       Because this object is hidden behind a code reference,
       it's probably a bit mysterious to those whose background
       is more firmly rooted in standard procedural or object-
       based 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



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       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 principle
       virtue of a programmer, would come into play, here you
       have it. (More seriously, Hubris is just the pride in
       craftsmanship that comes from having written a sound bit
       of well-designed code.)

AUTOLOAD: Proxy Methods
       Autoloading is a way to intercept calls to undefined
       methods.  An autoload routine may choose to create a new
       function on the fly, either loaded from disk or perhaps
       just eval()ed right there.  This define-on-the-fly
       strategy is why it's called autoloading.

       But that's only one possible approach.  Another one is to
       just have the autoloaded method itself directly provide
       the requested service.  When used in this way, you may
       think of autoloaded methods as "proxy" methods.

       When Perl tries to call an undefined function in a
       particular package and that function is not defined, it
       looks for a function in that same package called AUTOLOAD.
       If one exists, it's called with the same arguments as the
       original function would have had.  The fully-qualified
       name of the function is stored in that package's global
       variable $AUTOLOAD.  Once called, the function can do
       anything it would like, including defining a new function
       by the right name, and then doing a really fancy kind of
       "goto" right to it, erasing itself from the call stack.

       What does this have to do with objects?  After all, we
       keep talking about functions, not methods.  Well, since a
       method is just a function with an extra argument and some
       fancier semantics about where it's found, we can use
       autoloading for methods, too.  Perl doesn't start looking
       for an AUTOLOAD method until it has exhausted the
       recursive hunt up through @ISA, though.  Some programmers
       have even been known to define a UNIVERSAL::AUTOLOAD
       method to trap unresolved method calls to any kind of
       object.




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   Autoloaded Data Methods
       You probably began to get a little suspicious about the
       duplicated code way back earlier when we first showed you
       the Person class, and then later the Employee class.  Each
       method used to access the hash fields looked virtually
       identical.  This should have tickled that great
       programming virtue, Impatience, but for the time, we let
       Laziness win out, and so did nothing.  Proxy methods can
       cure this.

       Instead of writing a new function every time we want a new
       data field, we'll use the autoload mechanism to generate
       (actually, mimic) methods on the fly.  To verify that
       we're accessing a valid member, we will check against an
       "_permitted" (pronounced "under-permitted") field, which
       is a reference to a file-scoped lexical (like a C file
       static) hash of permitted fields in this record called
       %fields.  Why the underscore?  For the same reason as the
       _CENSUS field we once used: as a marker that means "for
       internal use only".

       Here's what the module initialization code and class
       constructor will look like when taking this approach:

           package Person;
           use Carp;
           our $AUTOLOAD;  # it's a package global

           my %fields = (
               name        => undef,
               age         => undef,
               peers       => undef,
           );

           sub new {
               my $class = shift;
               my $self  = {
                   _permitted => \%fields,
                   %fields,
               };
               bless $self, $class;
               return $self;
           }

       If we wanted our record to have default values, we could
       fill those in where current we have "undef" in the %fields
       hash.

       Notice how we saved a reference to our class data on the
       object itself?  Remember that it's important to access
       class data through the object itself instead of having any
       method reference %fields directly, or else you won't have
       a decent inheritance.




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       The real magic, though, is going to reside in our proxy
       method, which will handle all calls to undefined methods
       for objects of class Person (or subclasses of Person).  It
       has to be called AUTOLOAD.  Again, it's all caps because
       it's called for us implicitly by Perl itself, not by a
       user directly.

           sub AUTOLOAD {
               my $self = shift;
               my $type = ref($self)
                           or croak "$self is not an object";

               my $name = $AUTOLOAD;
               $name =~ s/.*://;   # strip fully-qualified portion

               unless (exists $self->{_permitted}->{$name} ) {
                   croak "Can't access `$name' field in class $type";
               }

               if (@_) {
                   return $self->{$name} = shift;
               } else {
                   return $self->{$name};
               }
           }

       Pretty nifty, eh?  All we have to do to add new data
       fields is modify %fields.  No new functions need be
       written.

       I could have avoided the "_permitted" field entirely, but
       I wanted to demonstrate how to store a reference to class
       data on the object so you wouldn't have to access that
       class data directly from an object method.

   Inherited Autoloaded Data Methods
       But what about inheritance?  Can we define our Employee
       class similarly?  Yes, so long as we're careful enough.

       Here's how to be careful:

           package Employee;
           use Person;
           use strict;
           our @ISA = qw(Person);

           my %fields = (
               id          => undef,
               salary      => undef,
           );

           sub new {
               my $class = shift;
               my $self  = $class->SUPER::new();



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               my($element);
               foreach $element (keys %fields) {
                   $self->{_permitted}->{$element} = $fields{$element};
               }
               @{$self}{keys %fields} = values %fields;
               return $self;
           }

       Once we've done this, we don't even need to have an
       AUTOLOAD function in the Employee package, because we'll
       grab Person's version of that via inheritance, and it will
       all work out just fine.

Metaclassical Tools
       Even though proxy methods can provide a more convenient
       approach to making more struct-like classes than tediously
       coding up data methods as functions, it still leaves a bit
       to be desired.  For one thing, it means you have to handle
       bogus calls that you don't mean to trap via your proxy.
       It also means you have to be quite careful when dealing
       with inheritance, as detailed above.

       Perl programmers have responded to this by creating
       several different class construction classes.  These
       metaclasses are classes that create other classes.  A
       couple worth looking at are Class::Struct and Alias.
       These and other related metaclasses can be found in the
       modules directory on CPAN.

   Class::Struct
       One of the older ones is Class::Struct.  In fact, its
       syntax and interface were sketched out long before perl5
       even solidified into a real thing.  What it does is
       provide you a way to "declare" a class as having objects
       whose fields are of a specific type.  The function that
       does this is called, not surprisingly enough, struct().
       Because structures or records are not base types in Perl,
       each time you want to create a class to provide a record-
       like data object, you yourself have to define a new()
       method, plus separate data-access methods for each of that
       record's fields.  You'll quickly become bored with this
       process.  The Class::Struct::struct() function alleviates
       this tedium.

       Here's a simple example of using it:

           use Class::Struct qw(struct);
           use Jobbie;  # user-defined; see below

           struct 'Fred' => {
               one        => '$',
               many       => '@',
               profession => 'Jobbie',  # does not call Jobbie->new()
           };



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           $ob = Fred->new(profession => Jobbie->new());
           $ob->one("hmmmm");

           $ob->many(0, "here");
           $ob->many(1, "you");
           $ob->many(2, "go");
           print "Just set: ", $ob->many(2), "\n";

           $ob->profession->salary(10_000);

       You can declare types in the struct to be basic Perl
       types, or user-defined types (classes).  User types will
       be initialized by calling that class's new() method.

       Take care that the "Jobbie" object is not created
       automatically by the "Fred" class's new() method, so you
       should specify a "Jobbie" object when you create an
       instance of "Fred".

       Here's a real-world example of using struct generation.
       Let's say you wanted to override Perl's idea of
       gethostbyname() and gethostbyaddr() so that they would
       return objects that acted like C structures.  We don't
       care about high-falutin' OO gunk.  All we want is for
       these objects to act like structs in the C sense.

           use Socket;
           use Net::hostent;
           $h = gethostbyname("perl.com");  # object return
           printf "perl.com's real name is %s, address %s\n",
               $h->name, inet_ntoa($h->addr);

       Here's how to do this using the Class::Struct module.  The
       crux is going to be this call:

           struct 'Net::hostent' => [          # note bracket
               name       => '$',
               aliases    => '@',
               addrtype   => '$',
               'length'   => '$',
               addr_list  => '@',
            ];

       Which creates object methods of those names and types.  It
       even creates a new() method for us.

       We could also have implemented our object this way:









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           struct 'Net::hostent' => {          # note brace
               name       => '$',
               aliases    => '@',
               addrtype   => '$',
               'length'   => '$',
               addr_list  => '@',
            };

       and then Class::Struct would have used an anonymous hash
       as the object type, instead of an anonymous array.  The
       array is faster and smaller, but the hash works out better
       if you eventually want to do inheritance.  Since for this
       struct-like object we aren't planning on inheritance, this
       time we'll opt for better speed and size over better
       flexibility.

       Here's the whole implementation:

           package Net::hostent;
           use strict;

           BEGIN {
               use Exporter   ();
               our @EXPORT      = qw(gethostbyname gethostbyaddr gethost);
               our @EXPORT_OK   = qw(
                                      $h_name         @h_aliases
                                      $h_addrtype     $h_length
                                      @h_addr_list    $h_addr
                                  );
               our %EXPORT_TAGS = ( FIELDS => [ @EXPORT_OK, @EXPORT ] );
           }
           our @EXPORT_OK;

           # Class::Struct forbids use of @ISA
           sub import { goto &Exporter::import }

           use Class::Struct qw(struct);
           struct 'Net::hostent' => [
              name        => '$',
              aliases     => '@',
              addrtype    => '$',
              'length'    => '$',
              addr_list   => '@',
           ];

           sub addr { shift->addr_list->[0] }

           sub populate (@) {
               return unless @_;
               my $hob = new();  # Class::Struct made this!
               $h_name     =    $hob->[0]              = $_[0];
               @h_aliases  = @{ $hob->[1] } = split ' ', $_[1];
               $h_addrtype =    $hob->[2]              = $_[2];
               $h_length   =    $hob->[3]              = $_[3];



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               $h_addr     =                             $_[4];
               @h_addr_list = @{ $hob->[4] } =         @_[ (4 .. $#_) ];
               return $hob;
           }

           sub gethostbyname ($)  { populate(CORE::gethostbyname(shift)) }

           sub gethostbyaddr ($;$) {
               my ($addr, $addrtype);
               $addr = shift;
               require Socket unless @_;
               $addrtype = @_ ? shift : Socket::AF_INET();
               populate(CORE::gethostbyaddr($addr, $addrtype))
           }

           sub gethost($) {
               if ($_[0] =~ /^\d+(?:\.\d+(?:\.\d+(?:\.\d+)?)?)?$/) {
                  require Socket;
                  &gethostbyaddr(Socket::inet_aton(shift));
               } else {
                  &gethostbyname;
               }
           }

           1;

       We've snuck in quite a fair bit of other concepts besides
       just dynamic class creation, like overriding core
       functions, import/export bits, function prototyping,
       short-cut function call via &whatever, and function
       replacement with "goto &whatever".  These all mostly make
       sense from the perspective of a traditional module, but as
       you can see, we can also use them in an object module.

       You can look at other object-based, struct-like overrides
       of core functions in the 5.004 release of Perl in
       File::stat, Net::hostent, Net::netent, Net::protoent,
       Net::servent, Time::gmtime, Time::localtime, User::grent,
       and User::pwent.  These modules have a final component
       that's all lowercase, by convention reserved for compiler
       pragmas, because they affect the compilation and change a
       builtin function.  They also have the type names that a C
       programmer would most expect.

   Data Members as Variables
       If you're used to C++ objects, then you're accustomed to
       being able to get at an object's data members as simple
       variables from within a method.  The Alias module provides
       for this, as well as a good bit more, such as the
       possibility of private methods that the object can call
       but folks outside the class cannot.

       Here's an example of creating a Person using the Alias
       module.  When you update these magical instance variables,



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       you automatically update value fields in the hash.
       Convenient, eh?

           package Person;

           # this is the same as before...
           sub new {
                my $class = shift;
                my $self = {
                   NAME  => undef,
                   AGE   => undef,
                   PEERS => [],
               };
               bless($self, $class);
               return $self;
           }

           use Alias qw(attr);
           our ($NAME, $AGE, $PEERS);

           sub name {
               my $self = attr shift;
               if (@_) { $NAME = shift; }
               return    $NAME;
           }

           sub age {
               my $self = attr shift;
               if (@_) { $AGE = shift; }
               return    $AGE;
           }

           sub peers {
               my $self = attr shift;
               if (@_) { @PEERS = @_; }
               return    @PEERS;
           }

           sub exclaim {
               my $self = attr shift;
               return sprintf "Hi, I'm %s, age %d, working with %s",
                   $NAME, $AGE, join(", ", @PEERS);
           }

           sub happy_birthday {
               my $self = attr shift;
               return ++$AGE;
           }

       The need for the "our" declaration is because what Alias
       does is play with package globals with the same name as
       the fields.  To use globals while "use strict" is in
       effect, you have to predeclare them.  These package
       variables are localized to the block enclosing the attr()



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       call just as if you'd used a local() on them.  However,
       that means that they're still considered global variables
       with temporary values, just as with any other local().

       It would be nice to combine Alias with something like
       Class::Struct or Class::MethodMaker.

NOTES
   Object Terminology
       In the various OO literature, it seems that a lot of
       different words are used to describe only a few different
       concepts.  If you're not already an object programmer,
       then you don't need to worry about all these fancy words.
       But if you are, then you might like to know how to get at
       the same concepts in Perl.

       For example, it's common to call an object an instance of
       a class and to call those objects' methods instance
       methods.  Data fields peculiar to each object are often
       called instance data or object attributes, and data fields
       common to all members of that class are class data, class
       attributes, or static data members.

       Also, base class, generic class, and superclass all
       describe the same notion, whereas derived class, specific
       class, and subclass describe the other related one.

       C++ programmers have static methods and virtual methods,
       but Perl only has class methods and object methods.
       Actually, Perl only has methods.  Whether a method gets
       used as a class or object method is by usage only.  You
       could accidentally call a class method (one expecting a
       string argument) on an object (one expecting a reference),
       or vice versa.

       From the C++ perspective, all methods in Perl are virtual.
       This, by the way, is why they are never checked for
       function prototypes in the argument list as regular
       builtin and user-defined functions can be.

       Because a class is itself something of an object, Perl's
       classes can be taken as describing both a "class as meta-
       object" (also called object factory) philosophy and the
       "class as type definition" (declaring behaviour, not
       defining mechanism) idea.  C++ supports the latter notion,
       but not the former.

SEE ALSO
       The following manpages will doubtless provide more
       background for this one: perlmod, perlref, perlobj,
       perlbot, perltie, and overload.

       perlboot is a kinder, gentler introduction to object-
       oriented programming.



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       perltooc provides more detail on class data.

       Some modules which might prove interesting are
       Class::Accessor, Class::Class, Class::Contract,
       Class::Data::Inheritable, Class::MethodMaker and
       Tie::SecureHash

AUTHOR AND COPYRIGHT
       Copyright (c) 1997, 1998 Tom Christiansen All rights
       reserved.

       This documentation is free; you can redistribute it and/or
       modify it under the same terms as Perl itself.

       Irrespective of its distribution, all code examples in
       this file are hereby placed into the public domain.  You
       are permitted and encouraged to use this code in your own
       programs for fun or for profit as you see fit.  A simple
       comment in the code giving credit would be courteous but
       is not required.

COPYRIGHT
   Acknowledgments
       Thanks to Larry Wall, Roderick Schertler, Gurusamy
       Sarathy, Dean Roehrich, Raphael Manfredi, Brent Halsey,
       Greg Bacon, Brad Appleton, and many others for their
       helpful comments.






























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