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perltoot ()
  • >> perltoot (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
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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 (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 MacOS 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 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");
             $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};
             }
    
             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.
    
         While we're at it, let's make our constructor a bit more
         flexible.  Rather than being uniquely a class method, we'll
         set it up so that it can be called as either a class method
         or an object method.  That way you can say:
    
             $me  = Person->new();
             $him = $me->new();
    
         To do this, all we have to do is check whether what was
         passed in was a reference or not.  If so, we were invoked as
         an object method, and we need to extract the package (class)
         using the ref() function.  If not, we just use the string
         passed in as the package name for blessing our referent.
    
             sub new {
                 my $proto = shift;
                 my $class = ref($proto) || $proto;
                 my $self  = {};
                 $self->{NAME}   = undef;
                 $self->{AGE}    = undef;
                 $self->{PEERS}  = [];
                 bless ($self, $class);
                 return $self;
             }
    
         That's about all there is for constructors.  These methods
         bring objects to life, returning neat little opaque bundles
         to the user to be used in subsequent method calls.
    
         Destructors
    
         Every story has a beginning and an end.  The beginning of
         the object's story is its constructor, explicitly called
         when the object comes into existence.  But the ending of its
         story is the destructor, a method implicitly called when an
         object leaves this life.  Any per-object clean-up code is
         placed in the destructor, which must (in Perl) be called
         DESTROY.
    
         If constructors can have arbitrary names, then why not
         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 the perltie manpage.
    
         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 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 the Data Members as Variables
         entry elsewhere in this document.)  Occasionally you'll see
         method names beginning or ending with an underscore or two.
         This marking is a convention indicating that the methods are
         private to that class alone and sometimes to its closest
         acquaintances, its immediate subclasses.  But this
         distinction is not enforced by Perl itself.  It's up to the
         programmer to behave.
    
         There's no reason to limit methods to those that simply
         access data.  Methods can do anything at all.  The key point
         is that they're invoked against an object or a class.  Let's
         say we'd like object methods that do more than fetch or set
         one particular field.
    
             sub exclaim {
                 my $self = shift;
                 return sprintf "Hi, I'm %s, age %d, working with %s",
                     $self->{NAME}, $self->{AGE}, join(", ", @{$self->{PEERS}});
             }
    
         Or maybe even one like this:
    
             sub happy_birthday {
                 my $self = shift;
                 return ++$self->{AGE};
             }
    
         Some might argue that one should go at these this way:
    
             sub exclaim {
                 my $self = shift;
                 return sprintf "Hi, I'm %s, age %d, working with %s",
                     $self->name, $self->age, join(", ", $self->peers);
             }
    
             sub happy_birthday {
                 my $self = shift;
                 return $self->age( $self->age() + 1 );
             }
    
         But since these methods are all executing in the class
         itself, this may not be critical.  There are tradeoffs to be
         made.  Using direct hash access is faster (about an order of
         magnitude faster, in fact), and it's more convenient when
         you want to interpolate in strings.  But using methods (the
         external interface) internally shields not just the users of
         your class but even you yourself from changes in your data
         representation.
    
    
    

    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 the perltie manpage, thereby intercepting all
         accesses.
    
         But more often than not, you just want to make your class
         data a file-scoped lexical.  To do so, simply put this at
         the top of the file:
    
             my $Census = 0;
    
         Even though the scope of a my() normally expires when the
         block in which it was declared is done (in this case the
         whole file being required or used), Perl's deep binding of
         lexical variables guarantees that the variable will not be
         deallocated, remaining accessible to functions declared
         within that scope.  This doesn't work with global variables
         given temporary values via local(), though.
    
         Irrespective of whether you leave $Census a package global
         or make it instead a file-scoped lexical, you should make
         these changes to your Person::new() constructor:
    
             sub new {
                 my $proto = shift;
                 my $class = ref($proto) || $proto;
                 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.
    
         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 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 $proto = shift;
                 my $class = ref($proto) || $proto;
                 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.
    
             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
         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;
    
    
    
              ### 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:
    
    
    
             sub new {
                 my $proto = shift;
                 my $class = ref($proto) || $proto;
                 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 $proto = shift;
                 my $class = ref($proto) || $proto;
                 my $self  = {
                     TITLE       => undef,
                     CHRISTIAN   => undef,
                     SURNAME     => undef,
                     NICK        => undef,
                 };
                 bless ($self, $class);
                 return $self;
             }
    
             sub christian {
                 my $self = shift;
                 if (@_) { $self->{CHRISTIAN} = shift }
                 return $self->{CHRISTIAN};
             }
    
             sub surname {
                 my $self = shift;
                 if (@_) { $self->{SURNAME} = shift }
                 return $self->{SURNAME};
             }
    
             sub nickname {
                 my $self = shift;
                 if (@_) { $self->{NICK} = shift }
                 return $self->{NICK};
             }
    
             sub title {
                 my $self = shift;
                 if (@_) { $self->{TITLE} = shift }
                 return $self->{TITLE};
             }
    
             sub as_string {
                 my $self = shift;
                 my $name = join(" ", @$self{'CHRISTIAN', 'SURNAME'});
                 if ($self->{TITLE}) {
                     $name = $self->{TITLE} . " " . $name;
                 }
                 return $name;
             }
    
             1;
    
         Finally, here's the test program:
    
             #!/usr/bin/perl -w
             use strict;
             use Person;
             sub END { show_census() }
    
             sub show_census ()  {
                 printf "Current population: %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;
             printf "%s's age: %d.\n", $him->name, $him->age;
             $him->happy_birthday;
             printf "%s's age: %d.\n", $him->name, $him->age;
    
             show_census();
    
    
    
    

    Inheritance

         Object-oriented programming systems all support some notion
         of inheritance.  Inheritance means allowing one class to
         piggy-back on top of another one so you don't have to write
         the same code again and again.  It's about software reuse,
         and therefore related to Laziness, the principal virtue of a
         programmer.  (The import/export mechanisms in traditional
         modules are also a form of code reuse, but a simpler one
         than the true inheritance that you find in object modules.)
    
         Sometimes the syntax of inheritance is built into the core
         of the language, and sometimes it's not.  Perl has no
         special syntax for specifying the class (or classes) to
         inherit from.  Instead, it's all strictly in the semantics.
         Each package can have a variable called @ISA, which governs
         (method) inheritance.  If you try to call a method on an
         object or class, and that method is not found in that
         object's package, Perl then looks to @ISA for other packages
         to go looking through in search of the missing method.
    
         Like the special per-package variables recognized by
         Exporter (such as @EXPORT, @EXPORT_OK, @EXPORT_FAIL,
         %EXPORT_TAGS, and $VERSION), the @ISA array must be a
         package-scoped global and not a file-scoped lexical created
         via my().  Most classes have just one item in their @ISA
         array.  In this case, we have what's called "single
         inheritance", or SI for short.
    
         Consider this class:
    
             package Employee;
             use Person;
             @ISA = ("Person");
             1;
    
         Not a lot to it, eh?  All it's doing so far is loading in
         another class and stating that this one will inherit methods
         from that other class if need be.  We have given it none of
         its own methods.  We rely upon an Employee to behave just
         like a Person.
    
         Setting up an empty class like this is called the "empty
         subclass test"; that is, making a derived class that does
         nothing but inherit from a base class.  If the original base
         class has been designed properly, then the new derived class
         can be used as a drop-in replacement for the old one.  This
         means you should be able to write a program like this:
    
             use Employee;
             my $empl = Employee->new();
             $empl->name("Jason");
             $empl->age(23);
             printf "%s is age %d.\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};
             }
    
             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:
    
             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.
         Things are getting a bit complicated here.  Have we done
         anything we shouldn't?  As before, one way to test whether
         we're designing a decent class is via the empty subclass
         test.  Since we already have an Employee class that we're
         trying to check, we'd better get a new empty subclass that
         can derive from Employee.  Here's one:
    
             package Boss;
             use Employee;        # :-)
             @ISA = qw(Employee);
    
         And here's the test program:
    
             #!/usr/bin/perl -w
             use strict;
             use Boss;
             Boss->debug(1);
    
             my $boss = Boss->new();
    
             $boss->fullname->title("Don");
             $boss->fullname->surname("Pichon Alvarez");
             $boss->fullname->christian("Federico Jesus");
             $boss->fullname->nickname("Fred");
    
             $boss->age(47);
             $boss->peers("Frank", "Felipe", "Faust");
    
             printf "%s is age %d.\n", $boss->fullname, $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
         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 $proto = shift;
                 my $class = ref($proto) || $proto;
                 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.
         Consider a bunch of @ISA arrays like this:
    
             @First::ISA    = qw( Alpha );
             @Second::ISA   = qw( Beta );
             @Third::ISA    = qw( First Second );
    
         If you have an object of class Third:
    
             my $ob = Third->new();
             $ob->spin();
    
         How do we find a spin() method (or a new() method for that
         matter)?  Because the search is depth-first, classes will be
         looked up in the following order: Third, First, Alpha,
         Second, and Beta.
    
         In practice, few class modules have been seen that actually
         make use of MI.  One nearly always chooses simple
         containership of one class within another over MI.  That's
         why our Person object contained a Fullname object.  That
         doesn't mean it was one.
    
         However, there is one particular area where MI in Perl is
         rampant:  borrowing another class's class methods.  This is
         rather common, especially with some bundled "objectless"
         classes, like Exporter, DynaLoader, AutoLoader, and
         SelfLoader.  These classes do not provide constructors; they
         exist only so you may inherit their class methods.  (It's
         not entirely clear why inheritance was done here rather than
         traditional module importation.)
    
         For example, here is the POSIX module's @ISA:
    
             package POSIX;
             @ISA = qw(Exporter DynaLoader);
    
         The POSIX module isn't really an object module, but then,
         neither are Exporter or DynaLoader.  They're just lending
         their classes' behaviours to POSIX.
    
         Why don't people use MI for object methods much?  One reason
         is that it can have complicated side-effects.  For one
         thing, your inheritance graph (no longer a tree) might
         converge back to the same base class.  Although Perl guards
         against recursive inheritance, merely having parents who are
         related to each other via a common ancestor, incestuous
         though it sounds, is not forbidden.  What if in our Third
         class shown above we wanted its new() method to also call
         both overridden constructors in its two parent classes?  The
         SUPER notation would only find the first one.  Also, what
         about if the Alpha and Beta classes both had a common
         ancestor, like Nought?  If you kept climbing up the
         inheritance tree calling overridden methods, you'd end up
         calling Nought::new() twice, which might well be a bad idea.
    
         UNIVERSAL: The Root of All Objects
    
         Wouldn't it be convenient if all objects were rooted at some
         ultimate base class?  That way you could give every object
         common methods without having to go and add it to each and
         every @ISA.  Well, it turns out that you can.  You don't see
         it, but Perl tacitly and irrevocably assumes that there's an
         extra element at the end of @ISA: the class UNIVERSAL.  In
         version 5.003, there were no predefined methods there, but
         you could put whatever you felt like into it.
    
         However, as of version 5.004 (or some subversive releases,
         like 5.003_08), UNIVERSAL has some methods in it already.
         These are builtin to your Perl binary, so they don't take
         any extra time to load.  Predefined methods include isa(),
         can(), and VERSION().  isa() tells you whether an object or
         class "is" another one without having to traverse the
         hierarchy yourself:
    
            $has_io = $fd->isa("IO::Handle");
            $itza_handle = IO::Socket->isa("IO::Handle");
    
         The can() method, called against that object or class,
         reports back whether its string argument is a callable
         method name in that class.  In fact, it gives you back a
         function reference to that method:
    
            $his_print_method = $obj->can('as_string');
    
         Finally, the VERSION method checks whether the class (or the
         object's class) has a package global called $VERSION that's
         high enough, as in:
    
             Some_Module->VERSION(3.0);
             $his_vers = $ob->VERSION();
    
         However, we don't usually call VERSION ourselves.  (Remember
         that an all uppercase function name is a Perl convention
         that indicates that the function will be automatically used
         by Perl in some way.)  In this case, it happens when you say
    
             use Some_Module 3.0;
    
         If you wanted to add version checking to your Person class
         explained above, just add this to Person.pm:
    
             our $VERSION = '1.1';
    
         and then in Employee.pm could you can say
    
             use Employee 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

         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
    
         If the user of your class honors the contract and sticks to
         the advertised interface, then you can change its underlying
         interface if you feel like it.  Here's another
         implementation that conforms to the same interface
         specification.  This time we'll use an array reference
         instead of a hash reference to represent the object.
    
             package Person;
             use strict;
    
             my($NAME, $AGE, $PEERS) = ( 0 .. 2 );
    
             ############################################
             ## the object constructor (array version) ##
             ############################################
             sub new {
                 my $self = [];
                 $self->[$NAME]   = undef;  # this is unnecessary
                 $self->[$AGE]    = undef;  # as is this
                 $self->[$PEERS]  = [];     # but this isn't, really
                 bless($self);
                 return $self;
             }
    
    
             sub name {
                 my $self = shift;
                 if (@_) { $self->[$NAME] = shift }
                 return $self->[$NAME];
             }
    
             sub age {
                 my $self = shift;
                 if (@_) { $self->[$AGE] = shift }
                 return $self->[$AGE];
             }
    
             sub peers {
                 my $self = shift;
                 if (@_) { @{ $self->[$PEERS] } = @_ }
                 return @{ $self->[$PEERS] };
             }
    
             1;  # so the require or use succeeds
    
         You might guess that the array access would be a lot faster
         than the hash access, but they're actually comparable.  The
         array is a little bit faster, but not more than ten or
         fifteen percent, even when you replace the variables above
         like $AGE with literal numbers, like 1.  A bigger difference
         between the two approaches can be found in memory use.  A
         hash representation takes up more memory than an array
         representation because you have to allocate memory for the
         keys as well as for the values.  However, it really isn't
         that bad, especially since as of version 5.004, memory is
         only allocated once for a given hash key, no matter how many
         hashes have that key.  It's expected that sometime in the
         future, even these differences will fade into obscurity as
         more efficient underlying representations are devised.
    
         Still, the tiny edge in speed (and somewhat larger one in
         memory) is enough to make some programmers choose an array
         representation for simple classes.  There's still a little
         problem with scalability, though, because later in life when
         you feel like creating subclasses, you'll find that hashes
         just work out better.
    
         Closures as Objects
    
         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 $that  = shift;
                  my $class = ref($that) || $that;
                  my $self = {
                     NAME  => undef,
                     AGE   => undef,
                     PEERS => [],
                  };
                  my $closure = sub {
                     my $field = shift;
                     if (@_) { $self->{$field} = shift }
                     return    $self->{$field};
                 };
                 bless($closure, $class);
                 return $closure;
             }
    
             sub name   { &{ $_[0] }("NAME",  @_[ 1 .. $#_ ] ) }
             sub age    { &{ $_[0] }("AGE",   @_[ 1 .. $#_ ] ) }
             sub peers  { &{ $_[0] }("PEERS", @_[ 1 .. $#_ ] ) }
    
             1;
    
         Because this object is hidden behind a code reference, it's
         probably a bit mysterious to those whose background is more
         firmly rooted in standard procedural or object-based
         programming languages than in functional programming
         languages whence closures derive.  The object created and
         returned by the new() method is itself not a data reference
         as we've seen before.  It's an anonymous code reference that
         has within it access to a specific version (lexical binding
         and instantiation) of the object's data, which are stored in
         the private variable $self.  Although this is the same
         function each time, it contains a different version of
         $self.
    
         When a method like `$him->name("Jason")' is called, its
         implicit zeroth argument is the invoking object--just as it
         is with all method calls.  But in this case, it's our code
         reference (something like a function pointer in C++, but
         with deep binding of lexical variables).  There's not a lot
         to be done with a code reference beyond calling it, so
         that's just what we do when we say `&{$_[0]}'.  This is just
         a regular function call, not a method call.  The initial
         argument is the string "NAME", and any remaining arguments
         are whatever had been passed to the method itself.
    
         Once we're executing inside the closure that had been
         created in new(), the $self hash reference suddenly becomes
         visible.  The closure grabs its first argument ("NAME" in
         this case because that's what the name() method passed it),
         and uses that string to subscript into the private hash
         hidden in its unique version of $self.
    
         Nothing under the sun will allow anyone outside the
         executing method to be able to get at this hidden data.
         Well, nearly nothing.  You could single step through the
         program using the debugger and find out the pieces while
         you're in the method, but everyone else is out of luck.
    
         There, if that doesn't excite the Scheme folks, then I just
         don't know what will.  Translation of this technique into
         C++, Java, or any other braindead-static language is left as
         a futile exercise for aficionados of those camps.
    
         You could even add a bit of nosiness via the caller()
         function and make the closure refuse to operate unless
         called via its own package.  This would no doubt satisfy
         certain fastidious concerns of programming police and
         related puritans.
    
         If you were wondering when Hubris, the third 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.
    
         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 $that  = shift;
                 my $class = ref($that) || $that;
                 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.
    
         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 $that  = shift;
                 my $class = ref($that) || $that;
                 my $self = bless $that->SUPER::new(), $class;
                 my($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,  # calls Jobbie->new()
             };
    
             $ob = Fred->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.
    
         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:
    
             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];
                 $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,
         you automatically update value fields in the hash.
         Convenient, eh?
    
             package Person;
    
    
    
             # this is the same as before...
             sub new {
                  my $that  = shift;
                  my $class = ref($that) || $that;
                  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() 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:  the perlmod manpage, the perlref
         manpage, the perlobj manpage, the perlbot manpage, the
         perltie manpage, and the overload manpage.
    
    
    

    AUTHOR AND COPYRIGHT

         Copyright (c) 1997, 1998 Tom Christiansen All rights
         reserved.
    
         When included as part of the Standard Version of Perl, or as
         part of its complete documentation whether printed or
         otherwise, this work may be distributed only under the terms
         of Perl's Artistic License.  Any distribution of this file
         or derivatives thereof outside of that package require that
         special arrangements be made with copyright holder.
    
         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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