perltoot


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
       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.
       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 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.

       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.

       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 "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};
           }
           }

       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 perltie, thereby intercepting all
       accesses.

       But more often than not, you just want to make your class data a file-
       scoped lexical.  To do so, simply put this at the top of the file:

           my $Census = 0;

       Even though the scope of a my() normally expires when the block in
       which it was declared is done (in this case the whole file being
       required or used), Perl's deep binding of lexical variables guarantees
       that the variable will not be deallocated, remaining accessible to
       functions declared within that scope.  This doesn't work with global
       variables given temporary values via local(), though.

       Irrespective of whether you leave $Census a package global or make it
       instead a file-scoped lexical, you should make these changes to your
       Person::new() constructor:

           sub new {
           }

       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
       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;
           }

           }

   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 {

   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");

           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 $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;
               my $class = shift;
               my $self  = {
                   TITLE       => undef,
                   CHRISTIAN   => undef,
                   SURNAME     => undef,
                   NICK        => undef,
               };
               bless ($self, $class);
               return $self;
           }

           sub christian {
               my $self = shift;
               if (@_) { $self->{CHRISTIAN} = shift }
               return $self->{CHRISTIAN};
           }

           sub surname {
               my $self = shift;
               if (@_) { $self->{SURNAME} = shift }
               return $self->{SURNAME};
           }

           sub nickname {
               my $self = shift;
               if (@_) { $self->{NICK} = shift }
               return $self->{NICK};
           }

           sub title {
               my $self = shift;
               if (@_) { $self->{TITLE} = shift }
               return $self->{TITLE};
           }

           sub as_string {
               my $self = shift;
               my $name = join(" ", @$self{'CHRISTIAN', 'SURNAME'});
               if ($self->{TITLE}) {
                   $name = $self->{TITLE} . " " . $name;
               }
               return $name;
           }

           1;

       Finally, here's the test program:

           #!/usr/bin/perl -w
           use strict;
           use Person;
           sub END { show_census() }

           sub show_census ()  {
           $him->fullname->title("St");
           $him->age(1);

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

           show_census();

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

       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:

       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
               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.

       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->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,
               $self->{START_DATE}    = undef;
               bless ($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.

       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;

       above we wanted its new() method to also call both overridden
       constructors in its two parent classes?  The SUPER notation would only
       find the first one.  Also, what about if the Alpha and Beta classes
       both had a common ancestor, like Nought?  If you kept climbing up the
       inheritance tree calling overridden methods, you'd end up calling
       Nought::new() twice, which might well be a bad idea.

   UNIVERSAL: The Root of All Objects
       Wouldn't it be convenient if all objects were rooted at some ultimate
       base class?  That way you could give every object common methods
       without having to go and add it to each and every @ISA.  Well, it turns
       out that you can.  You don't see it, but Perl tacitly and irrevocably
       assumes that there's an extra element at the end of @ISA: the class
       UNIVERSAL.  In version 5.003, there were no predefined methods there,
       but you could put whatever you felt like into it.

       However, as of version 5.004 (or some subversive releases, like
       5.003_08), UNIVERSAL has some methods in it already.  These are builtin
       to your Perl binary, so they don't take any extra time to load.
       Predefined methods include isa(), can(), and VERSION().  isa() tells
       you whether an object or class "is" another one without having to
       traverse the hierarchy yourself:

          $has_io = $fd->isa("IO::Handle");
          $itza_handle = IO::Socket->isa("IO::Handle");

       The can() method, called against that object or class, reports back
       whether its string argument is a callable method name in that class.
       In fact, it gives you back a function reference to that method:

          $his_print_method = $obj->can('as_string');

       Finally, the VERSION method checks whether the class (or the object's
       class) has a package global called $VERSION that's high enough, as in:

           Some_Module->VERSION(3.0);
           $his_vers = $ob->VERSION();

       However, we don't usually call VERSION ourselves.  (Remember that an
       all uppercase function name is a Perl convention that indicates that
       the function will be automatically used by Perl in some way.)  In this
       case, it happens when you say

           use Some_Module 3.0;

       If you wanted to add version checking to your Person class explained
       above, just add this to Person.pm:

           our $VERSION = '1.1';

       and then in Employee.pm you can say

           use Person 1.1;


           @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 long as its referent has been
       suitably blessed.  That means scalar, array, and code references are

           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,
       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 = {
                   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
       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

       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.

       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};

       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  = $class->SUPER::new();
               my($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.
           $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:

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

               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;
               }

       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 $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;
           }

       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: perlmod, perlref, perlobj, perlbot, perltie, and overload.

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

       perltooc provides more detail on class data.

       to use this code in your own programs for fun or for profit as you see
       fit.  A simple comment in the code giving credit would be courteous but
       is not required.

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



perl v5.14.2                      2011-09-26                       PERLTOOT(1)
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