perltooc


DESCRIPTION
       When designing an object class, you are sometimes faced with the
       situation of wanting common state shared by all objects of that class.
       Such class attributes act somewhat like global variables for the entire
       class, but unlike program-wide globals, class attributes have meaning
       only to the class itself.

       Here are a few examples where class attributes might come in handy:

       o   to keep a count of the objects you've created, or how many are
           still extant.

       o   to extract the name or file descriptor for a logfile used by a
           debugging method.

       o   to access collective data, like the total amount of cash dispensed
           by all ATMs in a network in a given day.

       o   to access the last object created by a class, or the most accessed
           object, or to retrieve a list of all objects.

       Unlike a true global, class attributes should not be accessed directly.
       Instead, their state should be inspected, and perhaps altered, only
       through the mediated access of class methods.  These class attributes
       accessor methods are similar in spirit and function to accessors used
       to manipulate the state of instance attributes on an object.  They
       provide a clear firewall between interface and implementation.

       You should allow access to class attributes through either the class
       name or any object of that class.  If we assume that $an_object is of
       type Some_Class, and the &Some_Class::population_count method accesses
       class attributes, then these two invocations should both be possible,
       and almost certainly equivalent.

           Some_Class->population_count()
           $an_object->population_count()

       The question is, where do you store the state which that method
       accesses?  Unlike more restrictive languages like C++, where these are
       called static data members, Perl provides no syntactic mechanism to
       declare class attributes, any more than it provides a syntactic
       mechanism to declare instance attributes.  Perl provides the developer
       with a broad set of powerful but flexible features that can be uniquely
       crafted to the particular demands of the situation.

       A class in Perl is typically implemented in a module.  A module
       consists of two complementary feature sets: a package for interfacing
       with the outside world, and a lexical file scope for privacy.  Either
       of these two mechanisms can be used to implement class attributes.
       That means you get to decide whether to put your class attributes in
       package variables or to put them in lexical variables.

       And those aren't the only decisions to make.  If you choose to use

Class Data as Package Variables
       Because a class in Perl is really just a package, using package
       variables to hold class attributes is the most natural choice.  This
       makes it simple for each class to have its own class attributes.  Let's
       say you have a class called Some_Class that needs a couple of different
       attributes that you'd like to be global to the entire class.  The
       simplest thing to do is to use package variables like
       $Some_Class::CData1 and $Some_Class::CData2 to hold these attributes.
       But we certainly don't want to encourage outsiders to touch those data
       directly, so we provide methods to mediate access.

       In the accessor methods below, we'll for now just ignore the first
       argument--that part to the left of the arrow on method invocation,
       which is either a class name or an object reference.

           package Some_Class;
           sub CData1 {
               shift;  # XXX: ignore calling class/object
               $Some_Class::CData1 = shift if @_;
               return $Some_Class::CData1;
           }
           sub CData2 {
               shift;  # XXX: ignore calling class/object
               $Some_Class::CData2 = shift if @_;
               return $Some_Class::CData2;
           }

       This technique is highly legible and should be completely
       straightforward to even the novice Perl programmer.  By fully
       qualifying the package variables, they stand out clearly when reading
       the code.  Unfortunately, if you misspell one of these, you've
       introduced an error that's hard to catch.  It's also somewhat
       disconcerting to see the class name itself hard-coded in so many
       places.

       Both these problems can be easily fixed.  Just add the "use strict"
       pragma, then pre-declare your package variables.  (The "our" operator
       will be new in 5.6, and will work for package globals just like "my"
       works for scoped lexicals.)

           package Some_Class;
           use strict;
           our($CData1, $CData2);      # our() is new to perl5.6
           sub CData1 {
               shift;  # XXX: ignore calling class/object
               $CData1 = shift if @_;
               return $CData1;
           }
           sub CData2 {
               shift;  # XXX: ignore calling class/object
               $CData2 = shift if @_;
               return $CData2;
           }

       programmer: Laziness, here manifesting as that innate urge every
       programmer feels to factor out duplicate code whenever possible.

       Here's what to do.  First, make just one hash to hold all class
       attributes.

           package Some_Class;
           use strict;
           our %ClassData = (          # our() is new to perl5.6
               CData1 => "",
               CData2 => "",
           );

       Using closures (see perlref) and direct access to the package symbol
       table (see perlmod), now clone an accessor method for each key in the
       %ClassData hash.  Each of these methods is used to fetch or store
       values to the specific, named class attribute.

           for my $datum (keys %ClassData) {
               no strict "refs";       # to register new methods in package
               *$datum = sub {
                   shift;      # XXX: ignore calling class/object
                   $ClassData{$datum} = shift if @_;
                   return $ClassData{$datum};
               }
           }

       It's true that you could work out a solution employing an &AUTOLOAD
       method, but this approach is unlikely to prove satisfactory.  Your
       function would have to distinguish between class attributes and object
       attributes; it could interfere with inheritance; and it would have to
       careful about DESTROY.  Such complexity is uncalled for in most cases,
       and certainly in this one.

       You may wonder why we're rescinding strict refs for the loop.  We're
       manipulating the package's symbol table to introduce new function names
       using symbolic references (indirect naming), which the strict pragma
       would otherwise forbid.  Normally, symbolic references are a dodgy
       notion at best.  This isn't just because they can be used accidentally
       when you aren't meaning to.  It's also because for most uses to which
       beginning Perl programmers attempt to put symbolic references, we have
       much better approaches, like nested hashes or hashes of arrays.  But
       there's nothing wrong with using symbolic references to manipulate
       something that is meaningful only from the perspective of the package
       symbol table, like method names or package variables.  In other words,
       when you want to refer to the symbol table, use symbol references.

       Clustering all the class attributes in one place has several
       advantages.  They're easy to spot, initialize, and change.  The
       aggregation also makes them convenient to access externally, such as
       from a debugger or a persistence package.  The only possible problem is
       that we don't automatically know the name of each class's class object,
       should it have one.  This issue is addressed below in "The Eponymous
       Meta-Object".
       class's objects, the version shown above is still run, so you'll access
       $Some_Class::CData1--or in the method cloning version,
       $Some_Class::ClassData{CData1}.

       Think of these class methods as executing in the context of their base
       class, not in that of their derived class.  Sometimes this is exactly
       what you want.  If Feline subclasses Carnivore, then the population of
       Carnivores in the world should go up when a new Feline is born.  But
       what if you wanted to figure out how many Felines you have apart from
       Carnivores?  The current approach doesn't support that.

       You'll have to decide on a case-by-case basis whether it makes any
       sense for class attributes to be package-relative.  If you want it to
       be so, then stop ignoring the first argument to the function.  Either
       it will be a package name if the method was invoked directly on a class
       name, or else it will be an object reference if the method was invoked
       on an object reference.  In the latter case, the ref() function
       provides the class of that object.

           package Some_Class;
           sub CData1 {
               my $obclass = shift;
               my $class   = ref($obclass) || $obclass;
               my $varname = $class . "::CData1";
               no strict "refs";       # to access package data symbolically
               $$varname = shift if @_;
               return $$varname;
           }

       And then do likewise for all other class attributes (such as CData2,
       etc.) that you wish to access as package variables in the invoking
       package instead of the compiling package as we had previously.

       Once again we temporarily disable the strict references ban, because
       otherwise we couldn't use the fully-qualified symbolic name for the
       package global.  This is perfectly reasonable: since all package
       variables by definition live in a package, there's nothing wrong with
       accessing them via that package's symbol table.  That's what it's there
       for (well, somewhat).

       What about just using a single hash for everything and then cloning
       methods?  What would that look like?  The only difference would be the
       closure used to produce new method entries for the class's symbol
       table.

           no strict "refs";
           *$datum = sub {
               my $obclass = shift;
               my $class   = ref($obclass) || $obclass;
               my $varname = $class . "::ClassData";
               $varname->{$datum} = shift if @_;
               return $varname->{$datum};
           }

       probably even exporting that name as well.  The StrNum class in Recipe
       13.14 in The Perl Cookbook does this, if you're looking for an
       example.)

       This predictable approach has many benefits, including having a well-
       known identifier to aid in debugging, transparent persistence, or
       checkpointing.  It's also the obvious name for monadic classes and
       translucent attributes, discussed later.

       Here's an example of such a class.  Notice how the name of the hash
       storing the meta-object is the same as the name of the package used to
       implement the class.

           package Some_Class;
           use strict;

           # create class meta-object using that most perfect of names
           our %Some_Class = (         # our() is new to perl5.6
               CData1 => "",
               CData2 => "",
           );

           # this accessor is calling-package-relative
           sub CData1 {
               my $obclass = shift;
               my $class   = ref($obclass) || $obclass;
               no strict "refs";       # to access eponymous meta-object
               $class->{CData1} = shift if @_;
               return $class->{CData1};
           }

           # but this accessor is not
           sub CData2 {
               shift;                  # XXX: ignore calling class/object
               no strict "refs";       # to access eponymous meta-object
               __PACKAGE__ -> {CData2} = shift if @_;
               return __PACKAGE__ -> {CData2};
           }

       In the second accessor method, the __PACKAGE__ notation was used for
       two reasons.  First, to avoid hardcoding the literal package name in
       the code in case we later want to change that name.  Second, to clarify
       to the reader that what matters here is the package currently being
       compiled into, not the package of the invoking object or class.  If the
       long sequence of non-alphabetic characters bothers you, you can always
       put the __PACKAGE__ in a variable first.

           sub CData2 {
               shift;                  # XXX: ignore calling class/object
               no strict "refs";       # to access eponymous meta-object
               my $class = __PACKAGE__;
               $class->{CData2} = shift if @_;
               return $class->{CData2};
           }
           package Some_Class;
           use strict;

           our %Some_Class = (         # our() is new to perl5.6
               CData1 => "",
               CData2 => "",
           );

           # tri-natured: function, class method, or object method
           sub _classobj {
               my $obclass = shift || __PACKAGE__;
               my $class   = ref($obclass) || $obclass;
               no strict "refs";   # to convert sym ref to real one
               return \%$class;
           }

           for my $datum (keys %{ _classobj() } ) {
               # turn off strict refs so that we can
               # register a method in the symbol table
               no strict "refs";
               *$datum = sub {
                   use strict "refs";
                   my $self = shift->_classobj();
                   $self->{$datum} = shift if @_;
                   return $self->{$datum};
               }
           }

   Indirect References to Class Data
       A reasonably common strategy for handling class attributes is to store
       a reference to each package variable on the object itself.  This is a
       strategy you've probably seen before, such as in perltoot and perlbot,
       but there may be variations in the example below that you haven't
       thought of before.

           package Some_Class;
           our($CData1, $CData2);              # our() is new to perl5.6

           sub new {
               my $obclass = shift;
               return bless my $self = {
                   ObData1 => "",
                   ObData2 => "",
                   CData1  => \$CData1,
                   CData2  => \$CData2,
               } => (ref $obclass || $obclass);
           }

           sub ObData1 {
               my $self = shift;
               $self->{ObData1} = shift if @_;
               return $self->{ObData1};
           }

               return $$dataref;
           }

           sub CData2 {
               my $self = shift;
               my $dataref = ref $self
                               ? $self->{CData2}
                               : \$CData2;
               $$dataref = shift if @_;
               return $$dataref;
           }

       As written above, a derived class will inherit these methods, which
       will consequently access package variables in the base class's package.
       This is not necessarily expected behavior in all circumstances.  Here's
       an example that uses a variable meta-object, taking care to access the
       proper package's data.

               package Some_Class;
               use strict;

               our %Some_Class = (     # our() is new to perl5.6
                   CData1 => "",
                   CData2 => "",
               );

               sub _classobj {
                   my $self  = shift;
                   my $class = ref($self) || $self;
                   no strict "refs";
                   # get (hard) ref to eponymous meta-object
                   return \%$class;
               }

               sub new {
                   my $obclass  = shift;
                   my $classobj = $obclass->_classobj();
                   bless my $self = {
                       ObData1 => "",
                       ObData2 => "",
                       CData1  => \$classobj->{CData1},
                       CData2  => \$classobj->{CData2},
                   } => (ref $obclass || $obclass);
                   return $self;
               }

               sub ObData1 {
                   my $self = shift;
                   $self->{ObData1} = shift if @_;
                   return $self->{ObData1};
               }

               sub ObData2 {
                   my $self = shift;

               sub CData2 {
                   my $self = shift;
                   $self = $self->_classobj() unless ref $self;
                   my $dataref = $self->{CData2};
                   $$dataref = shift if @_;
                   return $$dataref;
               }

       Not only are we now strict refs clean, using an eponymous meta-object
       seems to make the code cleaner.  Unlike the previous version, this one
       does something interesting in the face of inheritance: it accesses the
       class meta-object in the invoking class instead of the one into which
       the method was initially compiled.

       You can easily access data in the class meta-object, making it easy to
       dump the complete class state using an external mechanism such as when
       debugging or implementing a persistent class.  This works because the
       class meta-object is a package variable, has a well-known name, and
       clusters all its data together.  (Transparent persistence is not always
       feasible, but it's certainly an appealing idea.)

       There's still no check that object accessor methods have not been
       invoked on a class name.  If strict ref checking is enabled, you'd blow
       up.  If not, then you get the eponymous meta-object.  What you do
       with--or about--this is up to you.  The next two sections demonstrate
       innovative uses for this powerful feature.

   Monadic Classes
       Some of the standard modules shipped with Perl provide class interfaces
       without any attribute methods whatsoever.  The most commonly used
       module not numbered amongst the pragmata, the Exporter module, is a
       class with neither constructors nor attributes.  Its job is simply to
       provide a standard interface for modules wishing to export part of
       their namespace into that of their caller.  Modules use the Exporter's
       &import method by setting their inheritance list in their package's
       @ISA array to mention "Exporter".  But class Exporter provides no
       constructor, so you can't have several instances of the class.  In
       fact, you can't have any--it just doesn't make any sense.  All you get
       is its methods.  Its interface contains no statefulness, so state data
       is wholly superfluous.

       Another sort of class that pops up from time to time is one that
       supports a unique instance.  Such classes are called monadic classes,
       or less formally, singletons or highlander classes.

       If a class is monadic, where do you store its state, that is, its
       attributes?  How do you make sure that there's never more than one
       instance?  While you could merely use a slew of package variables, it's
       a lot cleaner to use the eponymously named hash.  Here's a complete
       example of a monadic class:

           package Cosmos;
           %Cosmos = ();

           }

           # accessor method for "stars" attribute
           sub stars {
               my $self = shift;
               $self->{stars} = shift if @_;
               return $self->{stars};
           }

           # oh my - one of our stars just went out!
           sub supernova {
               my $self = shift;
               my $count = $self->stars();
               $self->stars($count - 1) if $count > 0;
           }

           # constructor/initializer method - fix by reboot
           sub bigbang {
               my $self = shift;
               %$self = (
                   name         => "the world according to tchrist",
                   birthday     => time(),
                   stars        => 0,
               );
               return $self;       # yes, it's probably a class.  SURPRISE!
           }

           # After the class is compiled, but before any use or require
           # returns, we start off the universe with a bang.
           __PACKAGE__ -> bigbang();

       Hold on, that doesn't look like anything special.  Those attribute
       accessors look no different than they would if this were a regular
       class instead of a monadic one.  The crux of the matter is there's
       nothing that says that $self must hold a reference to a blessed object.
       It merely has to be something you can invoke methods on.  Here the
       package name itself, Cosmos, works as an object.  Look at the
       &supernova method.  Is that a class method or an object method?  The
       answer is that static analysis cannot reveal the answer.  Perl doesn't
       care, and neither should you.  In the three attribute methods, %$self
       is really accessing the %Cosmos package variable.

       If like Stephen Hawking, you posit the existence of multiple,
       sequential, and unrelated universes, then you can invoke the &bigbang
       method yourself at any time to start everything all over again.  You
       might think of &bigbang as more of an initializer than a constructor,
       since the function doesn't allocate new memory; it only initializes
       what's already there.  But like any other constructor, it does return a
       scalar value to use for later method invocations.

       Imagine that some day in the future, you decide that one universe just
       isn't enough.  You could write a new class from scratch, but you
       already have an existing class that does what you want--except that
       it's monadic, and you want more than just one cosmos.
               return bless($self, $class)->bigbang();
           }
           1;

       Because we were careful to be good little creators when we designed our
       Cosmos class, we can now reuse it without touching a single line of
       code when it comes time to write our Multiverse class.  The same code
       that worked when invoked as a class method continues to work perfectly
       well when invoked against separate instances of a derived class.

       The astonishing thing about the Cosmos class above is that the value
       returned by the &bigbang "constructor" is not a reference to a blessed
       object at all.  It's just the class's own name.  A class name is, for
       virtually all intents and purposes, a perfectly acceptable object.  It
       has state, behavior, and identity, the three crucial components of an
       object system.  It even manifests inheritance, polymorphism, and
       encapsulation.  And what more can you ask of an object?

       To understand object orientation in Perl, it's important to recognize
       the unification of what other programming languages might think of as
       class methods and object methods into just plain methods.  "Class
       methods" and "object methods" are distinct only in the
       compartmentalizing mind of the Perl programmer, not in the Perl
       language itself.

       Along those same lines, a constructor is nothing special either, which
       is one reason why Perl has no pre-ordained name for them.
       "Constructor" is just an informal term loosely used to describe a
       method that returns a scalar value that you can make further method
       calls against.  So long as it's either a class name or an object
       reference, that's good enough.  It doesn't even have to be a reference
       to a brand new object.

       You can have as many--or as few--constructors as you want, and you can
       name them whatever you care to.  Blindly and obediently using new() for
       each and every constructor you ever write is to speak Perl with such a
       severe C++ accent that you do a disservice to both languages.  There's
       no reason to insist that each class have but one constructor, or that a
       constructor be named new(), or that a constructor be used solely as a
       class method and not an object method.

       The next section shows how useful it can be to further distance
       ourselves from any formal distinction between class method calls and
       object method calls, both in constructors and in accessor methods.

   Translucent Attributes
       A package's eponymous hash can be used for more than just containing
       per-class, global state data.  It can also serve as a sort of template
       containing default settings for object attributes.  These default
       settings can then be used in constructors for initialization of a
       particular object.  The class's eponymous hash can also be used to
       implement translucent attributes.  A translucent attribute is one that
       has a class-wide default.  Each object can set its own value for the
       attribute, in which case "$object->attribute()" returns that value.

       Let's look at some concrete examples of using these properties before
       we show how to implement them.  Suppose that a class named Some_Class
       had a translucent data attribute called "color".  First you set the
       color in the meta-object, then you create three objects using a
       constructor that happens to be named &spawn.

           use Vermin;
           Vermin->color("vermilion");

           $ob1 = Vermin->spawn();     # so that's where Jedi come from
           $ob2 = Vermin->spawn();
           $ob3 = Vermin->spawn();

           print $obj3->color();       # prints "vermilion"

       Each of these objects' colors is now "vermilion", because that's the
       meta-object's value for that attribute, and these objects do not have
       individual color values set.

       Changing the attribute on one object has no effect on other objects
       previously created.

           $ob3->color("chartreuse");
           print $ob3->color();        # prints "chartreuse"
           print $ob1->color();        # prints "vermilion", translucently

       If you now use $ob3 to spawn off another object, the new object will
       take the color its parent held, which now happens to be "chartreuse".
       That's because the constructor uses the invoking object as its template
       for initializing attributes.  When that invoking object is the class
       name, the object used as a template is the eponymous meta-object.  When
       the invoking object is a reference to an instantiated object, the
       &spawn constructor uses that existing object as a template.

           $ob4 = $ob3->spawn();       # $ob3 now template, not %Vermin
           print $ob4->color();        # prints "chartreuse"

       Any actual values set on the template object will be copied to the new
       object.  But attributes undefined in the template object, being
       translucent, will remain undefined and consequently translucent in the
       new one as well.

       Now let's change the color attribute on the entire class:

           Vermin->color("azure");
           print $ob1->color();        # prints "azure"
           print $ob2->color();        # prints "azure"
           print $ob3->color();        # prints "chartreuse"
           print $ob4->color();        # prints "chartreuse"

       That color change took effect only in the first pair of objects, which
       were still translucently accessing the meta-object's values.  The
       second pair had per-object initialized colors, and so didn't change.
       do that.  Good taste aside, we want the answer to the question posed in
       the comment above to be "chartreuse", not "amethyst".  So we'll treat
       these attributes similar to the way process attributes like environment
       variables, user and group IDs, or the current working directory are
       treated across a fork().  You can change only yourself, but you will
       see those changes reflected in your unspawned children.  Changes to one
       object will propagate neither up to the parent nor down to any existing
       child objects.  Those objects made later, however, will see the
       changes.

       If you have an object with an actual attribute value, and you want to
       make that object's attribute value translucent again, what do you do?
       Let's design the class so that when you invoke an accessor method with
       "undef" as its argument, that attribute returns to translucency.

           $ob4->color(undef);         # back to "azure"

       Here's a complete implementation of Vermin as described above.

           package Vermin;

           # here's the class meta-object, eponymously named.
           # it holds all class attributes, and also all instance attributes
           # so the latter can be used for both initialization
           # and translucency.

           our %Vermin = (             # our() is new to perl5.6
               PopCount => 0,          # capital for class attributes
               color    => "beige",    # small for instance attributes
           );

           # constructor method
           # invoked as class method or object method
           sub spawn {
               my $obclass = shift;
               my $class   = ref($obclass) || $obclass;
               my $self = {};
               bless($self, $class);
               $class->{PopCount}++;
               # init fields from invoking object, or omit if
               # invoking object is the class to provide translucency
               %$self = %$obclass if ref $obclass;
               return $self;
           }

           # translucent accessor for "color" attribute
           # invoked as class method or object method
           sub color {
               my $self  = shift;
               my $class = ref($self) || $self;

               # handle class invocation
               unless (ref $self) {
                   $class->{color} = shift if @_;

           # accessor for "PopCount" class attribute
           # invoked as class method or object method
           # but uses object solely to locate meta-object
           sub population {
               my $obclass = shift;
               my $class   = ref($obclass) || $obclass;
               return $class->{PopCount};
           }

           # instance destructor
           # invoked only as object method
           sub DESTROY {
               my $self  = shift;
               my $class = ref $self;
               $class->{PopCount}--;
           }

       Here are a couple of helper methods that might be convenient.  They
       aren't accessor methods at all.  They're used to detect accessibility
       of data attributes.  The &is_translucent method determines whether a
       particular object attribute is coming from the meta-object.  The
       &has_attribute method detects whether a class implements a particular
       property at all.  It could also be used to distinguish undefined
       properties from non-existent ones.

           # detect whether an object attribute is translucent
           # (typically?) invoked only as object method
           sub is_translucent {
               my($self, $attr)  = @_;
               return !defined $self->{$attr};
           }

           # test for presence of attribute in class
           # invoked as class method or object method
           sub has_attribute {
               my($self, $attr)  = @_;
               my $class = ref($self) || $self;
               return exists $class->{$attr};
           }

       If you prefer to install your accessors more generically, you can make
       use of the upper-case versus lower-case convention to register into the
       package appropriate methods cloned from generic closures.

           for my $datum (keys %{ +__PACKAGE__ }) {
               *$datum = ($datum =~ /^[A-Z]/)
                   ? sub {  # install class accessor
                           my $obclass = shift;
                           my $class   = ref($obclass) || $obclass;
                           return $class->{$datum};
                         }
                   : sub { # install translucent accessor
                           my $self  = shift;
                           my $class = ref($self) || $self;

       have been left as exercises for the reader.  Be sure to send us mail as
       soon as you're done.

Class Data as Lexical Variables
   Privacy and Responsibility
       Unlike conventions used by some Perl programmers, in the previous
       examples, we didn't prefix the package variables used for class
       attributes with an underscore, nor did we do so for the names of the
       hash keys used for instance attributes.  You don't need little markers
       on data names to suggest nominal privacy on attribute variables or hash
       keys, because these are already notionally private!  Outsiders have no
       business whatsoever playing with anything within a class save through
       the mediated access of its documented interface; in other words,
       through method invocations.  And not even through just any method,
       either.  Methods that begin with an underscore are traditionally
       considered off-limits outside the class.  If outsiders skip the
       documented method interface to poke around the internals of your class
       and end up breaking something, that's not your fault--it's theirs.

       Perl believes in individual responsibility rather than mandated
       control.  Perl respects you enough to let you choose your own preferred
       level of pain, or of pleasure.  Perl believes that you are creative,
       intelligent, and capable of making your own decisions--and fully
       expects you to take complete responsibility for your own actions.  In a
       perfect world, these admonitions alone would suffice, and everyone
       would be intelligent, responsible, happy, and creative.  And careful.
       One probably shouldn't forget careful, and that's a good bit harder to
       expect.  Even Einstein would take wrong turns by accident and end up
       lost in the wrong part of town.

       Some folks get the heebie-jeebies when they see package variables
       hanging out there for anyone to reach over and alter them.  Some folks
       live in constant fear that someone somewhere might do something wicked.
       The solution to that problem is simply to fire the wicked, of course.
       But unfortunately, it's not as simple as all that.  These cautious
       types are also afraid that they or others will do something not so much
       wicked as careless, whether by accident or out of desperation.  If we
       fire everyone who ever gets careless, pretty soon there won't be
       anybody left to get any work done.

       Whether it's needless paranoia or sensible caution, this uneasiness can
       be a problem for some people.  We can take the edge off their
       discomfort by providing the option of storing class attributes as
       lexical variables instead of as package variables.  The my() operator
       is the source of all privacy in Perl, and it is a powerful form of
       privacy indeed.

       It is widely perceived, and indeed has often been written, that Perl
       provides no data hiding, that it affords the class designer no privacy
       nor isolation, merely a rag-tag assortment of weak and unenforceable
       social conventions instead.  This perception is demonstrably false and
       easily disproven.  In the next section, we show how to implement forms
       of privacy that are far stronger than those provided in nearly any
       other object-oriented language.
           sub CData1 {
               shift;  # XXX: ignore calling class/object
               $CData1 = shift if @_;
               return $CData1;
           }
           sub CData2 {
               shift;  # XXX: ignore calling class/object
               $CData2 = shift if @_;
               return $CData2;
           }

       So much for that old $Some_Class::CData1 package variable and its
       brethren!  Those are gone now, replaced with lexicals.  No one outside
       the scope can reach in and alter the class state without resorting to
       the documented interface.  Not even subclasses or superclasses of this
       one have unmediated access to $CData1.  They have to invoke the &CData1
       method against Some_Class or an instance thereof, just like anybody
       else.

       To be scrupulously honest, that last statement assumes you haven't
       packed several classes together into the same file scope, nor strewn
       your class implementation across several different files.
       Accessibility of those variables is based uniquely on the static file
       scope.  It has nothing to do with the package.  That means that code in
       a different file but the same package (class) could not access those
       variables, yet code in the same file but a different package (class)
       could.  There are sound reasons why we usually suggest a one-to-one
       mapping between files and packages and modules and classes.  You don't
       have to stick to this suggestion if you really know what you're doing,
       but you're apt to confuse yourself otherwise, especially at first.

       If you'd like to aggregate your class attributes into one lexically
       scoped, composite structure, you're perfectly free to do so.

           package Some_Class;
           my %ClassData = (
               CData1 => "",
               CData2 => "",
           );
           sub CData1 {
               shift;  # XXX: ignore calling class/object
               $ClassData{CData1} = shift if @_;
               return $ClassData{CData1};
           }
           sub CData2 {
               shift;  # XXX: ignore calling class/object
               $ClassData{CData2} = shift if @_;
               return $ClassData{CData2};
           }

       To make this more scalable as other class attributes are added, we can
       again register closures into the package symbol table to create
       accessor methods for them.

           }

       Requiring even your own class to use accessor methods like anybody else
       is probably a good thing.  But demanding and expecting that everyone
       else, be they subclass or superclass, friend or foe, will all come to
       your object through mediation is more than just a good idea.  It's
       absolutely critical to the model.  Let there be in your mind no such
       thing as "public" data, nor even "protected" data, which is a seductive
       but ultimately destructive notion.  Both will come back to bite at you.
       That's because as soon as you take that first step out of the solid
       position in which all state is considered completely private, save from
       the perspective of its own accessor methods, you have violated the
       envelope.  And, having pierced that encapsulating envelope, you shall
       doubtless someday pay the price when future changes in the
       implementation break unrelated code.  Considering that avoiding this
       infelicitous outcome was precisely why you consented to suffer the
       slings and arrows of obsequious abstraction by turning to object
       orientation in the first place, such breakage seems unfortunate in the
       extreme.

   More Inheritance Concerns
       Suppose that Some_Class were used as a base class from which to derive
       Another_Class.  If you invoke a &CData method on the derived class or
       on an object of that class, what do you get?  Would the derived class
       have its own state, or would it piggyback on its base class's versions
       of the class attributes?

       The answer is that under the scheme outlined above, the derived class
       would not have its own state data.  As before, whether you consider
       this a good thing or a bad one depends on the semantics of the classes
       involved.

       The cleanest, sanest, simplest way to address per-class state in a
       lexical is for the derived class to override its base class's version
       of the method that accesses the class attributes.  Since the actual
       method called is the one in the object's derived class if this exists,
       you automatically get per-class state this way.  Any urge to provide an
       unadvertised method to sneak out a reference to the %ClassData hash
       should be strenuously resisted.

       As with any other overridden method, the implementation in the derived
       class always has the option of invoking its base class's version of the
       method in addition to its own.  Here's an example:

           package Another_Class;
           @ISA = qw(Some_Class);

           my %ClassData = (
               CData1 => "",
           );

           sub CData1 {
               my($self, $newvalue) = @_;
               if (@_ > 1) {

       Those dabbling in multiple inheritance might be concerned about there
       being more than one override.

           for my $parent (@ISA) {
               my $methname = $parent . "::CData1";
               if ($self->can($methname)) {
                   $self->$methname($newvalue);
               }
           }

       Because the &UNIVERSAL::can method returns a reference to the function
       directly, you can use this directly for a significant performance
       improvement:

           for my $parent (@ISA) {
               if (my $coderef = $self->can($parent . "::CData1")) {
                   $self->$coderef($newvalue);
               }
           }

       If you override "UNIVERSAL::can" in your own classes, be sure to return
       the reference appropriately.

   Locking the Door and Throwing Away the Key
       As currently implemented, any code within the same scope as the file-
       scoped lexical %ClassData can alter that hash directly.  Is that ok?
       Is it acceptable or even desirable to allow other parts of the
       implementation of this class to access class attributes directly?

       That depends on how careful you want to be.  Think back to the Cosmos
       class.  If the &supernova method had directly altered $Cosmos::Stars or
       $Cosmos::Cosmos{stars}, then we wouldn't have been able to reuse the
       class when it came to inventing a Multiverse.  So letting even the
       class itself access its own class attributes without the mediating
       intervention of properly designed accessor methods is probably not a
       good idea after all.

       Restricting access to class attributes from the class itself is usually
       not enforceable even in strongly object-oriented languages.  But in
       Perl, you can.

       Here's one way:

           package Some_Class;

           {  # scope for hiding $CData1
               my $CData1;
               sub CData1 {
                   shift;      # XXX: unused
                   $CData1 = shift if @_;
                   return $CData1;
               }
           }

       This use of mediated access to class attributes is a form of privacy
       far stronger than most OO languages provide.

       The repetition of code used to create per-datum accessor methods chafes
       at our Laziness, so we'll again use closures to create similar methods.

           package Some_Class;

           {  # scope for ultra-private meta-object for class attributes
               my %ClassData = (
                   CData1 => "",
                   CData2 => "",
               );

               for my $datum (keys %ClassData ) {
                   no strict "refs";
                   *$datum = sub {
                       use strict "refs";
                       my ($self, $newvalue) = @_;
                       $ClassData{$datum} = $newvalue if @_ > 1;
                       return $ClassData{$datum};
                   }
               }

           }

       The closure above can be modified to take inheritance into account
       using the &UNIVERSAL::can method and SUPER as shown previously.

   Translucency Revisited
       The Vermin class demonstrates translucency using a package variable,
       eponymously named %Vermin, as its meta-object.  If you prefer to use
       absolutely no package variables beyond those necessary to appease
       inheritance or possibly the Exporter, this strategy is closed to you.
       That's too bad, because translucent attributes are an appealing
       technique, so it would be valuable to devise an implementation using
       only lexicals.

       There's a second reason why you might wish to avoid the eponymous
       package hash.  If you use class names with double-colons in them, you
       would end up poking around somewhere you might not have meant to poke.

           package Vermin;
           $class = "Vermin";
           $class->{PopCount}++;
           # accesses $Vermin::Vermin{PopCount}

           package Vermin::Noxious;
           $class = "Vermin::Noxious";
           $class->{PopCount}++;
           # accesses $Vermin::Noxious{PopCount}

       In the first case, because the class name had no double-colons, we got
       the hash in the current package.  But in the second case, instead of
       has a class named Eponymous::Vermin with its own %Noxious hash, but
       this kind of thing is always true.  There's no arbiter of package
       names.  It's always the case that globals like @Cwd::ISA would collide
       if more than one class uses the same Cwd package.

       If this still leaves you with an uncomfortable twinge of paranoia, we
       have another solution for you.  There's nothing that says that you have
       to have a package variable to hold a class meta-object, either for
       monadic classes or for translucent attributes.  Just code up the
       methods so that they access a lexical instead.

       Here's another implementation of the Vermin class with semantics
       identical to those given previously, but this time using no package
       variables.

           package Vermin;


           # Here's the class meta-object, eponymously named.
           # It holds all class data, and also all instance data
           # so the latter can be used for both initialization
           # and translucency.  it's a template.
           my %ClassData = (
               PopCount => 0,          # capital for class attributes
               color    => "beige",    # small for instance attributes
           );

           # constructor method
           # invoked as class method or object method
           sub spawn {
               my $obclass = shift;
               my $class   = ref($obclass) || $obclass;
               my $self = {};
               bless($self, $class);
               $ClassData{PopCount}++;
               # init fields from invoking object, or omit if
               # invoking object is the class to provide translucency
               %$self = %$obclass if ref $obclass;
               return $self;
           }

           # translucent accessor for "color" attribute
           # invoked as class method or object method
           sub color {
               my $self  = shift;

               # handle class invocation
               unless (ref $self) {
                   $ClassData{color} = shift if @_;
                   return $ClassData{color}
               }

               # handle object invocation
               $self->{color} = shift if @_;

           # instance destructor; invoked only as object method
           sub DESTROY {
               $ClassData{PopCount}--;
           }

           # detect whether an object attribute is translucent
           # (typically?) invoked only as object method
           sub is_translucent {
               my($self, $attr)  = @_;
               $self = \%ClassData if !ref $self;
               return !defined $self->{$attr};
           }

           # test for presence of attribute in class
           # invoked as class method or object method
           sub has_attribute {
               my($self, $attr)  = @_;
               return exists $ClassData{$attr};
           }

NOTES
       Inheritance is a powerful but subtle device, best used only after
       careful forethought and design.  Aggregation instead of inheritance is
       often a better approach.

       You can't use file-scoped lexicals in conjunction with the SelfLoader
       or the AutoLoader, because they alter the lexical scope in which the
       module's methods wind up getting compiled.

       The usual mealy-mouthed package-munging doubtless applies to setting up
       names of object attributes.  For example, "$self->{ObData1}" should
       probably be "$self->{ __PACKAGE__ . "_ObData1" }", but that would just
       confuse the examples.

SEE ALSO
       perltoot, perlobj, perlmod, and perlbot.

       The Tie::SecureHash and Class::Data::Inheritable modules from CPAN are
       worth checking out.

AUTHOR AND COPYRIGHT
       Copyright (c) 1999 Tom Christiansen.  All rights reserved.

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

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

ACKNOWLEDGEMENTS

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