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+=head1 NAME
+
+perltooc - Tom's OO Tutorial for Class Data in Perl
+
+=head1 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 I<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:
+
+=over 4
+
+=item *
+
+to keep a count of the objects you've created, or how many are
+still extant.
+
+=item *
+
+to extract the name or file descriptor for a logfile used by a debugging
+method.
+
+=item *
+
+to access collective data, like the total amount of cash dispensed by
+all ATMs in a network in a given day.
+
+=item *
+
+to access the last object created by a class, or the most accessed object,
+or to retrieve a list of all objects.
+
+=back
+
+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 I<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 package
+variables, you can make your class attribute accessor methods either ignorant
+of inheritance or sensitive to it. If you choose lexical variables,
+you can elect to permit access to them from anywhere in the entire file
+scope, or you can limit direct data access exclusively to the methods
+implementing those attributes.
+
+=head1 Class Data in a Can
+
+One of the easiest ways to solve a hard problem is to let someone else
+do it for you! In this case, Class::Data::Inheritable (available on a
+CPAN near you) offers a canned solution to the class data problem
+using closures. So before you wade into this document, consider
+having a look at that module.
+
+
+=head1 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 $Some_Class::CData2;
+ }
+
+This technique is highly legible and should be completely straightforward
+to even the novice Perl programmer. By fully qualifying the package
+variables, they stand out clearly when reading the code. Unfortunately,
+if you misspell one of these, you've introduced an error that's hard
+to catch. It's also somewhat disconcerting to see the class name itself
+hard-coded in so many places.
+
+Both these problems can be easily fixed. Just add the C<use strict>
+pragma, then pre-declare your package variables. (The C<our> operator
+will be new in 5.6, and will work for package globals just like C<my>
+works for scoped lexicals.)
+
+ package Some_Class;
+ use strict;
+ our($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;
+ }
+
+
+As with any other global variable, some programmers prefer to start their
+package variables with capital letters. This helps clarity somewhat, but
+by no longer fully qualifying the package variables, their significance
+can be lost when reading the code. You can fix this easily enough by
+choosing better names than were used here.
+
+=head2 Putting All Your Eggs in One Basket
+
+Just as the mindless enumeration of accessor methods for instance attributes
+grows tedious after the first few (see L<perltoot>), so too does the
+repetition begin to grate when listing out accessor methods for class
+data. Repetition runs counter to the primary virtue of a 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 L<perlref>) and direct access to the package symbol
+table (see L<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 L<"The Eponymous Meta-Object">.
+
+=head2 Inheritance Concerns
+
+Suppose you have an instance of a derived class, and you access class
+data using an inherited method call. Should that end up referring
+to the base class's attributes, or to those in the derived class?
+How would it work in the earlier examples? The derived class inherits
+all the base class's methods, including those that access class attributes.
+But what package are the class attributes stored in?
+
+The answer is that, as written, class attributes are stored in the package into
+which those methods were compiled. When you invoke the &CData1 method
+on the name of the derived class or on one of that class's objects, the
+version shown above is still run, so you'll access $Some_Class::CData1--or
+in the method cloning version, C<$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};
+ }
+
+=head2 The Eponymous Meta-Object
+
+It could be argued that the %ClassData hash in the previous example is
+neither the most imaginative nor the most intuitive of names. Is there
+something else that might make more sense, be more useful, or both?
+
+As it happens, yes, there is. For the "class meta-object", we'll use
+a package variable of the same name as the package itself. Within the
+scope of a package Some_Class declaration, we'll use the eponymously
+named hash %Some_Class as that class's meta-object. (Using an eponymously
+named hash is somewhat reminiscent of classes that name their constructors
+eponymously in the Python or C++ fashion. That is, class Some_Class would
+use &Some_Class::Some_Class as a constructor, probably even exporting that
+name as well. The StrNum class in Recipe 13.14 in I<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};
+ }
+
+Even though we're using symbolic references for good not evil, some
+folks tend to become unnerved when they see so many places with strict
+ref checking disabled. Given a symbolic reference, you can always
+produce a real reference (the reverse is not true, though). So we'll
+create a subroutine that does this conversion for us. If invoked as a
+function of no arguments, it returns a reference to the compiling class's
+eponymous hash. Invoked as a class method, it returns a reference to
+the eponymous hash of its caller. And when invoked as an object method,
+this function returns a reference to the eponymous hash for whatever
+class the object belongs to.
+
+ 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};
+ }
+ }
+
+=head2 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 L<perltoot> and
+L<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};
+ }
+
+ sub ObData2 {
+ my $self = shift;
+ $self->{ObData2} = shift if @_;
+ return $self->{ObData2};
+ }
+
+ sub CData1 {
+ my $self = shift;
+ my $dataref = ref $self
+ ? $self->{CData1}
+ : \$CData1;
+ $$dataref = shift if @_;
+ return $$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;
+ $self->{ObData2} = shift if @_;
+ return $self->{ObData2};
+ }
+
+ sub CData1 {
+ my $self = shift;
+ $self = $self->_classobj() unless ref $self;
+ my $dataref = $self->{CData1};
+ $$dataref = shift if @_;
+ return $$dataref;
+ }
+
+ 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.
+
+=head2 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 I<monadic classes>, or less
+formally, I<singletons> or I<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 "name" attribute
+ sub name {
+ my $self = shift;
+ $self->{name} = shift if @_;
+ return $self->{name};
+ }
+
+ # read-only accessor method for "birthday" attribute
+ sub birthday {
+ my $self = shift;
+ die "can't reset birthday" if @_; # XXX: croak() is better
+ return $self->{birthday};
+ }
+
+ # 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 $self must hold a reference to a blessed object. It merely
+has to be something you can invoke methods on. Here the package name
+itself, Cosmos, works as an object. Look at the &supernova method. Is that
+a class method or an object method? The answer is that static analysis
+cannot reveal the answer. Perl doesn't care, and neither should you.
+In the three attribute methods, C<%$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.
+
+That's what code reuse via subclassing is all about. Look how short
+the new code is:
+
+ package Multiverse;
+ use Cosmos;
+ @ISA = qw(Cosmos);
+
+ sub new {
+ my $protoverse = shift;
+ my $class = ref($protoverse) || $protoverse;
+ my $self = {};
+ return bless($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.
+
+=head2 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 I<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 C<< $object->attribute() >> returns that value.
+But if no value has been set, then C<< $object->attribute() >> returns
+the class-wide default.
+
+We'll apply something of a copy-on-write approach to these translucent
+attributes. If you're just fetching values from them, you get
+translucency. But if you store a new value to them, that new value is
+set on the current object. On the other hand, if you use the class as
+an object and store the attribute value directly on the class, then the
+meta-object's value changes, and later fetch operations on objects with
+uninitialized values for those attributes will retrieve the meta-object's
+new values. Objects with their own initialized values, however, won't
+see any change.
+
+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.
+
+One important question remains. Changes to the meta-object are reflected
+in translucent attributes in the entire class, but what about
+changes to discrete objects? If you change the color of $ob3, does the
+value of $ob4 see that change? Or vice-versa. If you change the color
+of $ob4, does then the value of $ob3 shift?
+
+ $ob3->color("amethyst");
+ print $ob3->color(); # prints "amethyst"
+ print $ob4->color(); # hmm: "chartreuse" or "amethyst"?
+
+While one could argue that in certain rare cases it should, let's not
+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
+C<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 @_;
+ return $class->{color}
+ }
+
+ # handle object invocation
+ $self->{color} = shift if @_;
+ if (defined $self->{color}) { # not exists!
+ return $self->{color};
+ } else {
+ return $class->{color};
+ }
+ }
+
+ # 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;
+ unless (ref $self) {
+ $class->{$datum} = shift if @_;
+ return $class->{$datum}
+ }
+ $self->{$datum} = shift if @_;
+ return defined $self->{$datum}
+ ? $self -> {$datum}
+ : $class -> {$datum}
+ }
+ }
+
+Translations of this closure-based approach into C++, Java, and Python
+have been left as exercises for the reader. Be sure to send us mail as
+soon as you're done.
+
+=head1 Class Data as Lexical Variables
+
+=head2 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 B<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.
+
+=head2 File-Scoped Lexicals
+
+A lexical variable is visible only through the end of its static scope.
+That means that the only code able to access that variable is code
+residing textually below the my() operator through the end of its block
+if it has one, or through the end of the current file if it doesn't.
+
+Starting again with our simplest example given at the start of this
+document, we replace our() variables with my() versions.
+
+ package Some_Class;
+ my($CData1, $CData2); # file scope, not in any package
+ 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.
+
+ package Some_Class;
+ my %ClassData = (
+ CData1 => "",
+ CData2 => "",
+ );
+ for my $datum (keys %ClassData) {
+ no strict "refs";
+ *$datum = sub {
+ shift; # XXX: ignore calling class/object
+ $ClassData{$datum} = shift if @_;
+ return $ClassData{$datum};
+ };
+ }
+
+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.
+
+=head2 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 B<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) {
+ # set locally first
+ $ClassData{CData1} = $newvalue;
+
+ # then pass the buck up to the first
+ # overridden version, if there is one
+ if ($self->can("SUPER::CData1")) {
+ $self->SUPER::CData1($newvalue);
+ }
+ }
+ return $ClassData{CData1};
+ }
+
+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 C<UNIVERSAL::can> in your own classes, be sure to return the
+reference appropriately.
+
+=head2 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 $Cosmos::Stars or
+C<$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;
+ }
+ }
+
+ { # scope for hiding $CData2
+ my $CData2;
+ sub CData2 {
+ shift; # XXX: unused
+ $CData2 = shift if @_;
+ return $CData2;
+ }
+ }
+
+No one--absolutely no one--is allowed to read or write the class
+attributes without the mediation of the managing accessor method, since
+only that method has access to the lexical variable it's managing.
+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.
+
+=head2 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
+getting some hash in the current package, we got the hash %Noxious in
+the Vermin package. (The noxious vermin just invaded another package and
+sprayed their data around it. :-) Perl doesn't support relative packages
+in its naming conventions, so any double-colons trigger a fully-qualified
+lookup instead of just looking in the current package.
+
+In practice, it is unlikely that the Vermin class had an existing
+package variable named %Noxious that you just blew away. If you're
+still mistrustful, you could always stake out your own territory
+where you know the rules, such as using Eponymous::Vermin::Noxious or
+Hieronymus::Vermin::Boschious or Leave_Me_Alone::Vermin::Noxious as class
+names instead. Sure, it's in theory possible that someone else 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 @_;
+ if (defined $self->{color}) { # not exists!
+ return $self->{color};
+ } else {
+ return $ClassData{color};
+ }
+ }
+
+ # class attribute accessor for "PopCount" attribute
+ # invoked as class method or object method
+ sub population {
+ return $ClassData{PopCount};
+ }
+
+ # 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};
+ }
+
+=head1 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, C<< $self->{ObData1} >>
+should probably be C<< $self->{ __PACKAGE__ . "_ObData1" } >>, but that
+would just confuse the examples.
+
+=head1 SEE ALSO
+
+L<perltoot>, L<perlobj>, L<perlmod>, and L<perlbot>.
+
+The Tie::SecureHash and Class::Data::Inheritable modules from CPAN are
+worth checking out.
+
+=head1 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.
+
+=head1 ACKNOWLEDGEMENTS
+
+Russ Allbery, Jon Orwant, Randy Ray, Larry Rosler, Nat Torkington,
+and Stephen Warren all contributed suggestions and corrections to this
+piece. Thanks especially to Damian Conway for his ideas and feedback,
+and without whose indirect prodding I might never have taken the time
+to show others how much Perl has to offer in the way of objects once
+you start thinking outside the tiny little box that today's "popular"
+object-oriented languages enforce.
+
+=head1 HISTORY
+
+Last edit: Sun Feb 4 20:50:28 EST 2001