source: trunk/third/perl/pod/perltooc.pod @ 20075

=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 unenforcible
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);
        }
    } 

=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 enforcible 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-mungeing 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