PERLTOOTC(1) Perl Programmers Reference Guide PERLTOOTC(1)NAMEperltootc - Tom's OO Tutorial for Class Data in Perl
DESCRIPTION
When designing an object class, you are sometimes faced
with the situation of wanting common state shared by all
objects of that class. Such class attributes act somewhat
like global variables for the entire class, but unlike
program-wide globals, class attributes have meaning only
to the class itself.
Here are a few examples where class attributes might come
in handy:
to keep a count of the objects you've created, or how
many are still extant.
to extract the name or file descriptor for a logfile
used by a debugging method.
to access collective data, like the total amount of
cash dispensed by all ATMs in a network in a given
day.
to access the last object created by a class, or the
most accessed object, or to retrieve a list of all
objects.
Unlike a true global, class attributes should not be
accessed directly. Instead, their state should be
inspected, and perhaps altered, only through the mediated
access of class methods. These class attributes accessor
methods are similar in spirit and function to accessors
used to manipulate the state of instance attributes on an
object. They provide a clear firewall between interface
and implementation.
You should allow access to class attributes through either
the class name or any object of that class. If we assume
that $an_object is of type Some_Class, and the
&Some_Class::population_count method accesses class
attributes, then these two invocations should both be pos
sible, 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 pro
vides 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 situa
tion.
A class in Perl is typically implemented in a module. A
module consists of two complementary feature sets: a pack
age 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.
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.
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 vari
ables 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 $Some_Class::CData2;
}
This technique is highly legible and should be completely
straightforward to even the novice Perl programmer. By
fully qualifying the package variables, they stand out
clearly when reading the code. Unfortunately, if you mis
spell one of these, you've introduced an error that's hard
to catch. It's also somewhat disconcerting to see the
class name itself hard-coded in so many places.
Both these problems can be easily fixed. Just add the
"use strict" pragma, then pre-declare your package vari
ables. (The "our" operator will be new in 5.6, and will
work for package globals just like "my" works for scoped
lexicals.)
package Some_Class;
use strict;
our($CData1, $CData2); # our() is new to perl5.6
sub CData1 {
shift; # XXX: ignore calling class/object
$CData1 = shift if @_;
return $CData1;
}
sub CData2 {
shift; # XXX: ignore calling class/object
$CData2 = shift if @_;
return $CData2;
}
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 quali
fying 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.
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
the perltoot manpage), 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 pro
grammer: Laziness, here manifesting as that innate urge
every programmer feels to factor out duplicate code when
ever 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 the perlref manpage) and direct access
to the package symbol table (see the perlmod manpage), 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 acci
dentally 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 per
spective 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 sev
eral 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 persis
tence package. The only possible problem is that we don't
automatically know the name of each class's class object,
should it have one. This issue is addressed below in the
section on "The Eponymous Meta-Object".
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 com
piled. 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
$Some_Class::CData1--or in the method cloning version,
"$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 sub
classes 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 sup
port that.
You'll have to decide on a case-by-case basis whether it
makes any sense for class attributes to be package-rela
tive. If you want it to be so, then stop ignoring the
first argument to the function. Either it will be a pack
age 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 vari
ables 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};
}
The Eponymous Meta-Object
It could be argued that the %ClassData hash in the previ
ous 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 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, trans
parent persistence, or checkpointing. It's also the obvi
ous 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 refer
ence (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};
}
}
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 the perltoot manpage and the perlbot
manpage, 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 meth
ods, 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, mak
ing 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 innova
tive uses for this powerful feature.
Monadic Classes
Some of the standard modules shipped with Perl provide
class interfaces without any attribute methods whatsoever.
The most commonly used module not numbered amongst the
pragmata, the Exporter module, is a class with neither
constructors nor attributes. Its job is simply to provide
a standard interface for modules wishing to export part of
their namespace into that of their caller. Modules use
the Exporter's &import method by setting their inheritance
list in their package's @ISA array to mention "Exporter".
But class Exporter provides no constructor, so you can't
have several instances of the class. In fact, you can't
have any--it just doesn't make any sense. All you get is
its methods. Its interface contains no statefulness, so
state data is wholly superfluous.
Another sort of class that pops up from time to time is
one that supports a unique instance. Such classes are
called monadic classes, or less formally, singletons or
highlander classes.
If a class is monadic, where do you store its state, that
is, its attributes? How do you make sure that there's
never more than one instance? While you could merely use
a slew of package variables, it's a lot cleaner to use the
eponymously named hash. Here's a complete example of a
monadic class:
package Cosmos;
%Cosmos = ();
# accessor method for "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 $self
must hold a reference to a blessed object. It merely has
to be something you can invoke methods on. Here the pack
age name itself, Cosmos, works as an object. Look at the
&supernova method. Is that a class method or an object
method? The answer is that static analysis cannot reveal
the answer. Perl doesn't care, and neither should you.
In the three attribute methods, "%$self" is really access
ing the %Cosmos package variable.
If like Stephen Hawking, you posit the existence of multi
ple, 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 func
tion 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 invoca
tions.
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 identify, the three crucial com
ponents of an object system. It even manifests inheri
tance, 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 meth
ods 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 refer
ence 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 that constructor be named new(), or that that con
structor 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 con
structors and in accessor methods.
Translucent Attributes
A package's eponymous hash can be used for more than just
containing per-class, global state data. It can also
serve as a sort of template containing default settings
for object attributes. These default settings can then be
used in constructors for initialization of a particular
object. The class's eponymous hash can also be used to
implement translucent attributes. A translucent attribute
is one that has a class-wide default. Each object can set
its own value for the attribute, in which case
"$object->attribute()" returns that value. But if no
value has been set, then "$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 val
ues. Objects with their own initialized values, however,
won't see any change.
Let's look at some concrete examples of using these prop
erties 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 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 hap
pens 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 ini
tialized 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 "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;
$class->{PopCount}--;
}
Here are a couple of helper methods that might be conve
nient. They aren't accessor methods at all. They're used
to detect accessibility of data attributes. The
&is_translucent method determines whether a particular
object attribute is coming from the meta-object. The
&has_attribute method detects whether a class implements a
particular property at all. It could also be used to dis
tinguish undefined properties from non-existent ones.
# detect whether an object attribute is translucent
# (typically?) invoked only as object method
sub is_translucent {
my($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 if $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 con
vention 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.
Class Data as Lexical Variables
Privacy and Responsibility
Unlike conventions used by some Perl programmers, in the
previous examples, we didn't prefix the package variables
used for class attributes with an underscore, nor did we
do so for the names of the hash keys used for instance
attributes. You don't need little markers on data names
to suggest nominal privacy on attribute variables or hash
keys, because these are already notionally private! Out
siders have no business whatsoever playing with anything
within a class save through the mediated access of its
documented interface; in other words, through method invo
cations. 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 cau
tious types are also afraid that they or others will do
something not so much wicked as careless, whether by acci
dent 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.
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 lexi
cals. 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 differ
ent 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 super
class, 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 encapsu
lating 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 ori
entation in the first place, such breakage seems unfortu
nate in the extreme.
More Inheritance Concerns
Suppose that Some_Class were used as a base class from
which to derive Another_Class. If you invoke a &CData
method on the derived class or on an object of that class,
what do you get? Would the derived class have its own
state, or would it piggyback on its base class's versions
of the class attributes?
The answer is that under the scheme outlined above, the
derived class would not have its own state data. As
before, whether you consider this a good thing or a bad
one depends on the semantics of the classes involved.
The cleanest, sanest, simplest way to address per-class
state in a lexical is for the derived class to override
its base class's version of the method that accesses the
class attributes. Since the actual method called is the
one in the object's derived class if this exists, you
automatically get per-class state this way. Any urge to
provide an unadvertised method to sneak out a reference to
the %ClassData hash should be strenuously resisted.
As with any other overridden method, the implementation in
the derived class always has the option of invoking its
base class's version of the method in addition to its own.
Here's an example:
package Another_Class;
@ISA = qw(Some_Class);
my %ClassData = (
CData1 => "",
);
sub CData1 {
my($self, $newvalue) = @_;
if (@_ > 1) {
# 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);
}
}
Locking the Door and Throwing Away the Key
As currently implemented, any code within the same scope
as the file-scoped lexical %ClassData can alter that hash
directly. Is that ok? Is it acceptable or even desirable
to allow other parts of the implementation of this class
to access class attributes directly?
That depends on how careful you want to be. Think back to
the Cosmos class. If the &supernova method had directly
altered $Cosmos::Stars or "$Cosmos::Cosmos{stars}", then
we wouldn't have been able to reuse the class when it came
to inventing a Multiverse. So letting even the class
itself access its own class attributes without the mediat
ing 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 clo
sures to create similar methods.
package Some_Class;
{ # scope for ultra-private meta-object for class attributes
my %ClassData = (
CData1 => "",
CData2 => "",
);
for my $datum (keys %ClassData ) {
no strict "refs";
*$datum = sub {
use strict "refs";
my ($self, $newvalue) = @_;
$ClassData{$datum} = $newvalue if @_ > 1;
return $ClassData{$datum};
}
}
}
The closure above can be modified to take inheritance into
account using the &UNIVERSAL::can method and SUPER as
shown previously.
Translucency Revisited
The Vermin class demonstrates translucency using a package
variable, eponymously named %Vermin, as its meta-object.
If you prefer to use absolutely no package variables
beyond those necessary to appease inheritance or possibly
the Exporter, this strategy is closed to you. That's too
bad, because translucent attributes are an appealing tech
nique, 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 dou
ble-colons in them, you would end up poking around some
where 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 cur
rent package, we got the hash %Noxious in the Vermin pack
age. (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 dou
ble-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 Hierony
mus::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 noth
ing 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};
}
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.
We use the hypothetical our() syntax for package vari
ables. It works like "use vars", but looks like my(). It
should be in this summer's major release (5.6) of perl--we
hope.
You can't use file-scoped lexicals in conjunction with the
SelfLoader or the AutoLoader, because they alter the lexi
cal 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,
"$self->{ObData1}" should probably be "$self->{ __PACK
AGE__ . "_ObData1" }", but that would just confuse the
examples.
SEE ALSO
the perltoot manpage, the perlobj manpage, the perlmod
manpage, and the perlbot manpage.
The Tie::SecureHash and Class::Data::Inheritable modules
from CPAN are worth checking out.
AUTHOR AND COPYRIGHT
Copyright (c) 1999 Tom Christiansen. All rights reserved.
When included as part of the Standard Version of Perl, or
as part of its complete documentation whether printed or
otherwise, this work may be distributed only under the
terms of Perl's Artistic License. Any distribution of
this file or derivatives thereof outside of that package
require that special arrangements be made with copyright
holder.
Irrespective of its distribution, all code examples in
this file are hereby placed into the public domain. You
are permitted and encouraged to use this code in your own
programs for fun or for profit as you see fit. A simple
comment in the code giving credit would be courteous but
is not required.
ACKNOWLEDGEMENTS
Russ Albery, 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.
HISTORY
Last edit: Sun Feb 4 20:50:28 EST 2001
2001-03-18 perl v5.6.1 PERLTOOTC(1)
