perltooc(1)
NAME
perltooc - Tom's OO Tutorial for Class Data in Perl
DESCRIPTION
When designing an object class, you are sometimes faced with the situa-
tion of wanting common state shared by all objects of that class. Such
class attributes act somewhat like global variables for the entire
class, but unlike program-wide globals, class attributes have meaning
only to the class itself.
Here are a few examples where class attributes might come in handy:
o to keep a count of the objects you've created, or how many are
still extant.
o to extract the name or file descriptor for a logfile used by a
debugging method.
o to access collective data, like the total amount of cash dispensed
by all ATMs in a network in a given day.
o to access the last object created by a class, or the most accessed
object, or to retrieve a list of all objects.
Unlike a true global, class attributes should not be accessed directly.
Instead, their state should be inspected, and perhaps altered, only
through the mediated access of class methods. These class attributes
accessor methods are similar in spirit and function to accessors used
to manipulate the state of instance attributes on an object. They pro-
vide 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 mecha-
nism 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 con-
sists 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 lexi-
cal 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 vari-
ables 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 sim-
plest 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 $Some_Class::CData2;
}
This technique is highly legible and should be completely straightfor-
ward to even the novice Perl programmer. By fully qualifying the pack-
age variables, they stand out clearly when reading the code. Unfortu-
nately, if you misspell one of these, you've introduced an error that's
hard to catch. It's also somewhat disconcerting to see the class name
itself hard-coded in so many places.
Both these problems can be easily fixed. Just add the "use strict"
pragma, then pre-declare your package variables. (The "our" operator
will be new in 5.6, and will work for package globals just like "my"
works for scoped lexicals.)
package Some_Class;
use strict;
our($CData1, $CData2); # our() is new to perl5.6
sub CData1 {
shift; # XXX: ignore calling class/object
$CData1 = shift if @_;
return $CData1;
}
sub CData2 {
shift; # XXX: ignore calling class/object
$CData2 = shift if @_;
return $CData2;
}
As with any other global variable, some programmers prefer to start
their package variables with capital letters. This helps clarity some-
what, but by no longer fully qualifying the package variables, their
significance can be lost when reading the code. You can fix this eas-
ily 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 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 pro-
grammer feels to factor out duplicate code whenever possible.
Here's what to do. First, make just one hash to hold all class
attributes.
package Some_Class;
use strict;
our %ClassData = ( # our() is new to perl5.6
CData1 => "",
CData2 => "",
);
Using closures (see perlref) and direct access to the package symbol
table (see perlmod), now clone an accessor method for each key in the
%ClassData hash. Each of these methods is used to fetch or store val-
ues 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 advan-
tages. They're easy to spot, initialize, and change. The aggregation
also makes them convenient to access externally, such as from a debug-
ger or a persistence package. The only possible problem is that we
don't automatically know the name of each class's class object, should
it have one. This issue is addressed below in "The Eponymous
Meta-Object".
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,
$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 pro-
vides 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 vari-
ables 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 ta-
ble.
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 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 epony-
mously 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, 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 pro-
duce a real reference (the reverse is not true, though). So we'll cre-
ate 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 refer-
ence 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 perltoot and perlbot,
but there may be variations in the example below that you haven't
thought of before.
package Some_Class;
our($CData1, $CData2); # our() is new to perl5.6
sub new {
my $obclass = shift;
return bless my $self = {
ObData1 => "",
ObData2 => "",
CData1 => \$CData1,
CData2 => \$CData2,
} => (ref $obclass || $obclass);
}
sub ObData1 {
my $self = shift;
$self->{ObData1} = shift if @_;
return $self->{ObData1};
}
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.
Monadic Classes
Some of the standard modules shipped with Perl provide class interfaces
without any attribute methods whatsoever. The most commonly used mod-
ule 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 names-
pace 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 superflu-
ous.
Another sort of class that pops up from time to time is one that sup-
ports 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 $self must hold a reference to a blessed object.
It merely has to be something you can invoke methods on. Here the
package name itself, Cosmos, works as an object. Look at the &super-
nova method. Is that a class method or an object method? The answer
is that static analysis cannot reveal the answer. Perl doesn't care,
and neither should you. In the three attribute methods, %$self is
really accessing the %Cosmos package variable.
If like Stephen Hawking, you posit the existence of multiple, sequen-
tial, 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 encap-
sulation. 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 meth-
ods" 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. "Construc-
tor" 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 our-
selves from any formal distinction between class method calls and
object method calls, both in constructors and in accessor methods.
Translucent Attributes
A package's eponymous hash can be used for more than just containing
per-class, global state data. It can also serve as a sort of template
containing default settings for object attributes. These default set-
tings can then be used in constructors for initialization of a particu-
lar 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 translu-
cency. 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 con-
structor 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 sec-
ond 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
"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;
$class->{PopCount}--;
}
Here are a couple of helper methods that might be convenient. They
aren't accessor methods at all. They're used to detect accessibility
of data attributes. The &is_translucent method determines whether a
particular object attribute is coming from the meta-object. The
&has_attribute method detects whether a class implements a particular
property at all. It could also be used to distinguish undefined prop-
erties from non-existent ones.
# detect whether an object attribute is translucent
# (typically?) invoked only as object method
sub is_translucent {
my($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.
Class Data as Lexical Variables
Privacy and Responsibility
Unlike conventions used by some Perl programmers, in the previous exam-
ples, we didn't prefix the package variables used for class attributes
with an underscore, nor did we do so for the names of the hash keys
used for instance attributes. You don't need little markers on data
names to suggest nominal privacy on attribute variables or hash keys,
because these are already notionally private! Outsiders have no busi-
ness 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 break-
ing something, that's not your fault--it's theirs.
Perl believes in individual responsibility rather than mandated con-
trol. 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 hang-
ing 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 any-
body 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 discom-
fort by providing the option of storing class attributes as lexical
variables instead of as package variables. The my() operator is the
source of all privacy in Perl, and it is a powerful form of privacy
indeed.
It is widely perceived, and indeed has often been written, that Perl
provides no data hiding, that it affords the class designer no privacy
nor isolation, merely a rag-tag assortment of weak and unenforceable
social conventions instead. This perception is demonstrably false and
easily disproven. In the next section, we show how to implement forms
of privacy that are far stronger than those provided in nearly any
other object-oriented language.
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. Accessibil-
ity 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 acces-
sor 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 implementa-
tion break unrelated code. Considering that avoiding this infelicitous
outcome was precisely why you consented to suffer the slings and arrows
of obsequious abstraction by turning to object orientation in the first
place, such breakage seems unfortunate in the extreme.
More Inheritance Concerns
Suppose that Some_Class were used as a base class from which to derive
Another_Class. If you invoke a &CData method on the derived class or
on an object of that class, what do you get? Would the derived class
have its own state, or would it piggyback on its base class's versions
of the class attributes?
The answer is that under the scheme outlined above, the derived class
would not have its own state data. As before, whether you consider
this a good thing or a bad one depends on the semantics of the classes
involved.
The cleanest, sanest, simplest way to address per-class state in a lex-
ical is for the derived class to override its base class's version of
the method that accesses the class attributes. Since the actual method
called is the one in the object's derived class if this exists, you
automatically get per-class state this way. Any urge to provide an
unadvertised method to sneak out a reference to the %ClassData hash
should be strenuously resisted.
As with any other overridden method, the implementation in the derived
class always has the option of invoking its base class's version of the
method in addition to its own. Here's an example:
package Another_Class;
@ISA = qw(Some_Class);
my %ClassData = (
CData1 => "",
);
sub CData1 {
my($self, $newvalue) = @_;
if (@_ > 1) {
# set locally first
$ClassData{CData1} = $newvalue;
# then pass the buck up to the first
# overridden version, if there is one
if ($self->can("SUPER::CData1")) {
$self->SUPER::CData1($newvalue);
}
}
return $ClassData{CData1};
}
Those dabbling in multiple inheritance might be concerned about there
being more than one override.
for my $parent (@ISA) {
my $methname = $parent . "::CData1";
if ($self->can($methname)) {
$self->$methname($newvalue);
}
}
Because the &UNIVERSAL::can method returns a reference to the function
directly, you can use this directly for a significant performance
improvement:
for my $parent (@ISA) {
if (my $coderef = $self->can($parent . "::CData1")) {
$self->$coderef($newvalue);
}
}
If you override "UNIVERSAL::can" in your own classes, be sure to return
the reference appropriately.
Locking the Door and Throwing Away the Key
As currently implemented, any code within the same scope as the file-
scoped lexical %ClassData can alter that hash directly. Is that ok?
Is it acceptable or even desirable to allow other parts of the imple-
mentation of this class to access class attributes directly?
That depends on how careful you want to be. Think back to the Cosmos
class. If the &supernova method had directly altered $Cosmos::Stars or
$Cosmos::Cosmos{stars}, then we wouldn't have been able to reuse the
class when it came to inventing a Multiverse. So letting even the
class itself access its own class attributes without the mediating
intervention of properly designed accessor methods is probably not a
good idea after all.
Restricting access to class attributes from the class itself is usually
not enforceable even in strongly object-oriented languages. But in
Perl, you can.
Here's one way:
package Some_Class;
{ # scope for hiding $CData1
my $CData1;
sub CData1 {
shift; # XXX: unused
$CData1 = shift if @_;
return $CData1;
}
}
{ # 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.
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 pack-
age 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 pack-
age 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 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 meth-
ods so that they access a lexical instead.
Here's another implementation of the Vermin class with semantics iden-
tical to those given previously, but this time using no package vari-
ables.
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 care-
ful forethought and design. Aggregation instead of inheritance is
often a better approach.
You can't use file-scoped lexicals in conjunction with the SelfLoader
or the AutoLoader, because they alter the lexical scope in which the
module's methods wind up getting compiled.
The usual mealy-mouthed package-munging doubtless applies to setting up
names of object attributes. For example, "$self->{ObData1}" should
probably be "$self->{ __PACKAGE__ . "_ObData1" }", but that would just
confuse the examples.
SEE ALSO
perltoot, perlobj, perlmod, and perlbot.
The Tie::SecureHash and Class::Data::Inheritable modules from CPAN are
worth checking out.
AUTHOR AND COPYRIGHT
Copyright (c) 1999 Tom Christiansen. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file are
hereby placed into the public domain. You are permitted and encouraged
to use this code in your own programs for fun or for profit as you see
fit. A simple comment in the code giving credit would be courteous but
is not required.
ACKNOWLEDGEMENTS
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.
HISTORY
Last edit: Sun Feb 4 20:50:28 EST 2001
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