perltooc
(1)
Name
perltooc - Tom's OO Tutorial for Class Data in Perl
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide 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 situation of wanting common state shared by all objects
of that class. Such class attributes act somewhat like
global variables for the entire class, but unlike program-
wide globals, class attributes have meaning only to the
class itself.
Here are a few examples where class attributes might come in
handy:
o to keep a count of the objects you've created, or how
many are still extant.
o to extract the name or file descriptor for a logfile
used by a debugging method.
o to access collective data, like the total amount of cash
dispensed by all ATMs in a network in a given day.
o to access the last object created by a class, or the
most accessed object, or to retrieve a list of all
objects.
Unlike a true global, class attributes should not be
accessed directly. Instead, their state should be
inspected, and perhaps altered, only through the mediated
access of class methods. These class attributes accessor
methods are similar in spirit and function to accessors used
to manipulate the state of instance attributes on an object.
They provide a clear firewall between interface and
implementation.
You should allow access to class attributes through either
the class name or any object of that class. If we assume
that $an_object is of type Some_Class, and the
&Some_Class::population_count method accesses class
attributes, then these two invocations should both be
possible, and almost certainly equivalent.
Some_Class->population_count()
$an_object->population_count()
The question is, where do you store the state which that
method accesses? Unlike more restrictive languages like
C++, where these are called static data members, Perl
provides no syntactic mechanism to declare class attributes,
any more than it provides a syntactic mechanism to declare
instance attributes. Perl provides the developer with a
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broad set of powerful but flexible features that can be
uniquely crafted to the particular demands of the situation.
A class in Perl is typically implemented in a module. A
module consists of two complementary feature sets: a package
for interfacing with the outside world, and a lexical file
scope for privacy. Either of these two mechanisms can be
used to implement class attributes. That means you get to
decide whether to put your class attributes in package
variables or to put them in lexical variables.
And those aren't the only decisions to make. If you choose
to use package variables, you can make your class attribute
accessor methods either ignorant of inheritance or sensitive
to it. If you choose lexical variables, you can elect to
permit access to them from anywhere in the entire file
scope, or you can limit direct data access exclusively to
the methods implementing those attributes.
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 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.
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package Some_Class;
sub CData1 {
shift; # XXX: ignore calling class/object
$Some_Class::CData1 = shift if @_;
return $Some_Class::CData1;
}
sub CData2 {
shift; # XXX: ignore calling class/object
$Some_Class::CData2 = shift if @_;
return $Some_Class::CData2;
}
This technique is highly legible and should be completely
straightforward to even the novice Perl programmer. By
fully qualifying the package variables, they stand out
clearly when reading the code. Unfortunately, if you
misspell one of these, you've introduced an error that's
hard to catch. It's also somewhat disconcerting to see the
class name itself hard-coded in so many places.
Both these problems can be easily fixed. Just add the "use
strict" pragma, then pre-declare your package variables.
(The "our" operator will be new in 5.6, and will work for
package globals just like "my" works for scoped lexicals.)
package Some_Class;
use strict;
our($CData1, $CData2); # our() is new to perl5.6
sub CData1 {
shift; # XXX: ignore calling class/object
$CData1 = shift if @_;
return $CData1;
}
sub CData2 {
shift; # XXX: ignore calling class/object
$CData2 = shift if @_;
return $CData2;
}
As with any other global variable, some programmers prefer
to start their package variables with capital letters. This
helps clarity somewhat, but by no longer fully qualifying
the package variables, their significance can be lost when
reading the code. You can fix this easily enough by
choosing better names than were used here.
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:
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Laziness, here manifesting as that innate urge every
programmer feels to factor out duplicate code whenever
possible.
Here's what to do. First, make just one hash to hold all
class attributes.
package Some_Class;
use strict;
our %ClassData = ( # our() is new to perl5.6
CData1 => "",
CData2 => "",
);
Using closures (see perlref) and direct access to the
package symbol table (see perlmod), now clone an accessor
method for each key in the %ClassData hash. Each of these
methods is used to fetch or store values to the specific,
named class attribute.
for my $datum (keys %ClassData) {
no strict "refs"; # to register new methods in package
*$datum = sub {
shift; # XXX: ignore calling class/object
$ClassData{$datum} = shift if @_;
return $ClassData{$datum};
}
}
It's true that you could work out a solution employing an
&AUTOLOAD method, but this approach is unlikely to prove
satisfactory. Your function would have to distinguish
between class attributes and object attributes; it could
interfere with inheritance; and it would have to careful
about DESTROY. Such complexity is uncalled for in most
cases, and certainly in this one.
You may wonder why we're rescinding strict refs for the
loop. We're manipulating the package's symbol table to
introduce new function names using symbolic references
(indirect naming), which the strict pragma would otherwise
forbid. Normally, symbolic references are a dodgy notion at
best. This isn't just because they can be used accidentally
when you aren't meaning to. It's also because for most uses
to which beginning Perl programmers attempt to put symbolic
references, we have much better approaches, like nested
hashes or hashes of arrays. But there's nothing wrong with
using symbolic references to manipulate something that is
meaningful only from the perspective of the package symbol
table, like method names or package variables. In other
words, when you want to refer to the symbol table, use
symbol references.
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Clustering all the class attributes in one place has several
advantages. They're easy to spot, initialize, and change.
The aggregation also makes them convenient to access
externally, such as from a debugger or a persistence
package. The only possible problem is that we don't
automatically know the name of each class's class object,
should it have one. This issue is addressed below in "The
Eponymous Meta-Object".
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
provides the class of that object.
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package Some_Class;
sub CData1 {
my $obclass = shift;
my $class = ref($obclass) || $obclass;
my $varname = $class . "::CData1";
no strict "refs"; # to access package data symbolically
$$varname = shift if @_;
return $$varname;
}
And then do likewise for all other class attributes (such as
CData2, etc.) that you wish to access as package variables
in the invoking package instead of the compiling package as
we had previously.
Once again we temporarily disable the strict references ban,
because otherwise we couldn't use the fully-qualified
symbolic name for the package global. This is perfectly
reasonable: since all package variables by definition live
in a package, there's nothing wrong with accessing them via
that package's symbol table. That's what it's there for
(well, somewhat).
What about just using a single hash for everything and then
cloning methods? What would that look like? The only
difference would be the closure used to produce new method
entries for the class's symbol table.
no strict "refs";
*$datum = sub {
my $obclass = shift;
my $class = ref($obclass) || $obclass;
my $varname = $class . "::ClassData";
$varname->{$datum} = shift if @_;
return $varname->{$datum};
}
The Eponymous Meta-Object
It could be argued that the %ClassData hash in the previous
example is neither the most imaginative nor the most
intuitive of names. Is there something else that might make
more sense, be more useful, or both?
As it happens, yes, there is. For the "class meta-object",
we'll use a package variable of the same name as the package
itself. Within the scope of a package Some_Class
declaration, we'll use the eponymously named hash
%Some_Class as that class's meta-object. (Using an
eponymously named hash is somewhat reminiscent of classes
that name their constructors eponymously in the Python or
C++ fashion. That is, class Some_Class would use
&Some_Class::Some_Class as a constructor, probably even
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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.
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sub CData2 {
shift; # XXX: ignore calling class/object
no strict "refs"; # to access eponymous meta-object
my $class = __PACKAGE__;
$class->{CData2} = shift if @_;
return $class->{CData2};
}
Even though we're using symbolic references for good not
evil, some folks tend to become unnerved when they see so
many places with strict ref checking disabled. Given a
symbolic reference, you can always produce a real reference
(the reverse is not true, though). So we'll create a
subroutine that does this conversion for us. If invoked as
a function of no arguments, it returns a reference to the
compiling class's eponymous hash. Invoked as a class
method, it returns a reference to the eponymous hash of its
caller. And when invoked as an object method, this function
returns a reference to the eponymous hash for whatever class
the object belongs to.
package Some_Class;
use strict;
our %Some_Class = ( # our() is new to perl5.6
CData1 => "",
CData2 => "",
);
# tri-natured: function, class method, or object method
sub _classobj {
my $obclass = shift || __PACKAGE__;
my $class = ref($obclass) || $obclass;
no strict "refs"; # to convert sym ref to real one
return \%$class;
}
for my $datum (keys %{ _classobj() } ) {
# turn off strict refs so that we can
# register a method in the symbol table
no strict "refs";
*$datum = sub {
use strict "refs";
my $self = shift->_classobj();
$self->{$datum} = shift if @_;
return $self->{$datum};
}
}
Indirect References to Class Data
A reasonably common strategy for handling class attributes
is to store a reference to each package variable on the
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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
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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 {
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my $self = shift;
$self = $self->_classobj() unless ref $self;
my $dataref = $self->{CData2};
$$dataref = shift if @_;
return $$dataref;
}
Not only are we now strict refs clean, using an eponymous
meta-object seems to make the code cleaner. Unlike the
previous version, this one does something interesting in the
face of inheritance: it accesses the class meta-object in
the invoking class instead of the one into which the method
was initially compiled.
You can easily access data in the class meta-object, making
it easy to dump the complete class state using an external
mechanism such as when debugging or implementing a
persistent class. This works because the class meta-object
is a package variable, has a well-known name, and clusters
all its data together. (Transparent persistence is not
always feasible, but it's certainly an appealing idea.)
There's still no check that object accessor methods have not
been invoked on a class name. If strict ref checking is
enabled, you'd blow up. If not, then you get the eponymous
meta-object. What you do with--or about--this is up to you.
The next two sections demonstrate innovative uses for this
powerful feature.
Monadic Classes
Some of the standard modules shipped with Perl provide class
interfaces without any attribute methods whatsoever. The
most commonly used module not numbered amongst the pragmata,
the Exporter module, is a class with neither constructors
nor attributes. Its job is simply to provide a standard
interface for modules wishing to export part of their
namespace into that of their caller. Modules use the
Exporter's &import method by setting their inheritance list
in their package's @ISA array to mention "Exporter". But
class Exporter provides no constructor, so you can't have
several instances of the class. In fact, you can't have
any--it just doesn't make any sense. All you get is its
methods. Its interface contains no statefulness, so state
data is wholly superfluous.
Another sort of class that pops up from time to time is one
that supports a unique instance. Such classes are called
monadic classes, or less formally, singletons or highlander
classes.
If a class is monadic, where do you store its state, that
is, its attributes? How do you make sure that there's never
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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();
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Hold on, that doesn't look like anything special. Those
attribute accessors look no different than they would if
this were a regular class instead of a monadic one. The
crux of the matter is there's nothing that says that $self
must hold a reference to a blessed object. It merely has to
be something you can invoke methods on. Here the package
name itself, Cosmos, works as an object. Look at the
&supernova method. Is that a class method or an object
method? The answer is that static analysis cannot reveal
the answer. Perl doesn't care, and neither should you. In
the three attribute methods, %$self is really accessing the
%Cosmos package variable.
If like Stephen Hawking, you posit the existence of
multiple, sequential, and unrelated universes, then you can
invoke the &bigbang method yourself at any time to start
everything all over again. You might think of &bigbang as
more of an initializer than a constructor, since the
function doesn't allocate new memory; it only initializes
what's already there. But like any other constructor, it
does return a scalar value to use for later method
invocations.
Imagine that some day in the future, you decide that one
universe just isn't enough. You could write a new class
from scratch, but you already have an existing class that
does what you want--except that it's monadic, and you want
more than just one cosmos.
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.
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The astonishing thing about the Cosmos class above is that
the value returned by the &bigbang "constructor" is not a
reference to a blessed object at all. It's just the class's
own name. A class name is, for virtually all intents and
purposes, a perfectly acceptable object. It has state,
behavior, and identity, the three crucial components of an
object system. It even manifests inheritance, polymorphism,
and encapsulation. And what more can you ask of an object?
To understand object orientation in Perl, it's important to
recognize the unification of what other programming
languages might think of as class methods and object methods
into just plain methods. "Class methods" and "object
methods" are distinct only in the compartmentalizing mind of
the Perl programmer, not in the Perl language itself.
Along those same lines, a constructor is nothing special
either, which is one reason why Perl has no pre-ordained
name for them. "Constructor" is just an informal term
loosely used to describe a method that returns a scalar
value that you can make further method calls against. So
long as it's either a class name or an object reference,
that's good enough. It doesn't even have to be a reference
to a brand new object.
You can have as many--or as few--constructors as you want,
and you can name them whatever you care to. Blindly and
obediently using new() for each and every constructor you
ever write is to speak Perl with such a severe C++ accent
that you do a disservice to both languages. There's no
reason to insist that each class have but one constructor,
or that a constructor be named new(), or that a constructor
be used solely as a class method and not an object method.
The next section shows how useful it can be to further
distance ourselves from any formal distinction between class
method calls and object method calls, both in constructors
and in accessor methods.
Translucent Attributes
A package's eponymous hash can be used for more than just
containing per-class, global state data. It can also serve
as a sort of template containing default settings for object
attributes. These default settings can then be used in
constructors for initialization of a particular object. The
class's eponymous hash can also be used to implement
translucent attributes. A translucent attribute is one that
has a class-wide default. Each object can set its own value
for the attribute, in which case "$object->attribute()"
returns that value. But if no value has been set, then
"$object->attribute()" returns the class-wide default.
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We'll apply something of a copy-on-write approach to these
translucent attributes. If you're just fetching values from
them, you get translucency. But if you store a new value to
them, that new value is set on the current object. On the
other hand, if you use the class as an object and store the
attribute value directly on the class, then the meta-
object's value changes, and later fetch operations on
objects with uninitialized values for those attributes will
retrieve the meta-object's new values. Objects with their
own initialized values, however, won't see any change.
Let's look at some concrete examples of using these
properties before we show how to implement them. Suppose
that a class named Some_Class had a translucent data
attribute called "color". First you set the color in the
meta-object, then you create three objects using a
constructor that happens to be named &spawn.
use Vermin;
Vermin->color("vermilion");
$ob1 = Vermin->spawn(); # so that's where Jedi come from
$ob2 = Vermin->spawn();
$ob3 = Vermin->spawn();
print $obj3->color(); # prints "vermilion"
Each of these objects' colors is now "vermilion", because
that's the meta-object's value for that attribute, and these
objects do not have individual color values set.
Changing the attribute on one object has no effect on other
objects previously created.
$ob3->color("chartreuse");
print $ob3->color(); # prints "chartreuse"
print $ob1->color(); # prints "vermilion", translucently
If you now use $ob3 to spawn off another object, the new
object will take the color its parent held, which now
happens to be "chartreuse". That's because the constructor
uses the invoking object as its template for initializing
attributes. When that invoking object is the class name,
the object used as a template is the eponymous meta-object.
When the invoking object is a reference to an instantiated
object, the &spawn constructor uses that existing object as
a template.
$ob4 = $ob3->spawn(); # $ob3 now template, not %Vermin
print $ob4->color(); # prints "chartreuse"
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Any actual values set on the template object will be copied
to the new object. But attributes undefined in the template
object, being translucent, will remain undefined and
consequently translucent in the new one as well.
Now let's change the color attribute on the entire class:
Vermin->color("azure");
print $ob1->color(); # prints "azure"
print $ob2->color(); # prints "azure"
print $ob3->color(); # prints "chartreuse"
print $ob4->color(); # prints "chartreuse"
That color change took effect only in the first pair of
objects, which were still translucently accessing the meta-
object's values. The second pair had per-object initialized
colors, and so didn't change.
One important question remains. Changes to the meta-object
are reflected in translucent attributes in the entire class,
but what about changes to discrete objects? If you change
the color of $ob3, does the value of $ob4 see that change?
Or vice-versa. If you change the color of $ob4, does then
the value of $ob3 shift?
$ob3->color("amethyst");
print $ob3->color(); # prints "amethyst"
print $ob4->color(); # hmm: "chartreuse" or "amethyst"?
While one could argue that in certain rare cases it should,
let's not do that. Good taste aside, we want the answer to
the question posed in the comment above to be "chartreuse",
not "amethyst". So we'll treat these attributes similar to
the way process attributes like environment variables, user
and group IDs, or the current working directory are treated
across a fork(). You can change only yourself, but you will
see those changes reflected in your unspawned children.
Changes to one object will propagate neither up to the
parent nor down to any existing child objects. Those
objects made later, however, will see the changes.
If you have an object with an actual attribute value, and
you want to make that object's attribute value translucent
again, what do you do? Let's design the class so that when
you invoke an accessor method with "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.
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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;
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my $class = ref($obclass) || $obclass;
return $class->{PopCount};
}
# instance destructor
# invoked only as object method
sub DESTROY {
my $self = shift;
my $class = ref $self;
$class->{PopCount}--;
}
Here are a couple of helper methods that might be
convenient. They aren't accessor methods at all. They're
used to detect accessibility of data attributes. The
&is_translucent method determines whether a particular
object attribute is coming from the meta-object. The
&has_attribute method detects whether a class implements a
particular property at all. It could also be used to
distinguish undefined properties from non-existent ones.
# detect whether an object attribute is translucent
# (typically?) invoked only as object method
sub is_translucent {
my($self, $attr) = @_;
return !defined $self->{$attr};
}
# test for presence of attribute in class
# invoked as class method or object method
sub has_attribute {
my($self, $attr) = @_;
my $class = ref($self) || $self;
return exists $class->{$attr};
}
If you prefer to install your accessors more generically,
you can make use of the upper-case versus lower-case
convention to register into the package appropriate methods
cloned from generic closures.
for my $datum (keys %{ +__PACKAGE__ }) {
*$datum = ($datum =~ /^[A-Z]/)
? sub { # install class accessor
my $obclass = shift;
my $class = ref($obclass) || $obclass;
return $class->{$datum};
}
: sub { # install translucent accessor
my $self = shift;
my $class = ref($self) || $self;
unless (ref $self) {
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$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! Outsiders
have no business whatsoever playing with anything within a
class save through the mediated access of its documented
interface; in other words, through method invocations. And
not even through just any method, either. Methods that
begin with an underscore are traditionally considered off-
limits outside the class. If outsiders skip the documented
method interface to poke around the internals of your class
and end up breaking something, that's not your fault--it's
theirs.
Perl believes in individual responsibility rather than
mandated control. Perl respects you enough to let you
choose your own preferred level of pain, or of pleasure.
Perl believes that you are creative, intelligent, and
capable of making your own decisions--and fully expects you
to take complete responsibility for your own actions. In a
perfect world, these admonitions alone would suffice, and
everyone would be intelligent, responsible, happy, and
creative. And careful. One probably shouldn't forget
careful, and that's a good bit harder to expect. Even
Einstein would take wrong turns by accident and end up lost
in the wrong part of town.
Some folks get the heebie-jeebies when they see package
variables hanging out there for anyone to reach over and
alter them. Some folks live in constant fear that someone
somewhere might do something wicked. The solution to that
problem is simply to fire the wicked, of course. But
unfortunately, it's not as simple as all that. These
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cautious types are also afraid that they or others will do
something not so much wicked as careless, whether by
accident or out of desperation. If we fire everyone who
ever gets careless, pretty soon there won't be anybody left
to get any work done.
Whether it's needless paranoia or sensible caution, this
uneasiness can be a problem for some people. We can take
the edge off their discomfort by providing the option of
storing class attributes as lexical variables instead of as
package variables. The my() operator is the source of all
privacy in Perl, and it is a powerful form of privacy
indeed.
It is widely perceived, and indeed has often been written,
that Perl provides no data hiding, that it affords the class
designer no privacy nor isolation, merely a rag-tag
assortment of weak and unenforceable social conventions
instead. This perception is demonstrably false and easily
disproven. In the next section, we show how to implement
forms of privacy that are far stronger than those provided
in nearly any other object-oriented language.
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
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have unmediated access to $CData1. They have to invoke the
&CData1 method against Some_Class or an instance thereof,
just like anybody else.
To be scrupulously honest, that last statement assumes you
haven't packed several classes together into the same file
scope, nor strewn your class implementation across several
different files. Accessibility of those variables is based
uniquely on the static file scope. It has nothing to do
with the package. That means that code in a different file
but the same package (class) could not access those
variables, yet code in the same file but a different package
(class) could. There are sound reasons why we usually
suggest a one-to-one mapping between files and packages and
modules and classes. You don't have to stick to this
suggestion if you really know what you're doing, but you're
apt to confuse yourself otherwise, especially at first.
If you'd like to aggregate your class attributes into one
lexically scoped, composite structure, you're perfectly free
to do so.
package Some_Class;
my %ClassData = (
CData1 => "",
CData2 => "",
);
sub CData1 {
shift; # XXX: ignore calling class/object
$ClassData{CData1} = shift if @_;
return $ClassData{CData1};
}
sub CData2 {
shift; # XXX: ignore calling class/object
$ClassData{CData2} = shift if @_;
return $ClassData{CData2};
}
To make this more scalable as other class attributes are
added, we can again register closures into the package
symbol table to create accessor methods for them.
package Some_Class;
my %ClassData = (
CData1 => "",
CData2 => "",
);
for my $datum (keys %ClassData) {
no strict "refs";
*$datum = sub {
shift; # XXX: ignore calling class/object
$ClassData{$datum} = shift if @_;
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return $ClassData{$datum};
};
}
Requiring even your own class to use accessor methods like
anybody else is probably a good thing. But demanding and
expecting that everyone else, be they subclass or
superclass, friend or foe, will all come to your object
through mediation is more than just a good idea. It's
absolutely critical to the model. Let there be in your mind
no such thing as "public" data, nor even "protected" data,
which is a seductive but ultimately destructive notion.
Both will come back to bite at you. That's because as soon
as you take that first step out of the solid position in
which all state is considered completely private, save from
the perspective of its own accessor methods, you have
violated the envelope. And, having pierced that
encapsulating envelope, you shall doubtless someday pay the
price when future changes in the implementation break
unrelated code. Considering that avoiding this infelicitous
outcome was precisely why you consented to suffer the slings
and arrows of obsequious abstraction by turning to object
orientation in the first place, such breakage seems
unfortunate in the extreme.
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:
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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 implementation of this class to
access class attributes directly?
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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 => "",
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);
for my $datum (keys %ClassData ) {
no strict "refs";
*$datum = sub {
use strict "refs";
my ($self, $newvalue) = @_;
$ClassData{$datum} = $newvalue if @_ > 1;
return $ClassData{$datum};
}
}
}
The closure above can be modified to take inheritance into
account using the &UNIVERSAL::can method and SUPER as shown
previously.
Translucency Revisited
The Vermin class demonstrates translucency using a package
variable, eponymously named %Vermin, as its meta-object. If
you prefer to use absolutely no package variables beyond
those necessary to appease inheritance or possibly the
Exporter, this strategy is closed to you. That's too bad,
because translucent attributes are an appealing technique,
so it would be valuable to devise an implementation using
only lexicals.
There's a second reason why you might wish to avoid the
eponymous package hash. If you use class names with double-
colons in them, you would end up poking around somewhere you
might not have meant to poke.
package Vermin;
$class = "Vermin";
$class->{PopCount}++;
# accesses $Vermin::Vermin{PopCount}
package Vermin::Noxious;
$class = "Vermin::Noxious";
$class->{PopCount}++;
# accesses $Vermin::Noxious{PopCount}
In the first case, because the class name had no double-
colons, we got the hash in the current package. But in the
second case, instead of 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.
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In practice, it is unlikely that the Vermin class had an
existing package variable named %Noxious that you just blew
away. If you're still mistrustful, you could always stake
out your own territory where you know the rules, such as
using Eponymous::Vermin::Noxious or
Hieronymus::Vermin::Boschious or
Leave_Me_Alone::Vermin::Noxious as class names instead.
Sure, it's in theory possible that someone else has a class
named Eponymous::Vermin with its own %Noxious hash, but this
kind of thing is always true. There's no arbiter of package
names. It's always the case that globals like @Cwd::ISA
would collide if more than one class uses the same Cwd
package.
If this still leaves you with an uncomfortable twinge of
paranoia, we have another solution for you. There's nothing
that says that you have to have a package variable to hold a
class meta-object, either for monadic classes or for
translucent attributes. Just code up the methods so that
they access a lexical instead.
Here's another implementation of the Vermin class with
semantics identical to those given previously, but this time
using no package variables.
package Vermin;
# Here's the class meta-object, eponymously named.
# It holds all class data, and also all instance data
# so the latter can be used for both initialization
# and translucency. it's a template.
my %ClassData = (
PopCount => 0, # capital for class attributes
color => "beige", # small for instance attributes
);
# constructor method
# invoked as class method or object method
sub spawn {
my $obclass = shift;
my $class = ref($obclass) || $obclass;
my $self = {};
bless($self, $class);
$ClassData{PopCount}++;
# init fields from invoking object, or omit if
# invoking object is the class to provide translucency
%$self = %$obclass if ref $obclass;
return $self;
}
# translucent accessor for "color" attribute
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# 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};
}
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
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+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
NOTES
Inheritance is a powerful but subtle device, best used only
after careful forethought and design. Aggregation instead
of inheritance is often a better approach.
You can't use file-scoped lexicals in conjunction with the
SelfLoader or the AutoLoader, because they alter the lexical
scope in which the module's methods wind up getting
compiled.
The usual mealy-mouthed package-munging doubtless applies to
setting up names of object attributes. For example,
"$self->{ObData1}" should probably be "$self->{ __PACKAGE__
. "_ObData1" }", but that would just confuse the examples.
SEE ALSO
perltoot, perlobj, perlmod, and perlbot.
The Tie::SecureHash and Class::Data::Inheritable modules
from CPAN are worth checking out.
AUTHOR AND COPYRIGHT
Copyright (c) 1999 Tom Christiansen. All rights reserved.
This documentation is free; you can redistribute it and/or
modify it under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this
file are hereby placed into the public domain. You are
permitted and encouraged to use this code in your own
programs for fun or for profit as you see fit. A simple
comment in the code giving credit would be courteous but is
not required.
ACKNOWLEDGEMENTS
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.
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HISTORY
Last edit: Sun Feb 4 20:50:28 EST 2001
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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