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PERLTOOC(1) 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:
• to keep a count of the objects you've created, or how many are still extant.
• to extract the name or file descriptor for a logfile used by a debugging method.
• to access collective data, like the total amount of cash dispensed by all ATMs in a network in a
given day.
• to access the last object created by a class, or the most accessed object, or to retrieve a list
of all objects.
Unlike a true global, class attributes should not be accessed directly. Instead, their state should
be inspected, and perhaps altered, only through the mediated access of class methods. These class
attributes accessor methods are similar in spirit and function to accessors used to manipulate the
state of instance attributes on an object. They provide a clear firewall between interface and
implementation.
You should allow access to class attributes through either the class name or any object of that
class. If we assume that $an_object is of type Some_Class, and the &Some_Class::population_count
method accesses class attributes, then these two invocations should both be possible, and almost
certainly equivalent.
Some_Class->population_count()
$an_object->population_count()
The question is, where do you store the state which that method accesses? Unlike more restrictive
languages like C++, where these are called static data members, Perl provides no syntactic mechanism
to declare class attributes, any more than it provides a syntactic mechanism to declare instance
attributes. Perl provides the developer with a broad set of powerful but flexible features that can
be uniquely crafted to the particular demands of the situation.
A class in Perl is typically implemented in a module. A module consists of two complementary feature
sets: a package for interfacing with the outside world, and a lexical file scope for privacy. Either
of these two mechanisms can be used to implement class attributes. That means you get to decide
whether to put your class attributes in package variables or to put them in lexical variables.
And those aren't the only decisions to make. If you choose to use package variables, you can make
your class attribute accessor methods either ignorant of inheritance or sensitive to it. If you
choose lexical variables, you can elect to permit access to them from anywhere in the entire file
scope, or you can limit direct data access exclusively to the methods implementing those attributes.
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.
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: 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.
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.
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
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 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 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 module not numbered amongst the pragmata, the Exporter module, is
a class with neither constructors nor attributes. Its job is simply to provide a standard interface
for modules wishing to export part of their namespace into that of their caller. Modules use the
Exporter's &import method by setting their inheritance list in their package's @ISA array to mention
"Exporter". But class Exporter provides no constructor, so you can't have several instances of the
class. In fact, you can't have any--it just doesn't make any sense. All you get is its methods.
Its interface contains no statefulness, so state data is wholly superfluous.
Another sort of class that pops up from time to time is one that supports a unique instance. Such
classes are called monadic classes, or less formally, singletons or highlander classes.
If a class is monadic, where do you store its state, that is, its attributes? How do you make sure
that there's never more than one instance? While you could merely use a slew of package variables,
it's a lot cleaner to use the eponymously named hash. Here's a complete example of a monadic class:
package Cosmos;
%Cosmos = ();
# accessor method for "name" attribute
sub name {
my $self = shift;
$self->{name} = shift if @_;
return $self->{name};
}
# read-only accessor method for "birthday" attribute
sub birthday {
my $self = shift;
die "can't reset birthday" if @_; # XXX: croak() is better
return $self->{birthday};
}
# accessor method for "stars" attribute
sub stars {
my $self = shift;
$self->{stars} = shift if @_;
return $self->{stars};
}
# oh my - one of our stars just went out!
sub supernova {
my $self = shift;
my $count = $self->stars();
$self->stars($count - 1) if $count > 0;
}
# constructor/initializer method - fix by reboot
sub bigbang {
my $self = shift;
%$self = (
name => "the world according to tchrist",
birthday => time(),
stars => 0,
);
return $self; # yes, it's probably a class. SURPRISE!
}
# After the class is compiled, but before any use or require
# returns, we start off the universe with a bang.
__PACKAGE__ -> bigbang();
Hold on, that doesn't look like anything special. Those attribute accessors look no different than
they would if this were a regular class instead of a monadic one. The crux of the matter is there's
nothing that says that $self must hold a reference to a blessed object. It merely has to be
something you can invoke methods on. Here the package name itself, Cosmos, works as an object. Look
at the &supernova method. Is that a class method or an object method? The answer is that static
analysis cannot reveal the answer. Perl doesn't care, and neither should you. In the three
attribute methods, %$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.
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.
We'll apply something of a copy-on-write approach to these translucent attributes. If you're just
fetching values from them, you get translucency. But if you store a new value to them, that new
value is set on the current object. On the other hand, if you use the class as an object and store
the attribute value directly on the class, then the meta-object's value changes, and later fetch
operations on objects with uninitialized values for those attributes will retrieve the meta-object's
new values. Objects with their own initialized values, however, won't see any change.
Let's look at some concrete examples of using these properties before we show how to implement them.
Suppose that a class named Some_Class had a translucent data attribute called "color". First you set
the color in the meta-object, then you create three objects using a constructor that happens to be
named &spawn.
use Vermin;
Vermin->color("vermilion");
$ob1 = Vermin->spawn(); # so that's where Jedi come from
$ob2 = Vermin->spawn();
$ob3 = Vermin->spawn();
print $obj3->color(); # prints "vermilion"
Each of these objects' colors is now "vermilion", because that's the meta-object's value for that
attribute, and these objects do not have individual color values set.
Changing the attribute on one object has no effect on other objects previously created.
$ob3->color("chartreuse");
print $ob3->color(); # prints "chartreuse"
print $ob1->color(); # prints "vermilion", translucently
If you now use $ob3 to spawn off another object, the new object will take the color its parent held,
which now happens to be "chartreuse". That's because the constructor uses the invoking object as its
template for initializing attributes. When that invoking object is the class name, the object used
as a template is the eponymous meta-object. When the invoking object is a reference to an
instantiated object, the &spawn constructor uses that existing object as a template.
$ob4 = $ob3->spawn(); # $ob3 now template, not %Vermin
print $ob4->color(); # prints "chartreuse"
Any actual values set on the template object will be copied to the new object. But attributes
undefined in the template object, being translucent, will remain undefined and consequently
translucent in the new one as well.
Now let's change the color attribute on the entire class:
Vermin->color("azure");
print $ob1->color(); # prints "azure"
print $ob2->color(); # prints "azure"
print $ob3->color(); # prints "chartreuse"
print $ob4->color(); # prints "chartreuse"
That color change took effect only in the first pair of objects, which were still translucently
accessing the meta-object's values. The second pair had per-object initialized colors, and so didn't
change.
One important question remains. Changes to the meta-object are reflected in translucent attributes
in the entire class, but what about changes to discrete objects? If you change the color of $ob3,
does the value of $ob4 see that change? Or vice-versa. If you change the color of $ob4, does then
the value of $ob3 shift?
$ob3->color("amethyst");
print $ob3->color(); # prints "amethyst"
print $ob4->color(); # hmm: "chartreuse" or "amethyst"?
While one could argue that in certain rare cases it should, let's not do that. Good taste aside, we
want the answer to the question posed in the comment above to be "chartreuse", not "amethyst". So
we'll treat these attributes similar to the way process attributes like environment variables, user
and group IDs, or the current working directory are treated across a fork(). You can change only
yourself, but you will see those changes reflected in your unspawned children. Changes to one object
will propagate neither up to the parent nor down to any existing child objects. Those objects made
later, however, will see the changes.
If you have an object with an actual attribute value, and you want to make that object's attribute
value translucent again, what do you do? Let's design the class so that when you invoke an accessor
method with "undef" as its argument, that attribute returns to translucency.
$ob4->color(undef); # back to "azure"
Here's a complete implementation of Vermin as described above.
package Vermin;
# here's the class meta-object, eponymously named.
# it holds all class attributes, and also all instance attributes
# so the latter can be used for both initialization
# and translucency.
our %Vermin = ( # our() is new to perl5.6
PopCount => 0, # capital for class attributes
color => "beige", # small for instance attributes
);
# constructor method
# invoked as class method or object method
sub spawn {
my $obclass = shift;
my $class = ref($obclass) || $obclass;
my $self = {};
bless($self, $class);
$class->{PopCount}++;
# init fields from invoking object, or omit if
# invoking object is the class to provide translucency
%$self = %$obclass if ref $obclass;
return $self;
}
# translucent accessor for "color" attribute
# invoked as class method or object method
sub color {
my $self = shift;
my $class = ref($self) || $self;
# handle class invocation
unless (ref $self) {
$class->{color} = shift if @_;
return $class->{color}
}
# handle object invocation
$self->{color} = shift if @_;
if (defined $self->{color}) { # not exists!
return $self->{color};
} else {
return $class->{color};
}
}
# accessor for "PopCount" class attribute
# invoked as class method or object method
# but uses object solely to locate meta-object
sub population {
my $obclass = shift;
my $class = ref($obclass) || $obclass;
return $class->{PopCount};
}
# instance destructor
# invoked only as object method
sub DESTROY {
my $self = shift;
my $class = ref $self;
$class->{PopCount}--;
}
Here are a couple of helper methods that might be convenient. They aren't accessor methods at all.
They're used to detect accessibility of data attributes. The &is_translucent method determines
whether a particular object attribute is coming from the meta-object. The &has_attribute method
detects whether a class implements a particular property at all. It could also be used to
distinguish undefined properties from non-existent ones.
# detect whether an object attribute is translucent
# (typically?) invoked only as object method
sub is_translucent {
my($self, $attr) = @_;
return !defined $self->{$attr};
}
# test for presence of attribute in class
# invoked as class method or object method
sub has_attribute {
my($self, $attr) = @_;
my $class = ref($self) || $self;
return exists $class->{$attr};
}
If you prefer to install your accessors more generically, you can make use of the upper-case versus
lower-case convention to register into the package appropriate methods cloned from generic closures.
for my $datum (keys %{ +__PACKAGE__ }) {
*$datum = ($datum =~ /^[A-Z]/)
? sub { # install class accessor
my $obclass = shift;
my $class = ref($obclass) || $obclass;
return $class->{$datum};
}
: sub { # install translucent accessor
my $self = shift;
my $class = ref($self) || $self;
unless (ref $self) {
$class->{$datum} = shift if @_;
return $class->{$datum}
}
$self->{$datum} = shift if @_;
return defined $self->{$datum}
? $self -> {$datum}
: $class -> {$datum}
}
}
Translations of this closure-based approach into C++, Java, and Python have been left as exercises
for the reader. Be sure to send us mail as soon as you're done.
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 offlimits
limits outside the class. If outsiders skip the documented method interface to poke around the
internals of your class and end up breaking something, that's not your fault--it's theirs.
Perl believes in individual responsibility rather than mandated control. Perl respects you enough to
let you choose your own preferred level of pain, or of pleasure. Perl believes that you are
creative, intelligent, and capable of making your own decisions--and fully expects you to take
complete responsibility for your own actions. In a perfect world, these admonitions alone would
suffice, and everyone would be intelligent, responsible, happy, and creative. And careful. One
probably shouldn't forget careful, and that's a good bit harder to expect. Even Einstein would take
wrong turns by accident and end up lost in the wrong part of town.
Some folks get the heebie-jeebies when they see package variables hanging out there for anyone to
reach over and alter them. Some folks live in constant fear that someone somewhere might do
something wicked. The solution to that problem is simply to fire the wicked, of course. But
unfortunately, it's not as simple as all that. These cautious types are also afraid that they or
others will do something not so much wicked as careless, whether by accident or out of desperation.
If we fire everyone who ever gets careless, pretty soon there won't be anybody left to get any work
done.
Whether it's needless paranoia or sensible caution, this uneasiness can be a problem for some people.
We can take the edge off their discomfort by providing the option of storing class attributes as
lexical variables instead of as package variables. The my() operator is the source of all privacy in
Perl, and it is a powerful form of privacy indeed.
It is widely perceived, and indeed has often been written, that Perl provides no data hiding, that it
affords the class designer no privacy nor isolation, merely a rag-tag assortment of weak and
unenforceable social conventions instead. This perception is demonstrably false and easily
disproven. In the next section, we show how to implement forms of privacy that are far stronger than
those provided in nearly any other object-oriented language.
File-Scoped Lexicals
A lexical variable is visible only through the end of its static scope. That means that the only
code able to access that variable is code residing textually below the my() operator through the end
of its block if it has one, or through the end of the current file if it doesn't.
Starting again with our simplest example given at the start of this document, we replace our()
variables with my() versions.
package Some_Class;
my($CData1, $CData2); # file scope, not in any package
sub CData1 {
shift; # XXX: ignore calling class/object
$CData1 = shift if @_;
return $CData1;
}
sub CData2 {
shift; # XXX: ignore calling class/object
$CData2 = shift if @_;
return $CData2;
}
So much for that old $Some_Class::CData1 package variable and its brethren! Those are gone now,
replaced with lexicals. No one outside the scope can reach in and alter the class state without
resorting to the documented interface. Not even subclasses or superclasses of this one have
unmediated access to $CData1. They have to invoke the &CData1 method against Some_Class or an
instance thereof, just like anybody else.
To be scrupulously honest, that last statement assumes you haven't packed several classes together
into the same file scope, nor strewn your class implementation across several different files.
Accessibility of those variables is based uniquely on the static file scope. It has nothing to do
with the package. That means that code in a different file but the same package (class) could not
access those variables, yet code in the same file but a different package (class) could. There are
sound reasons why we usually suggest a one-to-one mapping between files and packages and modules and
classes. You don't have to stick to this suggestion if you really know what you're doing, but you're
apt to confuse yourself otherwise, especially at first.
If you'd like to aggregate your class attributes into one lexically scoped, composite structure,
you're perfectly free to do so.
package Some_Class;
my %ClassData = (
CData1 => "",
CData2 => "",
);
sub CData1 {
shift; # XXX: ignore calling class/object
$ClassData{CData1} = shift if @_;
return $ClassData{CData1};
}
sub CData2 {
shift; # XXX: ignore calling class/object
$ClassData{CData2} = shift if @_;
return $ClassData{CData2};
}
To make this more scalable as other class attributes are added, we can again register closures into
the package symbol table to create accessor methods for them.
package Some_Class;
my %ClassData = (
CData1 => "",
CData2 => "",
);
for my $datum (keys %ClassData) {
no strict "refs";
*$datum = sub {
shift; # XXX: ignore calling class/object
$ClassData{$datum} = shift if @_;
return $ClassData{$datum};
};
}
Requiring even your own class to use accessor methods like anybody else is probably a good thing.
But demanding and expecting that everyone else, be they subclass or superclass, friend or foe, will
all come to your object through mediation is more than just a good idea. It's absolutely critical to
the model. Let there be in your mind no such thing as "public" data, nor even "protected" data,
which is a seductive but ultimately destructive notion. Both will come back to bite at you. That's
because as soon as you take that first step out of the solid position in which all state is
considered completely private, save from the perspective of its own accessor methods, you have
violated the envelope. And, having pierced that encapsulating envelope, you shall doubtless someday
pay the price when future changes in the implementation break unrelated code. Considering that
avoiding this infelicitous outcome was precisely why you consented to suffer the slings and arrows of
obsequious abstraction by turning to object orientation in the first place, such breakage seems
unfortunate in the extreme.
More Inheritance Concerns
Suppose that Some_Class were used as a base class from which to derive Another_Class. If you invoke
a &CData method on the derived class or on an object of that class, what do you get? Would the
derived class have its own state, or would it piggyback on its base class's versions of the class
attributes?
The answer is that under the scheme outlined above, the derived class would not have its own state
data. As before, whether you consider this a good thing or a bad one depends on the semantics of the
classes involved.
The cleanest, sanest, simplest way to address per-class state in a lexical is for the derived class
to override its base class's version of the method that accesses the class attributes. Since the
actual method called is the one in the object's derived class if this exists, you automatically get
per-class state this way. Any urge to provide an unadvertised method to sneak out a reference to the
%ClassData hash should be strenuously resisted.
As with any other overridden method, the implementation in the derived class always has the option of
invoking its base class's version of the method in addition to its own. Here's an example:
package Another_Class;
@ISA = qw(Some_Class);
my %ClassData = (
CData1 => "",
);
sub CData1 {
my($self, $newvalue) = @_;
if (@_ > 1) {
# set locally first
$ClassData{CData1} = $newvalue;
# then pass the buck up to the first
# overridden version, if there is one
if ($self->can("SUPER::CData1")) {
$self->SUPER::CData1($newvalue);
}
}
return $ClassData{CData1};
}
Those dabbling in multiple inheritance might be concerned about there being more than one override.
for my $parent (@ISA) {
my $methname = $parent . "::CData1";
if ($self->can($methname)) {
$self->$methname($newvalue);
}
}
Because the &UNIVERSAL::can method returns a reference to the function directly, you can use this
directly for a significant performance improvement:
for my $parent (@ISA) {
if (my $coderef = $self->can($parent . "::CData1")) {
$self->$coderef($newvalue);
}
}
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?
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 technique, so it would be valuable to devise an
implementation using only lexicals.
There's a second reason why you might wish to avoid the eponymous package hash. If you use class
names with double-colons in them, you would end up poking around somewhere you might not have meant
to poke.
package Vermin;
$class = "Vermin";
$class->{PopCount}++;
# accesses $Vermin::Vermin{PopCount}
package Vermin::Noxious;
$class = "Vermin::Noxious";
$class->{PopCount}++;
# accesses $Vermin::Noxious{PopCount}
In the first case, because the class name had no double-colons, we got the hash in the current
package. But in the second case, instead of getting some hash in the current package, we got the
hash %Noxious in the Vermin package. (The noxious vermin just invaded another package and sprayed
their data around it. :-) Perl doesn't support relative packages in its naming conventions, so any
double-colons trigger a fully-qualified lookup instead of just looking in the current package.
In practice, it is unlikely that the Vermin class had an existing package variable named %Noxious
that you just blew away. If you're still mistrustful, you could always stake out your own territory
where you know the rules, such as using Eponymous::Vermin::Noxious or Hieronymus::Vermin::Boschious
or Leave_Me_Alone::Vermin::Noxious as class names instead. Sure, it's in theory possible that
someone else has a class named Eponymous::Vermin with its own %Noxious hash, but this kind of thing
is always true. There's no arbiter of package names. It's always the case that globals like
@Cwd::ISA would collide if more than one class uses the same Cwd package.
If this still leaves you with an uncomfortable twinge of paranoia, we have another solution for you.
There's nothing that says that you have to have a package variable to hold a class meta-object,
either for monadic classes or for translucent attributes. Just code up the methods so that they
access a lexical instead.
Here's another implementation of the Vermin class with semantics identical to those given previously,
but this time using no package variables.
package Vermin;
# Here's the class meta-object, eponymously named.
# It holds all class data, and also all instance data
# so the latter can be used for both initialization
# and translucency. it's a template.
my %ClassData = (
PopCount => 0, # capital for class attributes
color => "beige", # small for instance attributes
);
# constructor method
# invoked as class method or object method
sub spawn {
my $obclass = shift;
my $class = ref($obclass) || $obclass;
my $self = {};
bless($self, $class);
$ClassData{PopCount}++;
# init fields from invoking object, or omit if
# invoking object is the class to provide translucency
%$self = %$obclass if ref $obclass;
return $self;
}
# translucent accessor for "color" attribute
# invoked as class method or object method
sub color {
my $self = shift;
# handle class invocation
unless (ref $self) {
$ClassData{color} = shift if @_;
return $ClassData{color}
}
# handle object invocation
$self->{color} = shift if @_;
if (defined $self->{color}) { # not exists!
return $self->{color};
} else {
return $ClassData{color};
}
}
# class attribute accessor for "PopCount" attribute
# invoked as class method or object method
sub population {
return $ClassData{PopCount};
}
# instance destructor; invoked only as object method
sub DESTROY {
$ClassData{PopCount}--;
}
# detect whether an object attribute is translucent
# (typically?) invoked only as object method
sub is_translucent {
my($self, $attr) = @_;
$self = \%ClassData if !ref $self;
return !defined $self->{$attr};
}
# test for presence of attribute in class
# invoked as class method or object method
sub has_attribute {
my($self, $attr) = @_;
return exists $ClassData{$attr};
}
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.
HISTORY
Last edit: Sun Feb 4 20:50:28 EST 2001
perl v5.12.5 2012-11-03 PERLTOOC(1)
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