NAME¶
perlobj - Perl object reference
DESCRIPTION¶
This document provides a reference for Perl's object orientation features. If
you're looking for an introduction to object-oriented programming in Perl,
please see perlootut.
In order to understand Perl objects, you first need to understand references in
Perl. See perlref for details.
This document describes all of Perl's object-oriented (OO) features from the
ground up. If you're just looking to write some object-oriented code of your
own, you are probably better served by using one of the object systems from
CPAN described in perlootut.
If you're looking to write your own object system, or you need to maintain code
which implements objects from scratch then this document will help you
understand exactly how Perl does object orientation.
There are a few basic principles which define object oriented Perl:
- 1.
- An object is simply a data structure that knows to which class it
belongs.
- 2.
- A class is simply a package. A class provides methods that expect to
operate on objects.
- 3.
- A method is simply a subroutine that expects a reference to an object (or
a package name, for class methods) as the first argument.
Let's look at each of these principles in depth.
An Object is Simply a Data Structure¶
Unlike many other languages which support object orientation, Perl does not
provide any special syntax for constructing an object. Objects are merely Perl
data structures (hashes, arrays, scalars, filehandles, etc.) that have been
explicitly associated with a particular class.
That explicit association is created by the built-in "bless" function,
which is typically used within the
constructor subroutine of the class.
Here is a simple constructor:
package File;
sub new {
my $class = shift;
return bless {}, $class;
}
The name "new" isn't special. We could name our constructor something
else:
package File;
sub load {
my $class = shift;
return bless {}, $class;
}
The modern convention for OO modules is to always use "new" as the
name for the constructor, but there is no requirement to do so. Any subroutine
that blesses a data structure into a class is a valid constructor in Perl.
In the previous examples, the "{}" code creates a reference to an
empty anonymous hash. The "bless" function then takes that reference
and associates the hash with the class in $class. In the simplest case, the
$class variable will end up containing the string "File".
We can also use a variable to store a reference to the data structure that is
being blessed as our object:
sub new {
my $class = shift;
my $self = {};
bless $self, $class;
return $self;
}
Once we've blessed the hash referred to by $self we can start calling methods on
it. This is useful if you want to put object initialization in its own
separate method:
sub new {
my $class = shift;
my $self = {};
bless $self, $class;
$self->_initialize();
return $self;
}
Since the object is also a hash, you can treat it as one, using it to store data
associated with the object. Typically, code inside the class can treat the
hash as an accessible data structure, while code outside the class should
always treat the object as opaque. This is called
encapsulation.
Encapsulation means that the user of an object does not have to know how it is
implemented. The user simply calls documented methods on the object.
Note, however, that (unlike most other OO languages) Perl does not ensure or
enforce encapsulation in any way. If you want objects to actually
be
opaque you need to arrange for that yourself. This can be done in a variety of
ways, including using "Inside-Out objects" or modules from CPAN.
Objects Are Blessed; Variables Are Not
When we bless something, we are not blessing the variable which contains a
reference to that thing, nor are we blessing the reference that the variable
stores; we are blessing the thing that the variable refers to (sometimes known
as the
referent). This is best demonstrated with this code:
use Scalar::Util 'blessed';
my $foo = {};
my $bar = $foo;
bless $foo, 'Class';
print blessed( $bar ); # prints "Class"
$bar = "some other value";
print blessed( $bar ); # prints undef
When we call "bless" on a variable, we are actually blessing the
underlying data structure that the variable refers to. We are not blessing the
reference itself, nor the variable that contains that reference. That's why
the second call to "blessed( $bar )" returns false. At that point
$bar is no longer storing a reference to an object.
You will sometimes see older books or documentation mention "blessing a
reference" or describe an object as a "blessed reference", but
this is incorrect. It isn't the reference that is blessed as an object; it's
the thing the reference refers to (i.e. the referent).
A Class is Simply a Package¶
Perl does not provide any special syntax for class definitions. A package is
simply a namespace containing variables and subroutines. The only difference
is that in a class, the subroutines may expect a reference to an object or the
name of a class as the first argument. This is purely a matter of convention,
so a class may contain both methods and subroutines which
don't operate
on an object or class.
Each package contains a special array called @ISA. The @ISA array contains a
list of that class's parent classes, if any. This array is examined when Perl
does method resolution, which we will cover later.
It is possible to manually set @ISA, and you may see this in older Perl code.
Much older code also uses the base pragma. For new code, we recommend that you
use the parent pragma to declare your parents. This pragma will take care of
setting @ISA. It will also load the parent classes and make sure that the
package doesn't inherit from itself.
However the parent classes are set, the package's @ISA variable will contain a
list of those parents. This is simply a list of scalars, each of which is a
string that corresponds to a package name.
All classes inherit from the UNIVERSAL class implicitly. The UNIVERSAL class is
implemented by the Perl core, and provides several default methods, such as
"isa()", "can()", and "VERSION()". The
"UNIVERSAL" class will
never appear in a package's @ISA
variable.
Perl
only provides method inheritance as a built-in feature. Attribute
inheritance is left up the class to implement. See the "Writing
Accessors" section for details.
A Method is Simply a Subroutine¶
Perl does not provide any special syntax for defining a method. A method is
simply a regular subroutine, and is declared with "sub". What makes
a method special is that it expects to receive either an object or a class
name as its first argument.
Perl
does provide special syntax for method invocation, the
"->" operator. We will cover this in more detail later.
Most methods you write will expect to operate on objects:
sub save {
my $self = shift;
open my $fh, '>', $self->path() or die $!;
print {$fh} $self->data() or die $!;
close $fh or die $!;
}
Method Invocation¶
Calling a method on an object is written as "$object->method".
The left hand side of the method invocation (or arrow) operator is the object
(or class name), and the right hand side is the method name.
my $pod = File->new( 'perlobj.pod', $data );
$pod->save();
The "->" syntax is also used when dereferencing a reference. It
looks like the same operator, but these are two different operations.
When you call a method, the thing on the left side of the arrow is passed as the
first argument to the method. That means when we call
"Critter->new()", the "new()" method receives the
string "Critter" as its first argument. When we call
"$fred->speak()", the $fred variable is passed as the first
argument to "speak()".
Just as with any Perl subroutine, all of the arguments passed in @_ are aliases
to the original argument. This includes the object itself. If you assign
directly to $_[0] you will change the contents of the variable that holds the
reference to the object. We recommend that you don't do this unless you know
exactly what you're doing.
Perl knows what package the method is in by looking at the left side of the
arrow. If the left hand side is a package name, it looks for the method in
that package. If the left hand side is an object, then Perl looks for the
method in the package that the object has been blessed into.
If the left hand side is neither a package name nor an object, then the method
call will cause an error, but see the section on "Method Call
Variations" for more nuances.
Inheritance¶
We already talked about the special @ISA array and the parent pragma.
When a class inherits from another class, any methods defined in the parent
class are available to the child class. If you attempt to call a method on an
object that isn't defined in its own class, Perl will also look for that
method in any parent classes it may have.
package File::MP3;
use parent 'File'; # sets @File::MP3::ISA = ('File');
my $mp3 = File::MP3->new( 'Andvari.mp3', $data );
$mp3->save();
Since we didn't define a "save()" method in the "File::MP3"
class, Perl will look at the "File::MP3" class's parent classes to
find the "save()" method. If Perl cannot find a "save()"
method anywhere in the inheritance hierarchy, it will die.
In this case, it finds a "save()" method in the "File"
class. Note that the object passed to "save()" in this case is still
a "File::MP3" object, even though the method is found in the
"File" class.
We can override a parent's method in a child class. When we do so, we can still
call the parent class's method with the "SUPER" pseudo-class.
sub save {
my $self = shift;
say 'Prepare to rock';
$self->SUPER::save();
}
The "SUPER" modifier can
only be used for method calls. You
can't use it for regular subroutine calls or class methods:
SUPER::save($thing); # FAIL: looks for save() sub in package SUPER
SUPER->save($thing); # FAIL: looks for save() method in class
# SUPER
$thing->SUPER::save(); # Okay: looks for save() method in parent
# classes
How SUPER is Resolved
The "SUPER" pseudo-class is resolved from the package where the call
is made. It is
not resolved based on the object's class. This is
important, because it lets methods at different levels within a deep
inheritance hierarchy each correctly call their respective parent methods.
package A;
sub new {
return bless {}, shift;
}
sub speak {
my $self = shift;
say 'A';
}
package B;
use parent -norequire, 'A';
sub speak {
my $self = shift;
$self->SUPER::speak();
say 'B';
}
package C;
use parent -norequire, 'B';
sub speak {
my $self = shift;
$self->SUPER::speak();
say 'C';
}
my $c = C->new();
$c->speak();
In this example, we will get the following output:
A
B
C
This demonstrates how "SUPER" is resolved. Even though the object is
blessed into the "C" class, the "speak()" method in the
"B" class can still call "SUPER::speak()" and expect it to
correctly look in the parent class of "B" (i.e the class the method
call is in), not in the parent class of "C" (i.e. the class the
object belongs to).
There are rare cases where this package-based resolution can be a problem. If
you copy a subroutine from one package to another, "SUPER"
resolution will be done based on the original package.
Multiple Inheritance
Multiple inheritance often indicates a design problem, but Perl always gives you
enough rope to hang yourself with if you ask for it.
To declare multiple parents, you simply need to pass multiple class names to
"use parent":
package MultiChild;
use parent 'Parent1', 'Parent2';
Method Resolution Order
Method resolution order only matters in the case of multiple inheritance. In the
case of single inheritance, Perl simply looks up the inheritance chain to find
a method:
Grandparent
|
Parent
|
Child
If we call a method on a "Child" object and that method is not defined
in the "Child" class, Perl will look for that method in the
"Parent" class and then, if necessary, in the
"Grandparent" class.
If Perl cannot find the method in any of these classes, it will die with an
error message.
When a class has multiple parents, the method lookup order becomes more
complicated.
By default, Perl does a depth-first left-to-right search for a method. That
means it starts with the first parent in the @ISA array, and then searches all
of its parents, grandparents, etc. If it fails to find the method, it then
goes to the next parent in the original class's @ISA array and searches from
there.
SharedGreatGrandParent
/ \
PaternalGrandparent MaternalGrandparent
\ /
Father Mother
\ /
Child
So given the diagram above, Perl will search "Child",
"Father", "PaternalGrandparent",
"SharedGreatGrandParent", "Mother", and finally
"MaternalGrandparent". This may be a problem because now we're
looking in "SharedGreatGrandParent"
before we've checked all
its derived classes (i.e. before we tried "Mother" and
"MaternalGrandparent").
It is possible to ask for a different method resolution order with the mro
pragma.
package Child;
use mro 'c3';
use parent 'Father', 'Mother';
This pragma lets you switch to the "C3" resolution order. In simple
terms, "C3" order ensures that shared parent classes are never
searched before child classes, so Perl will now search: "Child",
"Father", "PaternalGrandparent", "Mother"
"MaternalGrandparent", and finally
"SharedGreatGrandParent". Note however that this is not
"breadth-first" searching: All the "Father" ancestors
(except the common ancestor) are searched before any of the "Mother"
ancestors are considered.
The C3 order also lets you call methods in sibling classes with the
"next" pseudo-class. See the mro documentation for more details on
this feature.
Method Resolution Caching
When Perl searches for a method, it caches the lookup so that future calls to
the method do not need to search for it again. Changing a class's parent class
or adding subroutines to a class will invalidate the cache for that class.
The mro pragma provides some functions for manipulating the method cache
directly.
Writing Constructors¶
As we mentioned earlier, Perl provides no special constructor syntax. This means
that a class must implement its own constructor. A constructor is simply a
class method that returns a reference to a new object.
The constructor can also accept additional parameters that define the object.
Let's write a real constructor for the "File" class we used earlier:
package File;
sub new {
my $class = shift;
my ( $path, $data ) = @_;
my $self = bless {
path => $path,
data => $data,
}, $class;
return $self;
}
As you can see, we've stored the path and file data in the object itself.
Remember, under the hood, this object is still just a hash. Later, we'll write
accessors to manipulate this data.
For our File::MP3 class, we can check to make sure that the path we're given
ends with ".mp3":
package File::MP3;
sub new {
my $class = shift;
my ( $path, $data ) = @_;
die "You cannot create a File::MP3 without an mp3 extension\n"
unless $path =~ /\.mp3\z/;
return $class->SUPER::new(@_);
}
This constructor lets its parent class do the actual object construction.
Attributes¶
An attribute is a piece of data belonging to a particular object. Unlike most
object-oriented languages, Perl provides no special syntax or support for
declaring and manipulating attributes.
Attributes are often stored in the object itself. For example, if the object is
an anonymous hash, we can store the attribute values in the hash using the
attribute name as the key.
While it's possible to refer directly to these hash keys outside of the class,
it's considered a best practice to wrap all access to the attribute with
accessor methods.
This has several advantages. Accessors make it easier to change the
implementation of an object later while still preserving the original API.
An accessor lets you add additional code around attribute access. For example,
you could apply a default to an attribute that wasn't set in the constructor,
or you could validate that a new value for the attribute is acceptable.
Finally, using accessors makes inheritance much simpler. Subclasses can use the
accessors rather than having to know how a parent class is implemented
internally.
Writing Accessors
As with constructors, Perl provides no special accessor declaration syntax, so
classes must provide explicitly written accessor methods. There are two common
types of accessors, read-only and read-write.
A simple read-only accessor simply gets the value of a single attribute:
sub path {
my $self = shift;
return $self->{path};
}
A read-write accessor will allow the caller to set the value as well as get it:
sub path {
my $self = shift;
if (@_) {
$self->{path} = shift;
}
return $self->{path};
}
An Aside About Smarter and Safer Code¶
Our constructor and accessors are not very smart. They don't check that a $path
is defined, nor do they check that a $path is a valid filesystem path.
Doing these checks by hand can quickly become tedious. Writing a bunch of
accessors by hand is also incredibly tedious. There are a lot of modules on
CPAN that can help you write safer and more concise code, including the
modules we recommend in perlootut.
Method Call Variations¶
Perl supports several other ways to call methods besides the
"$object->method()" usage we've seen so far.
Method Names as Strings
Perl lets you use a scalar variable containing a string as a method name:
my $file = File->new( $path, $data );
my $method = 'save';
$file->$method();
This works exactly like calling "$file->save()". This can be very
useful for writing dynamic code. For example, it allows you to pass a method
name to be called as a parameter to another method.
Class Names as Strings
Perl also lets you use a scalar containing a string as a class name:
my $class = 'File';
my $file = $class->new( $path, $data );
Again, this allows for very dynamic code.
Subroutine References as Methods
You can also use a subroutine reference as a method:
my $sub = sub {
my $self = shift;
$self->save();
};
$file->$sub();
This is exactly equivalent to writing "$sub->($file)". You may see
this idiom in the wild combined with a call to "can":
if ( my $meth = $object->can('foo') ) {
$object->$meth();
}
Deferencing Method Call
Perl also lets you use a dereferenced scalar reference in a method call. That's
a mouthful, so let's look at some code:
$file->${ \'save' };
$file->${ returns_scalar_ref() };
$file->${ \( returns_scalar() ) };
$file->${ returns_ref_to_sub_ref() };
This works if the dereference produces a string
or a subroutine
reference.
Method Calls on Filehandles
Under the hood, Perl filehandles are instances of the "IO::Handle" or
"IO::File" class. Once you have an open filehandle, you can call
methods on it. Additionally, you can call methods on the "STDIN",
"STDOUT", and "STDERR" filehandles.
open my $fh, '>', 'path/to/file';
$fh->autoflush();
$fh->print('content');
STDOUT->autoflush();
Invoking Class Methods¶
Because Perl allows you to use barewords for package names and subroutine names,
it sometimes interprets a bareword's meaning incorrectly. For example, the
construct "Class->new()" can be interpreted as either
"'Class'->new()" or "Class()->new()". In English,
that second interpretation reads as "call a subroutine named
Class(), then call
new() as a method on the return value of
Class()". If there is a subroutine named "Class()" in
the current namespace, Perl will always interpret "Class->new()"
as the second alternative: a call to "new()" on the object returned
by a call to "Class()"
You can force Perl to use the first interpretation (i.e. as a method call on the
class named "Class") in two ways. First, you can append a
"::" to the class name:
Class::->new()
Perl will always interpret this as a method call.
Alternatively, you can quote the class name:
'Class'->new()
Of course, if the class name is in a scalar Perl will do the right thing as
well:
my $class = 'Class';
$class->new();
Indirect Object Syntax
Outside of the file handle case, use of this syntax is discouraged as
it can confuse the Perl interpreter. See below for more details.
Perl supports another method invocation syntax called "indirect
object" notation. This syntax is called "indirect" because the
method comes before the object it is being invoked on.
This syntax can be used with any class or object method:
my $file = new File $path, $data;
save $file;
We recommend that you avoid this syntax, for several reasons.
First, it can be confusing to read. In the above example, it's not clear if
"save" is a method provided by the "File" class or simply
a subroutine that expects a file object as its first argument.
When used with class methods, the problem is even worse. Because Perl allows
subroutine names to be written as barewords, Perl has to guess whether the
bareword after the method is a class name or subroutine name. In other words,
Perl can resolve the syntax as either "File->new( $path, $data )"
or "new( File( $path, $data ) )".
To parse this code, Perl uses a heuristic based on what package names it has
seen, what subroutines exist in the current package, what barewords it has
previously seen, and other input. Needless to say, heuristics can produce very
surprising results!
Older documentation (and some CPAN modules) encouraged this syntax, particularly
for constructors, so you may still find it in the wild. However, we encourage
you to avoid using it in new code.
You can force Perl to interpret the bareword as a class name by appending
"::" to it, like we saw earlier:
my $file = new File:: $path, $data;
"bless", "blessed", and "ref"¶
As we saw earlier, an object is simply a data structure that has been blessed
into a class via the "bless" function. The "bless"
function can take either one or two arguments:
my $object = bless {}, $class;
my $object = bless {};
In the first form, the anonymous hash is being blessed into the class in $class.
In the second form, the anonymous hash is blessed into the current package.
The second form is strongly discouraged, because it breaks the ability of a
subclass to reuse the parent's constructor, but you may still run across it in
existing code.
If you want to know whether a particular scalar refers to an object, you can use
the "blessed" function exported by Scalar::Util, which is shipped
with the Perl core.
use Scalar::Util 'blessed';
if ( defined blessed($thing) ) { ... }
If $thing refers to an object, then this function returns the name of the
package the object has been blessed into. If $thing doesn't contain a
reference to a blessed object, the "blessed" function returns
"undef".
Note that "blessed($thing)" will also return false if $thing has been
blessed into a class named "0". This is a possible, but quite
pathological. Don't create a class named "0" unless you know what
you're doing.
Similarly, Perl's built-in "ref" function treats a reference to a
blessed object specially. If you call "ref($thing)" and $thing holds
a reference to an object, it will return the name of the class that the object
has been blessed into.
If you simply want to check that a variable contains an object reference, we
recommend that you use "defined blessed($object)", since
"ref" returns true values for all references, not just objects.
The UNIVERSAL Class¶
All classes automatically inherit from the UNIVERSAL class, which is built-in to
the Perl core. This class provides a number of methods, all of which can be
called on either a class or an object. You can also choose to override some of
these methods in your class. If you do so, we recommend that you follow the
built-in semantics described below.
- isa($class)
- The "isa" method returns true if the object is a member
of the class in $class, or a member of a subclass of $class.
If you override this method, it should never throw an exception.
- DOES($role)
- The "DOES" method returns true if its object claims to
perform the role $role. By default, this is equivalent to "isa".
This method is provided for use by object system extensions that implement
roles, like "Moose" and "Role::Tiny".
You can also override "DOES" directly in your own classes. If you
override this method, it should never throw an exception.
- can($method)
- The "can" method checks to see if the class or object it was
called on has a method named $method. This checks for the method in the
class and all of its parents. If the method exists, then a reference to
the subroutine is returned. If it does not then "undef" is
returned.
If your class responds to method calls via "AUTOLOAD", you may
want to overload "can" to return a subroutine reference for
methods which your "AUTOLOAD" method handles.
If you override this method, it should never throw an exception.
- VERSION($need)
- The "VERSION" method returns the version number of the class
(package).
If the $need argument is given then it will check that the current version
(as defined by the $VERSION variable in the package) is greater than or
equal to $need; it will die if this is not the case. This method is called
automatically by the "VERSION" form of "use".
use Package 1.2 qw(some imported subs);
# implies:
Package->VERSION(1.2);
We recommend that you use this method to access another package's version,
rather than looking directly at $Package::VERSION. The package you are
looking at could have overridden the "VERSION" method.
We also recommend using this method to check whether a module has a
sufficient version. The internal implementation uses the version module to
make sure that different types of version numbers are compared
correctly.
AUTOLOAD ¶
If you call a method that doesn't exist in a class, Perl will throw an error.
However, if that class or any of its parent classes defines an
"AUTOLOAD" method, that "AUTOLOAD" method is called
instead.
"AUTOLOAD" is called as a regular method, and the caller will not know
the difference. Whatever value your "AUTOLOAD" method returns is
returned to the caller.
The fully qualified method name that was called is available in the $AUTOLOAD
package global for your class. Since this is a global, if you want to refer to
do it without a package name prefix under "strict 'vars'", you need
to declare it.
# XXX - this is a terrible way to implement accessors, but it makes
# for a simple example.
our $AUTOLOAD;
sub AUTOLOAD {
my $self = shift;
# Remove qualifier from original method name...
my $called = $AUTOLOAD =~ s/.*:://r;
# Is there an attribute of that name?
die "No such attribute: $called"
unless exists $self->{$called};
# If so, return it...
return $self->{$called};
}
sub DESTROY { } # see below
Without the "our $AUTOLOAD" declaration, this code will not compile
under the strict pragma.
As the comment says, this is not a good way to implement accessors. It's slow
and too clever by far. However, you may see this as a way to provide accessors
in older Perl code. See perlootut for recommendations on OO coding in Perl.
If your class does have an "AUTOLOAD" method, we strongly recommend
that you override "can" in your class as well. Your overridden
"can" method should return a subroutine reference for any method
that your "AUTOLOAD" responds to.
Destructors¶
When the last reference to an object goes away, the object is destroyed. If you
only have one reference to an object stored in a lexical scalar, the object is
destroyed when that scalar goes out of scope. If you store the object in a
package global, that object may not go out of scope until the program exits.
If you want to do something when the object is destroyed, you can define a
"DESTROY" method in your class. This method will always be called by
Perl at the appropriate time, unless the method is empty.
This is called just like any other method, with the object as the first
argument. It does not receive any additional arguments. However, the $_[0]
variable will be read-only in the destructor, so you cannot assign a value to
it.
If your "DESTROY" method throws an error, this error will be ignored.
It will not be sent to "STDERR" and it will not cause the program to
die. However, if your destructor is running inside an "eval {}"
block, then the error will change the value of $@.
Because "DESTROY" methods can be called at any time, you should
localize any global variables you might update in your "DESTROY". In
particular, if you use "eval {}" you should localize $@, and if you
use "system" or backticks you should localize $?.
If you define an "AUTOLOAD" in your class, then Perl will call your
"AUTOLOAD" to handle the "DESTROY" method. You can prevent
this by defining an empty "DESTROY", like we did in the autoloading
example. You can also check the value of $AUTOLOAD and return without doing
anything when called to handle "DESTROY".
Global Destruction
The order in which objects are destroyed during the global destruction before
the program exits is unpredictable. This means that any objects contained by
your object may already have been destroyed. You should check that a contained
object is defined before calling a method on it:
sub DESTROY {
my $self = shift;
$self->{handle}->close() if $self->{handle};
}
You can use the "${^GLOBAL_PHASE}" variable to detect if you are
currently in the global destruction phase:
sub DESTROY {
my $self = shift;
return if ${^GLOBAL_PHASE} eq 'DESTRUCT';
$self->{handle}->close();
}
Note that this variable was added in Perl 5.14.0. If you want to detect the
global destruction phase on older versions of Perl, you can use the
"Devel::GlobalDestruction" module on CPAN.
If your "DESTROY" method issues a warning during global destruction,
the Perl interpreter will append the string " during global
destruction" the warning.
During global destruction, Perl will always garbage collect objects before
unblessed references. See "PERL_DESTRUCT_LEVEL" in perlhacktips for
more information about global destruction.
Non-Hash Objects¶
All the examples so far have shown objects based on a blessed hash. However,
it's possible to bless any type of data structure or referent, including
scalars, globs, and subroutines. You may see this sort of thing when looking
at code in the wild.
Here's an example of a module as a blessed scalar:
package Time;
use strict;
use warnings;
sub new {
my $class = shift;
my $time = time;
return bless \$time, $class;
}
sub epoch {
my $self = shift;
return ${ $self };
}
my $time = Time->new();
print $time->epoch();
Inside-Out objects¶
In the past, the Perl community experimented with a technique called
"inside-out objects". An inside-out object stores its data outside
of the object's reference, indexed on a unique property of the object, such as
its memory address, rather than in the object itself. This has the advantage
of enforcing the encapsulation of object attributes, since their data is not
stored in the object itself.
This technique was popular for a while (and was recommended in Damian Conway's
Perl Best Practices), but never achieved universal adoption. The
Object::InsideOut module on CPAN provides a comprehensive implementation of
this technique, and you may see it or other inside-out modules in the wild.
Here is a simple example of the technique, using the Hash::Util::FieldHash core
module. This module was added to the core to support inside-out object
implementations.
package Time;
use strict;
use warnings;
use Hash::Util::FieldHash 'fieldhash';
fieldhash my %time_for;
sub new {
my $class = shift;
my $self = bless \( my $object ), $class;
$time_for{$self} = time;
return $self;
}
sub epoch {
my $self = shift;
return $time_for{$self};
}
my $time = Time->new;
print $time->epoch;
Pseudo-hashes¶
The pseudo-hash feature was an experimental feature introduced in earlier
versions of Perl and removed in 5.10.0. A pseudo-hash is an array reference
which can be accessed using named keys like a hash. You may run in to some
code in the wild which uses it. See the fields pragma for more information.
SEE ALSO¶
A kinder, gentler tutorial on object-oriented programming in Perl can be found
in perlootut. You should also check out perlmodlib for some style guides on
constructing both modules and classes.