Tweaks to perlootut by Damian

[perl5.git] / pod / perlobj.pod

=encoding utf8

=for comment
Consistent formatting of this file is achieved with:
  perl ./Porting/podtidy pod/perlobj.pod

=head1 NAME
X<object> X<OOP>

perlobj - Perl object reference

=head1 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 L<perlootut>.

In order to understand Perl objects, you first need to understand
references in Perl. See L<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 L<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:

=over 4

=item 1.

An object is simply a reference that knows to which class it belongs.

=item 2.

A class is simply a package. A class provides methods that expect to
operate on objects.

=item 3.

A method is simply a subroutine that expects an object reference (or a
package name, for class methods) as the first argument.

=back

Let's look at each of these principles in depth.

=head2 An Object is Simply a Reference
X<object> X<bless> X<constructor> X<new>

Unlike many other languages which support object orientation, Perl does
not provide any special syntax for constructing an object. A
constructor is just a subroutine that returns a reference which has
been "blessed" into a class.

Here is a simple constructor:

  package File;

  sub new {
      my $class = shift;

      return bless {}, $class;
  }

The name C<new> isn't special. We could name our constructor something
else:

  sub load {
      my $class = shift;

      return bless {}, $class;
  }

The modern convention for OO modules is to use C<new> as the
constructor, but you are free to use the name that works best for your
class.

The C<{}> code creates an empty anonymous hash reference. The C<bless>
function takes that reference and associates it with the class in
C<$class>. In the simplest case, the C<$class> variable will end up
containing the string "Critter".

We can also call bless with a variable:

  sub new {
      my $class = shift;

      my $self = {};
      bless $self, $class;

      return $self;
  }

Once we've blessed C<$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 reference, you can treat it as one,
using it store data associated with the object. Typically, code inside
the class can treat the hash as reference, while code outside the class
should always treat the object as opaque. This is called
B<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.

=head3 Objects Are Blessed; Variables Are Not

When we bless something, we are not blessing the variable which
contains that thing. This is best demonstrated with this code:

  my $foo = {};
  my $bar = $foo;

  bless $foo, 'Class';
  print blessed( $bar );

This will print out "Class". When we call C<bless> on a variable, we
are actually blessing the underlying reference, not the container.

=head2 A Class is Simply a Package
X<class> X<package> X<@ISA> X<inheritance>

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 an
object or class as the first argument. This is purely a matter of
convention, so a class may contain both methods and subroutines which
I<don't> operate on an object or class.

Each package contains a special array called C<@ISA>. The C<@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 C<@ISA>, and you may see this in older
Perl code. Much older code also uses the L<base> pragma. For new code,
we recommend that you use the L<parent> pragma to declare your parents.
This pragma will take care of setting C<@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 C<@ISA> variable will
contain a list of those parents.

All classes inherit from the L<UNIVERSAL> class implicitly. The
L<UNIVERSAL> class is implemented by the Perl core, and provides
several default methods, such as C<isa()>, C<can()>, and C<VERSION()>.
The C<UNIVERSAL> class will I<never> appear in a package's C<@ISA>
variable.

Perl I<only> provides method inheritance as a built-in feature.
Attribute inheritance is left up the class to implement. See the
L</Writing Accessors> section for details.

=head2 A Method is Simply a Subroutine
X<method>

Perl does not provide any special syntax for defining a method. A
method is simply a regular subroutine, and is declared with C<sub>.
What makes a method special is that it expects to receive either an
object or a class name as its first argument.

Perl I<does> provide special syntax for method invocation, the C<< ->
>> 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 $!;
  }

=head2 Method Invocation
X<invocation> X<method> X<arrow> X<< -> >>

Calling a method on an object is written as C<< $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 C<< -> >> syntax is also used when dereferencing a reference.  It's
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 C<<
Critter->new() >>, the C<new()> method receives the string C<"Critter">
as its first argument. When we call C<< $fred->speak() >>, the C<$fred>
variable is passed as the first argument to C<speak()>.

Just as with any Perl subroutine, all of the arguments passed in C<@_>
are aliases to the original argument. This includes the object itself.
If you assign directly to C<$_[0]> you will change the contents of the
variable which references 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 the package. If the left hand side is an object, then Perl
can tell what class that 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 L</Method Call
Variations> for more nuances.

=head2 Inheritance
X<inheritance>

We already talked about the special C<@ISA> array and the L<parent>
pragma.

When a class inherits from another class, any methods defined in the
parent class are available in 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 C<save()> method in the C<File::MP3> class,
Perl will look at the C<File::MP3> class's parent classes to find the
C<save()> method. If Perl cannot find a C<save()> method anywhere in
the inheritance hierarchy, it will die.

In this case, it finds it in the C<File> class. Note that the object
passed to C<save()> in this case is still a C<File::MP3> object, even
though the method is found in the C<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 C<SUPER>
pseudo-class.

  sub save {
      my $self = shift;

      say 'Prepare to rock';
      $self->SUPER::save();
  }

The C<SUPER> modifier can I<only> be used for method calls. You can't
use it for regular subroutine calls:

  # XXX - This will not work!
  SUPER::save($thing);
  # This won't work either!
  SUPER->save($thing);

=head3 How SUPER is Resolved
X<SUPER>

The C<SUPER> pseudo-class is resolved from the package where the call
is made. It is I<not> resolved based on the object's class. This is
important, because it lets an inheritance hierarchy several levels deep
all call their parent methods.

  package A;

  sub new {
      return bless {}, shift;
  }

  sub speak {
      my $self = shift;

      $self->SUPER::speak();

      say 'A';
  }

  package B;

  use parent 'A';

  sub speak {
      my $self = shift;

      $self->SUPER::speak();

      say 'B';
  }

  package C;

  use parent '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 outupt:

  A
  B
  C

This demonstrates how C<SUPER> is resolved. Even though the object is
blessed into the C<C> class, the C<speak()> method in the C<B> class
can still call C<SUPER::speak()>.

There are cases where this package-based resolution can be a problem.
If you copy a subroutine from one package to another, C<SUPER>
resolution will be done based on the original package.

=head3 Multiple Inheritance
X<multiple inheritance>

Multiple inheritance often indicates a design problem, but Perl always
give you enough rope to hang yourself with.

To declare multiple parents, you simply need to pass multiple class
names to C<use parent>:

  package MultiChild;

  use parent 'Parent1', 'Parent2';

=head3 Method Resolution Order
X<method resolution order> X<mro>

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 C<Child> object and that method is not defined
in the C<Child> class, Perl will look for that method. in the C<Parent>
class and then the C<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 C<@ISA> array, and
then searches all of its parents and so on. If it fails to find the
method, it then goes to the next parent in the original class's C<@ISA>
array and searches from there.

  PaternalGrandparent       MaternalGrandparent
            \                    /
             Father        Mother
                   \      /
                    Child

So given the diagram above, Perl will search C<Child>, C<Father>,
C<PaternalGrandparent>, C<Mother>, and finally C<MaternalGrandparent>

It is possible to ask for a different method resolution order with the
L<mro> pragma.

  package Child;

  use mro 'c3';
  use parent 'Father', 'Mother';

This pragma lets you switch to the "C3" resolution order. In simple
terms, this is a breadth-first order, so Perl will search C<Child>,
C<Father>, C<Mother>, C<PaternalGrandparent>, and finally
C<MaternalGrandparent>.

The C3 order also lets you call methods in sibling classes with the
C<next> pseudo-class. See the L<mro> documentation for more details on
this feature.

=head3 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 L<mro> pragma provides some functions for manipulating the method
cache directly.

=head2 Writing Constructors
X<constructor>

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 new object.

The constructor can also accept additional parameters that define the
object. Let's write a real constructor for the C<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
reference. 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.

=head2 Attributes
X<attribute>

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 reference, 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. Accessor 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 the inner details of the
object.

=head3 Writing Accessors
X<accessor>

As with constructors, Perl provides no special accessor declaration
syntax, so classes must write them by hand. 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};
  }

=head2 An Aside About Smarter and Safer Code

Our constructor and accessors are not very smart. They don't check that
a C<$path> is defined, nor do they check that a C<$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 L<perlootut>.

=head2 Method Call Variations
X<method>

Perl supports several other ways to call methods besides the typical
C<< $object->method() >> pattern we've seen so far.

=head3 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 C<< $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.

=head3 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.

=head3 Subroutine References as Methods

You can also call subroutine reference as a method:

  my $sub = sub {
      my $self = shift;

      $self->save();
  };

  $file->$sub();

This is exactly equivalent to writing C<< $sub->($file) >>. You may see
this idiom in the wild combined with a call to C<can>:

  if ( my $meth = $object->can('foo') ) {
      $object->$meth();
  }

=head3 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() ) };

This works if the dereference produces a string I<or> a subroutine
reference.

=head3 Method Calls on Filehandles

Under the hood, Perl filehandles are instances of the C<IO::Handle> or
C<IO::File> class. Once you have an open filehandle, you can call
methods on it. Additionally, you can call methods on the C<STDING>,
C<STDOUT>, and C<STDERR> filehandles.

  open my $fh, '>', 'path/to/file';
  $fh->autoflush();
  $fh->print('content');

  STDOUT->autoflush();

=head2 Invoking Class Methods
X<invocation>

Because Perl allows you to use barewords for package names and
subroutine names, it can sometimes guess wrong about what you intend a
bareword to be. The construct C<< Class->new() >> can be interpreted as
C<< Class()->new() >>. In English, that reads as "call a subroutine
named Class(), then call new() as a method on the return value".

You can force Perl to interpret this as a class method call in two
ways. First, you can append a C<::> to the class name:

    Class::->new()

Perl will always interpret this as a method call. You can also 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();

=head3 Indirect Object Syntax
X<indirect object>

B<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 suports 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 C<save> is a method 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 C<<
File->new( $path, $data ) >> B<or> C<< 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;

=head2 C<bless>, C<blessed>, and C<ref>

As we saw earlier, an object is simply a reference that has been
blessed into a class with the C<bless> function. The C<bless> function
can take either one or two arguments:

  my $object = bless {}, $class;
  my $object = bless {};

In the first form, the anonymous hash reference is being blessed into
the class in C<$class>. In the second form, the reference is blessed
into the current package.

The second form is 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 C<blessed> function exported by L<Scalar::Util>, which
is shipped with the Perl core.

  use Scalar::Util 'blessed';

  if ( blessed($thing) ) { ... }

If the C<$thing> has been blessed, then this function returns the name
of the package the object has been blessed into. Note that the example
above will return false if C<$thing> has been blessed into a class
named "0". If the C<$thing> is not a blessed reference, the C<blessed>
function returns false.

Similarly, Perl's built-in C<ref> function treats a blessed reference
specially. If you call C<ref($thing)> and C<$thing> is 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, we
recommend that you use C<defined blessed($object)>, since C<ref>
returns true values for all references, not just objects.

=head2 The UNIVERSAL Class
X<UNIVERSAL>

All classes automatically inherit from the L<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.

=over 4

=item isa($class)
X<isa>

The C<isa> method returns I<true> if the object is a member of the
class in C<$class>, or a member of a subclass of C<$class>.

=item DOES($role)
X<DOES>

The C<DOES> method returns I<true> if its object claims to perform the
role C<$role>. By default, this is equivalent to C<isa>. This method is
provided for use by object system extensions that implement roles, like
C<Moose> and C<Role::Tiny>.

You can also override C<DOES> directly in your own classes.

=item can($method)
X<can>

The C<can> method checks to see if the class or object it was called on
has a method named C<$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 C<undef> is returned.

If your class responds to method calls via C<AUTOLOAD>, you may want to
overload C<can> to return a subroutine reference for methods which your
C<AUTOLOAD> method handles.

=item VERSION($need)
X<VERSION>

The C<VERSION> method returns the version number of the class
(package).

If the C<$need> argument is given then it will check that the current
version (as defined by the $VERSION variable in the package) is greater
than or equal to C<$need>; it will die if this is not the case. This
method is called automatically by the C<VERSION> form of C<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 C<$Package::VERSION>. The
package you are looking at could have overridden the C<VERSION> method.

We also recommend using this method to check whether a module has a
sufficient version. The internal implementation uses the L<version>
module to make sure that different types of version numbers are
compared correctly.

=back

=head2 AUTOLOAD
X<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 defined an
C<AUTOLOAD> method, that method will be called instead.

This is called as a regular method, and the caller will not know the
difference. Whatever value your C<AUTOLOAD> method returns is given to
the caller.

The fully qualified method name that was called is available in the
C<$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 C<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;

      ( my $called = $AUTOLOAD ) =~ s/.*:://;

      die "No such attribute: $called"
          unless exists $self->{$called};

      return $self->{$called};
  }

  sub DESTROY { } # see below

Without the C<our $AUTOLOAD> declaration, this code will not compile
under the L<strict> pragma.

As the comment says, this is not a good way to implement accessors.
It's slow and too clever by far. See L<perlootut> for recommendations
on OO coding in Perl.

If your class does have an C<AUTOLOAD> method, we strongly recommend
that you override C<can> in your class as well. Your overridden C<can>
method should return a subroutine reference for any method that your
C<AUTOLOAD> responds to.

=head2 Destructors
X<destructor> X<DESTROY>

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 C<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
C<$_[0]> variable will be read-only in the destructor, so you cannot
assign a value to it.

If your C<DESTROY> method throws an error, this error will be ignored.
It will not be sent to C<STDERR> and it will not cause the program to
die. However, if your destructor is running inside an C<eval {}> block,
then the error will change the value of C<$@>.

Because C<DESTROY> methods can be called at any time, you should
localize any global variables you might update in your C<DESTROY>. In
particular, if you use C<eval {}> you should localize C<$@>, and if you
use C<system> or backticks, you should localize C<$?>.

If you define an C<AUTOLOAD> in your class, then Perl will call your
C<AUTOLOAD> to handle the C<DESTROY> method. You can prevent this by
defining an empty C<DESTROY>, like we did in the example above. You can
also check the value of C<$AUTOLOAD> and return without doing anything
when called to handle C<DESTROY>.

=head3 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 C<${^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
C<Devel::GlobalDestruction> module on CPAN.

If your C<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 L<perlhacktips/PERL_DESTRUCT_LEVEL>
for more information about global destruction.

=head2 Non-Hashref Objects

All the examples so far have shown objects based on a blessed hash
reference. However, it's possible to bless any type of reference,
including scalar refs, glob refs, and code refs. 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();

=head2 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 technique was popular for a while (and was recommended in Damian
Conway's I<Perl Best Practices>), but never achieved wide adoption due
to additional complexity.  The L<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
L<Hash::Util::FieldHash> core module. This module was added to the core
to support inside-out object implementations.

  package Time::InsideOut;

  use strict;
  use warnings;

  use Hash::Util::FieldHash 'fieldhash';

  fieldhash my %TIME;

  sub new {
      my $class = shift;
      my $self = bless \( my $empty ), $class;
      $TIME{$self} = time;

      $self;
  }

  sub epoch {
      my $self = shift;

      $TIME{$self};
  }

  my $time = Time::InsideOut->new;
  print $time->epoch;

=head2 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 L<fields>
pragma for more information.

=head1 SEE ALSO

A kinder, gentler tutorial on object-oriented programming in Perl can
be found in L<perlootut>. You should also check out L<perlmodlib> for
some style guides on constructing both modules and classes.