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perlobj(1)             Perl Programmers Reference Guide             perlobj(1)




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(1). You should also check out perlmodlib(1) for
       style guides on constructing both modules and classes.



perl v5.20.0                      2014-05-26                        perlobj(1)

perl 5.20.0 - Generated Sat May 31 07:57:21 CDT 2014