4 Consistent formatting of this file is achieved with:
5 perl ./Porting/podtidy pod/perlobj.pod
10 perlobj - Perl object reference
14 This document provides a reference for Perl's object orientation
15 features. If you're looking for an introduction to object-oriented
16 programming in Perl, please see L<perlootut>.
18 In order to understand Perl objects, you first need to understand
19 references in Perl. See L<perlref> for details.
21 This document describes all of Perl's object-oriented (OO) features
22 from the ground up. If you're just looking to write some
23 object-oriented code of your own, you are probably better served by
24 using one of the object systems from CPAN described in L<perlootut>.
26 If you're looking to write your own object system, or you need to
27 maintain code which implements objects from scratch then this document
28 will help you understand exactly how Perl does object orientation.
30 There are a few basic principles which define object oriented Perl:
36 An object is simply a data structure that knows to which class it
41 A class is simply a package. A class provides methods that expect to
46 A method is simply a subroutine that expects a reference to an object
47 (or a package name, for class methods) as the first argument.
51 Let's look at each of these principles in depth.
53 =head2 An Object is Simply a Data Structure
54 X<object> X<bless> X<constructor> X<new>
56 Unlike many other languages which support object orientation, Perl does
57 not provide any special syntax for constructing an object. Objects are
58 merely Perl data structures (hashes, arrays, scalars, filehandles,
59 etc.) that have been explicitly associated with a particular class.
61 That explicit association is created by the built-in C<bless> function,
62 which is typically used within the I<constructor> subroutine of the
65 Here is a simple constructor:
72 return bless {}, $class;
75 The name C<new> isn't special. We could name our constructor something
83 return bless {}, $class;
86 The modern convention for OO modules is to always use C<new> as the
87 name for the constructor, but there is no requirement to do so. Any
88 subroutine that blesses a data structure into a class is a valid
91 In the previous examples, the C<{}> code creates a reference to an
92 empty anonymous hash. The C<bless> function then takes that reference
93 and associates the hash with the class in C<$class>. In the simplest
94 case, the C<$class> variable will end up containing the string "File".
96 We can also use a variable to store a reference to the data structure
97 that is being blessed as our object:
108 Once we've blessed the hash referred to by C<$self> we can start
109 calling methods on it. This is useful if you want to put object
110 initialization in its own separate method:
118 $self->_initialize();
123 Since the object is also a hash, you can treat it as one, using it to
124 store data associated with the object. Typically, code inside the class
125 can treat the hash as an accessible data structure, while code outside
126 the class should always treat the object as opaque. This is called
127 B<encapsulation>. Encapsulation means that the user of an object does
128 not have to know how it is implemented. The user simply calls
129 documented methods on the object.
131 Note, however, that (unlike most other OO languages) Perl does not
132 ensure or enforce encapsulation in any way. If you want objects to
133 actually I<be> opaque you need to arrange for that yourself. This can
134 be done in a varierty of ways, including using L<"Inside-Out objects">
135 or modules from CPAN.
137 =head3 Objects Are Blessed; Variables Are Not
139 When we bless something, we are not blessing the variable which
140 contains a reference to that thing, nor are we blessing the reference
141 that the variable stores; we are blessing the thing that the variable
142 refers to (sometimes known as the I<referent>). This is best
143 demonstrated with this code:
145 use Scalar::Util 'blessed';
151 print blessed( $bar ); # prints "Class"
153 $bar = "some other value";
154 print blessed( $bar ); # prints undef
156 When we call C<bless> on a variable, we are actually blessing the
157 underlying data structure that the variable refers to. We are not
158 blessing the reference itself, nor the variable that contains that
159 reference. That's why the second call to C<blessed( $bar )> returns
160 false. At that point C<$bar> is no longer storing a reference to an
163 You will sometimes see older books or documentation mention "blessing a
164 reference" or describe an object as a "blessed reference", but this is
165 incorrect. It isn't the reference that is blessed as an object; it's
166 the thing the reference refers to (i.e. the referent).
168 =head2 A Class is Simply a Package
169 X<class> X<package> X<@ISA> X<inheritance>
171 Perl does not provide any special syntax for class definitions. A
172 package is simply a namespace containing variables and subroutines. The
173 only difference is that in a class, the subroutines may expect a
174 reference to an object or the name of a class as the first argument.
175 This is purely a matter of convention, so a class may contain both
176 methods and subroutines which I<don't> operate on an object or class.
178 Each package contains a special array called C<@ISA>. The C<@ISA> array
179 contains a list of that class's parent classes, if any. This array is
180 examined when Perl does method resolution, which we will cover later.
182 It is possible to manually set C<@ISA>, and you may see this in older
183 Perl code. Much older code also uses the L<base> pragma. For new code,
184 we recommend that you use the L<parent> pragma to declare your parents.
185 This pragma will take care of setting C<@ISA>. It will also load the
186 parent classes and make sure that the package doesn't inherit from
189 However the parent classes are set, the package's C<@ISA> variable will
190 contain a list of those parents.
192 All classes inherit from the L<UNIVERSAL> class implicitly. The
193 L<UNIVERSAL> class is implemented by the Perl core, and provides
194 several default methods, such as C<isa()>, C<can()>, and C<VERSION()>.
195 The C<UNIVERSAL> class will I<never> appear in a package's C<@ISA>
198 Perl I<only> provides method inheritance as a built-in feature.
199 Attribute inheritance is left up the class to implement. See the
200 L</Writing Accessors> section for details.
202 =head2 A Method is Simply a Subroutine
205 Perl does not provide any special syntax for defining a method. A
206 method is simply a regular subroutine, and is declared with C<sub>.
207 What makes a method special is that it expects to receive either an
208 object or a class name as its first argument.
210 Perl I<does> provide special syntax for method invocation, the C<< ->
211 >> operator. We will cover this in more detail later.
213 Most methods you write will expect to operate on objects:
218 open my $fh, '>', $self->path() or die $!;
219 print {$fh} $self->data() or die $!;
223 =head2 Method Invocation
224 X<invocation> X<method> X<arrow> X<< -> >>
226 Calling a method on an object is written as C<< $object->method >>.
228 The left hand side of the method invocation (or arrow) operator is the
229 object (or class name), and the right hand side is the method name.
231 my $pod = File->new( 'perlobj.pod', $data );
234 The C<< -> >> syntax is also used when dereferencing a reference. It
235 looks like the same operator, but these are two different operations.
237 When you call a method, the thing on the left side of the arrow is
238 passed as the first argument to the method. That means when we call C<<
239 Critter->new() >>, the C<new()> method receives the string C<"Critter">
240 as its first argument. When we call C<< $fred->speak() >>, the C<$fred>
241 variable is passed as the first argument to C<speak()>.
243 Just as with any Perl subroutine, all of the arguments passed in C<@_>
244 are aliases to the original argument. This includes the object itself.
245 If you assign directly to C<$_[0]> you will change the contents of the
246 variable that holds the reference to the object. We recommend that you
247 don't do this unless you know exactly what you're doing.
249 Perl knows what package the method is in by looking at the left side of
250 the arrow. If the left hand side is a package name, it looks for the
251 method in that package. If the left hand side is an object, then Perl
252 looks for the method in the package that the object has been blessed
255 If the left hand side is neither a package name nor an object, then the
256 method call will cause an error, but see the section on L</Method Call
257 Variations> for more nuances.
262 We already talked about the special C<@ISA> array and the L<parent>
265 When a class inherits from another class, any methods defined in the
266 parent class are available to the child class. If you attempt to call a
267 method on an object that isn't defined in its own class, Perl will also
268 look for that method in any parent classes it may have.
271 use parent 'File'; # sets @File::MP3::ISA = ('File');
273 my $mp3 = File::MP3->new( 'Andvari.mp3', $data );
276 Since we didn't define a C<save()> method in the C<File::MP3> class,
277 Perl will look at the C<File::MP3> class's parent classes to find the
278 C<save()> method. If Perl cannot find a C<save()> method anywhere in
279 the inheritance hierarchy, it will die.
281 In this case, it finds a C<save()> method in the C<File> class. Note
282 that the object passed to C<save()> in this case is still a
283 C<File::MP3> object, even though the method is found in the C<File>
286 We can override a parent's method in a child class. When we do so, we
287 can still call the parent class's method with the C<SUPER>
293 say 'Prepare to rock';
294 $self->SUPER::save();
297 The C<SUPER> modifier can I<only> be used for method calls. You can't
298 use it for regular subroutine calls or class methods:
300 SUPER::save($thing); # FAIL: looks for save() sub in package SUPER
302 SUPER->save($thing); # FAIL: looks for save() method in class
305 $thing->SUPER::save(); # Okay: looks for save() method in parent
309 =head3 How SUPER is Resolved
312 The C<SUPER> pseudo-class is resolved from the package where the call
313 is made. It is I<not> resolved based on the object's class. This is
314 important, because it lets methods at different levels within a deep
315 inheritance hierarchy each correctly call their respective parent
321 return bless {}, shift;
327 $self->SUPER::speak();
339 $self->SUPER::speak();
351 $self->SUPER::speak();
359 In this example, we will get the following output:
365 This demonstrates how C<SUPER> is resolved. Even though the object is
366 blessed into the C<C> class, the C<speak()> method in the C<B> class
367 can still call C<SUPER::speak()> and expect it to correctly look in the
368 parent class of C<B> (i.e the class the method call is in), not in the
369 parent class of C<C> (i.e. the class the object belongs to).
371 There are rare cases where this package-based resolution can be a
372 problem. If you copy a subroutine from one package to another, C<SUPER>
373 resolution will be done based on the original package.
375 =head3 Multiple Inheritance
376 X<multiple inheritance>
378 Multiple inheritance often indicates a design problem, but Perl always
379 give you enough rope to hang yourself with if you really need to.
381 To declare multiple parents, you simply need to pass multiple class
382 names to C<use parent>:
386 use parent 'Parent1', 'Parent2';
388 =head3 Method Resolution Order
389 X<method resolution order> X<mro>
391 Method resolution order only matters in the case of multiple
392 inheritance. In the case of single inheritance, Perl simply looks up
393 the inheritance chain to find a method:
401 If we call a method on a C<Child> object and that method is not defined
402 in the C<Child> class, Perl will look for that method in the C<Parent>
403 class and then, if necessary, in the C<Grandparent> class.
405 If Perl cannot find the method in any of these classes, it will die
406 with an error message.
408 When a class has multiple parents, the method lookup order becomes more
411 By default, Perl does a depth-first left-to-right search for a method.
412 That means it starts with the first parent in the C<@ISA> array, and
413 then searches all of its parents, grandparents, etc. If it fails to
414 find the method, it then goes to the next parent in the original
415 class's C<@ISA> array and searches from there.
417 SharedGreatGrandParent
419 PaternalGrandparent MaternalGrandparent
425 So given the diagram above, Perl will search C<Child>, C<Father>,
426 C<PaternalGrandparent>, C<SharedGreatGrandParent>, C<Mother>, and
427 finally C<MaternalGrandparent> This is a problem because now we're
428 looking in C<SharedGreatGrandParent> I<before> we've checked all its
429 derived classes (i.e. before we tried C<Mother> and
430 C<MaternalGrandparent>).
432 It is possible to ask for a different method resolution order with the
438 use parent 'Father', 'Mother';
440 This pragma lets you switch to the "C3" resolution order. In simple
441 terms, "C3" order ensures that parent classes are never searched before
442 child classes, so Perl will now search: C<Child>, C<Father>,
443 C<PaternalGrandparent>, C<Mother> C<MaternalGrandparent>, and finally
444 C<SharedGreatGrandParent>. Note however that this is not
445 "breadth-first" searching: All the C<Father> ancestors (except the
446 common ancestor) are searched before any of the C<Mother> ancestors are
449 The C3 order also lets you call methods in sibling classes with the
450 C<next> pseudo-class. See the L<mro> documentation for more details on
453 =head3 Method Resolution Caching
455 When Perl searches for a method, it caches the lookup so that future
456 calls to the method do not need to search for it again. Changing a
457 class's parent class or adding subroutines to a class will invalidate
458 the cache for that class.
460 The L<mro> pragma provides some functions for manipulating the method
463 =head2 Writing Constructors
466 As we mentioned earlier, Perl provides no special constructor syntax.
467 This means that a class must implement its own constructor. A
468 constructor is simply a class method that returns a reference to a new
471 The constructor can also accept additional parameters that define the
472 object. Let's write a real constructor for the C<File> class we used
479 my ( $path, $data ) = @_;
489 As you can see, we've stored the path and file data in the object
490 itself. Remember, under the hood, this object is still just a hash.
491 Later, we'll write accessors to manipulate this data.
493 For our File::MP3 class, we can check to make sure that the path we're
494 given ends with ".mp3":
500 my ( $path, $data ) = @_;
502 die "You cannot create a File::MP3 without an mp3 extension\n"
503 unless $path =~ /\.mp3\z/;
505 return $class->SUPER::new(@_);
508 This constructor lets its parent class do the actual object
514 An attribute is a piece of data belonging to a particular object.
515 Unlike most object-oriented languages, Perl provides no special syntax
516 or support for declaring and manipulating attributes.
518 Attributes are often stored in the object itself. For example, if the
519 object is an anonymous hash, we can store the attribute values in the
520 hash using the attribute name as the key.
522 While it's possible to refer directly to these hash keys outside of the
523 class, it's considered a best practice to wrap all access to the
524 attribute with accessor methods.
526 This has several advantages. Accessors make it easier to change the
527 implementation of an object later while still preserving the original
530 An accessor lets you add additional code around attribute access. For
531 example, you could apply a default to an attribute that wasn't set in
532 the constructor, or you could validate that a new value for the
533 attribute is acceptable.
535 Finally, using accessors makes inheritance much simpler. Subclasses can
536 use the accessors rather than having to know the inner details of the
539 =head3 Writing Accessors
542 As with constructors, Perl provides no special accessor declaration
543 syntax, so classes must write them by hand. There are two common types
544 of accessors, read-only and read-write.
546 A simple read-only accessor simply gets the value of a single
552 return $self->{path};
555 A read-write accessor will allow the caller to set the value as well as
562 $self->{path} = shift;
565 return $self->{path};
568 =head2 An Aside About Smarter and Safer Code
570 Our constructor and accessors are not very smart. They don't check that
571 a C<$path> is defined, nor do they check that a C<$path> is a valid
574 Doing these checks by hand can quickly become tedious. Writing a bunch
575 of accessors by hand is also incredibly tedious. There are a lot of
576 modules on CPAN that can help you write safer and more concise code,
577 including the modules we recommend in L<perlootut>.
579 =head2 Method Call Variations
582 Perl supports several other ways to call methods besides the typical
583 C<< $object->method() >> pattern we've seen so far.
585 =head3 Method Names as Strings
587 Perl lets you use a scalar variable containing a string as a method
590 my $file = File->new( $path, $data );
595 This works exactly like calling C<< $file->save() >>. This can be very
596 useful for writing dynamic code. For example, it allows you to pass a
597 method name to be called as a parameter to another method.
599 =head3 Class Names as Strings
601 Perl also lets you use a scalar containing a string as a class name:
605 my $file = $class->new( $path, $data );
607 Again, this allows for very dynamic code.
609 =head3 Subroutine References as Methods
611 You can also use a subroutine reference as a method:
621 This is exactly equivalent to writing C<< $sub->($file) >>. You may see
622 this idiom in the wild combined with a call to C<can>:
624 if ( my $meth = $object->can('foo') ) {
628 =head3 Deferencing Method Call
630 Perl also lets you use a dereferenced scalar reference in a method
631 call. That's a mouthful, so let's look at some code:
634 $file->${ returns_scalar_ref() };
635 $file->${ \( returns_scalar() ) };
637 This works if the dereference produces a string I<or> a subroutine
640 =head3 Method Calls on Filehandles
642 Under the hood, Perl filehandles are instances of the C<IO::Handle> or
643 C<IO::File> class. Once you have an open filehandle, you can call
644 methods on it. Additionally, you can call methods on the C<STDIN>,
645 C<STDOUT>, and C<STDERR> filehandles.
647 open my $fh, '>', 'path/to/file';
649 $fh->print('content');
653 =head2 Invoking Class Methods
656 Because Perl allows you to use barewords for package names and
657 subroutine names, it can sometimes guess wrong about what you intend a
658 bareword to be. For example, the construct C<< Class->new() >> can be
659 interpreted as either C<< 'Class'->new() >> or C<< Class()->new() >>.
660 In English, that second interpretation reads as "call a subroutine
661 named Class(), then call new() as a method on the return value". If
662 there is a subroutine named C<Class()> in the current namespace, Perl
663 will always interpret C<Class->new()> as the second alterative: a call
664 to C<new()> on the object returned by a call to C<Class()>
666 You can force Perl to use the first interpretation (i.e. as a method
667 call on the class named "Class") in two ways. First, you can append a
668 C<::> to the class name:
672 Perl will always interpret this as a method call.
674 Alternatively, you can quote the class name:
678 Of course, if the class name is in a scalar Perl will do the right
684 =head3 Indirect Object Syntax
687 B<Outside of the file handle case, use of this syntax is discouraged,
688 as it can confuse the Perl interpreter. See below for more details.>
690 Perl suports another method invocation syntax called "indirect object"
691 notation. This syntax is called "indirect" because the method comes
692 before the object it is being invoked on.
694 This syntax can be used with any class or object method:
696 my $file = new File $path, $data;
699 We recommend that you avoid this syntax, for several reasons.
701 First, it can be confusing to read. In the above example, it's not
702 clear if C<save> is a method or simply a subroutine that expects a file
703 object as its first argument.
705 When used with class methods, the problem is even worse. Because Perl
706 allows subroutine names to be written as barewords, Perl has to guess
707 whether the bareword after the method is a class name or subroutine
708 name. In other words, Perl can resolve the syntax as either C<<
709 File->new( $path, $data ) >> B<or> C<< new( File( $path, $data ) ) >>.
711 To parse this code, Perl uses a heuristic based on what package names
712 it has seen, what subroutines exist in the current package, what
713 barewords it has previously seen, and other input. Needless to say,
714 heuristics can produce very surprising results!
716 Older documentation (and some CPAN modules) encouraged this syntax,
717 particularly for constructors, so you may still find it in the wild.
718 However, we encourage you to avoid using it in new code.
720 You can force Perl to interpret the bareword as a class name by
721 appending "::" to it, like we saw earlier:
723 my $file = new File:: $path, $data;
725 =head2 C<bless>, C<blessed>, and C<ref>
727 As we saw earlier, an object is simply a data structure that has been
728 blessed into a class via the C<bless> function. The C<bless> function
729 can take either one or two arguments:
731 my $object = bless {}, $class;
732 my $object = bless {};
734 In the first form, the anonymous hash is being blessed into the class
735 in C<$class>. In the second form, the anonymous hash is blessed into
738 The second form is strongly discouraged, because it breaks the ability
739 of a subclass to reuse the parent's constructor, but you may still run
740 across it in existing code.
742 If you want to know whether a particular scalar refers to an object,
743 you can use the C<blessed> function exported by L<Scalar::Util>, which
744 is shipped with the Perl core.
746 use Scalar::Util 'blessed';
748 if ( blessed($thing) ) { ... }
750 If C<$thing> refers to an object, then this function returns the name
751 of the package the object has been blessed into. Note that the example
752 above will return false if C<$thing> has been blessed into a class
753 named "0". If C<$thing> doesn't contain a reference to a blessed
754 object, the C<blessed> function returns false (specifically: C<undef>).
756 Similarly, Perl's built-in C<ref> function treats a reference to a
757 blessed object specially. If you call C<ref($thing)> and C<$thing>
758 holds a reference to an object, it will return the name of the class
759 that the object has been blessed into.
761 If you simply want to check that a variable contains an object
762 reference, we recommend that you use C<defined blessed($object)>, since
763 C<ref> returns true values for all references, not just objects.
765 =head2 The UNIVERSAL Class
768 All classes automatically inherit from the L<UNIVERSAL> class, which is
769 built-in to the Perl core. This class provides a number of methods, all
770 of which can be called on either a class or an object. You can also
771 choose to override some of these methods in your class. If you do so,
772 we recommend that you follow the built-in semantics described below.
779 The C<isa> method returns I<true> if the object is a member of the
780 class in C<$class>, or a member of a subclass of C<$class>.
785 The C<DOES> method returns I<true> if its object claims to perform the
786 role C<$role>. By default, this is equivalent to C<isa>. This method is
787 provided for use by object system extensions that implement roles, like
788 C<Moose> and C<Role::Tiny>.
790 You can also override C<DOES> directly in your own classes.
795 The C<can> method checks to see if the class or object it was called on
796 has a method named C<$method>. This checks for the method in the class
797 and all of its parents. If the method exists, then a reference to the
798 subroutine is returned. If it does not then C<undef> is returned.
800 If your class responds to method calls via C<AUTOLOAD>, you may want to
801 overload C<can> to return a subroutine reference for methods which your
802 C<AUTOLOAD> method handles.
807 The C<VERSION> method returns the version number of the class
810 If the C<$need> argument is given then it will check that the current
811 version (as defined by the $VERSION variable in the package) is greater
812 than or equal to C<$need>; it will die if this is not the case. This
813 method is called automatically by the C<VERSION> form of C<use>.
815 use Package 1.2 qw(some imported subs);
817 Package->VERSION(1.2);
819 We recommend that you use this method to access another package's
820 version, rather than looking directly at C<$Package::VERSION>. The
821 package you are looking at could have overridden the C<VERSION> method.
823 We also recommend using this method to check whether a module has a
824 sufficient version. The internal implementation uses the L<version>
825 module to make sure that different types of version numbers are
833 If you call a method that doesn't exist in a class, Perl will throw an
834 error. However, if that class or any of its parent classes defined an
835 C<AUTOLOAD> method, that method will be called instead.
837 This is called as a regular method, and the caller will not know the
838 difference. Whatever value your C<AUTOLOAD> method returns is given to
841 The fully qualified method name that was called is available in the
842 C<$AUTOLOAD> package global for your class. Since this is a global, if
843 you want to refer to do it without a package name prefix under C<strict
844 'vars'>, you need to declare it.
846 # XXX - this is a terrible way to implement accessors, but it makes
847 # for a simple example.
852 # Remove qualifier from original method name...
853 my $called = $AUTOLOAD =~ s/.*:://r;
855 # Is there an attribute of that name?
856 die "No such attribute: $called"
857 unless exists $self->{$called};
859 # If so, return it...
860 return $self->{$called};
863 sub DESTROY { } # see below
865 Without the C<our $AUTOLOAD> declaration, this code will not compile
866 under the L<strict> pragma.
868 As the comment says, this is not a good way to implement accessors.
869 It's slow and too clever by far. See L<perlootut> for recommendations
870 on OO coding in Perl.
872 If your class does have an C<AUTOLOAD> method, we strongly recommend
873 that you override C<can> in your class as well. Your overridden C<can>
874 method should return a subroutine reference for any method that your
875 C<AUTOLOAD> responds to.
878 X<destructor> X<DESTROY>
880 When the last reference to an object goes away, the object is
881 destroyed. If you only have one reference to an object stored in a
882 lexical scalar, the object is destroyed when that scalar goes out of
883 scope. If you store the object in a package global, that object may not
884 go out of scope until the program exits.
886 If you want to do something when the object is destroyed, you can
887 define a C<DESTROY> method in your class. This method will always be
888 called by Perl at the appropriate time, unless the method is empty.
890 This is called just like any other method, with the object as the first
891 argument. It does not receive any additional arguments. However, the
892 C<$_[0]> variable will be read-only in the destructor, so you cannot
893 assign a value to it.
895 If your C<DESTROY> method throws an error, this error will be ignored.
896 It will not be sent to C<STDERR> and it will not cause the program to
897 die. However, if your destructor is running inside an C<eval {}> block,
898 then the error will change the value of C<$@>.
900 Because C<DESTROY> methods can be called at any time, you should
901 localize any global variables you might update in your C<DESTROY>. In
902 particular, if you use C<eval {}> you should localize C<$@>, and if you
903 use C<system> or backticks, you should localize C<$?>.
905 If you define an C<AUTOLOAD> in your class, then Perl will call your
906 C<AUTOLOAD> to handle the C<DESTROY> method. You can prevent this by
907 defining an empty C<DESTROY>, like we did in the autoloading example.
908 You can also check the value of C<$AUTOLOAD> and return without doing
909 anything when called to handle C<DESTROY>.
911 =head3 Global Destruction
913 The order in which objects are destroyed during the global destruction
914 before the program exits is unpredictable. This means that any objects
915 contained by your object may already have been destroyed. You should
916 check that a contained object is defined before calling a method on it:
921 $self->{handle}->close() if $self->{handle};
924 You can use the C<${^GLOBAL_PHASE}> variable to detect if you are
925 currently in the global destruction phase:
930 return if ${^GLOBAL_PHASE} eq 'DESTRUCT';
932 $self->{handle}->close();
935 Note that this variable was added in Perl 5.14.0. If you want to detect
936 the global destruction phase on older versions of Perl, you can use the
937 C<Devel::GlobalDestruction> module on CPAN.
939 If your C<DESTROY> method issues a warning during global destruction,
940 the Perl interpreter will append the string " during global
941 destruction" the warning.
943 During global destruction, Perl will always garbage collect objects
944 before unblessed references. See L<perlhacktips/PERL_DESTRUCT_LEVEL>
945 for more information about global destruction.
947 =head2 Non-Hash Objects
949 All the examples so far have shown objects based on a blessed hash.
950 However, it's possible to bless any type of data structure or referent,
951 including scalars, globs, and subroutines. You may see this sort of
952 thing when looking at code in the wild.
954 Here's an example of a module as a blessed scalar:
965 return bless \$time, $class;
973 my $time = Time->new();
974 print $time->epoch();
976 =head2 Inside-Out objects
978 In the past, the Perl community experimented with a technique called
979 "inside-out objects". An inside-out object stores its data outside of
980 the object's reference, indexed on a unique property of the object,
981 such as its memory address, rather than in the object itself. This has
982 the advantage of enforcing the encapsulation of object attributes,
983 since their data is not stored in the object itself.
985 This technique was popular for a while (and was recommended in Damian
986 Conway's I<Perl Best Practices>), but never achieved universal
987 adoption. The L<Object::InsideOut> module on CPAN provides a
988 comprehensive implementation of this technique, and you may see it or
989 other inside-out modules in the wild.
991 Here is a simple example of the technique, using the
992 L<Hash::Util::FieldHash> core module. This module was added to the core
993 to support inside-out object implementations.
1000 use Hash::Util::FieldHash 'fieldhash';
1002 fieldhash my %time_for;
1007 my $self = bless \( my $object ), $class;
1009 $time_for{$self} = time;
1017 return $time_for{$self};
1020 my $time = Time->new;
1023 =head2 Pseudo-hashes
1025 The pseudo-hash feature was an experimental feature introduced in
1026 earlier versions of Perl and removed in 5.10.0. A pseudo-hash is an
1027 array reference which can be accessed using named keys like a hash. You
1028 may run in to some code in the wild which uses it. See the L<fields>
1029 pragma for more information.
1033 A kinder, gentler tutorial on object-oriented programming in Perl can
1034 be found in L<perlootut>. You should also check out L<perlmodlib> for
1035 some style guides on constructing both modules and classes.