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<perlreftut> 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 variety 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 ) // 'not blessed'; # prints "Class"
153 $bar = "some other value";
154 print blessed( $bar ) // 'not blessed'; # prints "not blessed"
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 Calling methods from a package means it must be loaded, of course, so
183 you will often want to load a module and add it to C<@ISA> at the same
184 time. You can do so in a single step using the L<parent> pragma.
185 (In older code you may encounter the L<base> pragma, which is nowadays
186 discouraged except when you have to work with the equally discouraged
189 However the parent classes are set, the package's C<@ISA> variable will
190 contain a list of those parents. This is simply a list of scalars, each
191 of which is a string that corresponds to a package name.
193 All classes inherit from the L<UNIVERSAL> class implicitly. The
194 L<UNIVERSAL> class is implemented by the Perl core, and provides
195 several default methods, such as C<isa()>, C<can()>, and C<VERSION()>.
196 The C<UNIVERSAL> class will I<never> appear in a package's C<@ISA>
199 Perl I<only> provides method inheritance as a built-in feature.
200 Attribute inheritance is left up the class to implement. See the
201 L</Writing Accessors> section for details.
203 =head2 A Method is Simply a Subroutine
206 Perl does not provide any special syntax for defining a method. A
207 method is simply a regular subroutine, and is declared with C<sub>.
208 What makes a method special is that it expects to receive either an
209 object or a class name as its first argument.
211 Perl I<does> provide special syntax for method invocation, the C<< ->
212 >> operator. We will cover this in more detail later.
214 Most methods you write will expect to operate on objects:
219 open my $fh, '>', $self->path() or die $!;
220 print {$fh} $self->data() or die $!;
224 =head2 Method Invocation
225 X<invocation> X<method> X<arrow> X<< -> >>
227 Calling a method on an object is written as C<< $object->method >>.
229 The left hand side of the method invocation (or arrow) operator is the
230 object (or class name), and the right hand side is the method name.
232 my $pod = File->new( 'perlobj.pod', $data );
235 The C<< -> >> syntax is also used when dereferencing a reference. It
236 looks like the same operator, but these are two different operations.
238 When you call a method, the thing on the left side of the arrow is
239 passed as the first argument to the method. That means when we call C<<
240 Critter->new() >>, the C<new()> method receives the string C<"Critter">
241 as its first argument. When we call C<< $fred->speak() >>, the C<$fred>
242 variable is passed as the first argument to C<speak()>.
244 Just as with any Perl subroutine, all of the arguments passed in C<@_>
245 are aliases to the original argument. This includes the object itself.
246 If you assign directly to C<$_[0]> you will change the contents of the
247 variable that holds the reference to the object. We recommend that you
248 don't do this unless you know exactly what you're doing.
250 Perl knows what package the method is in by looking at the left side of
251 the arrow. If the left hand side is a package name, it looks for the
252 method in that package. If the left hand side is an object, then Perl
253 looks for the method in the package that the object has been blessed
256 If the left hand side is neither a package name nor an object, then the
257 method call will cause an error, but see the section on L</Method Call
258 Variations> for more nuances.
263 We already talked about the special C<@ISA> array and the L<parent>
266 When a class inherits from another class, any methods defined in the
267 parent class are available to the child class. If you attempt to call a
268 method on an object that isn't defined in its own class, Perl will also
269 look for that method in any parent classes it may have.
272 use parent 'File'; # sets @File::MP3::ISA = ('File');
274 my $mp3 = File::MP3->new( 'Andvari.mp3', $data );
277 Since we didn't define a C<save()> method in the C<File::MP3> class,
278 Perl will look at the C<File::MP3> class's parent classes to find the
279 C<save()> method. If Perl cannot find a C<save()> method anywhere in
280 the inheritance hierarchy, it will die.
282 In this case, it finds a C<save()> method in the C<File> class. Note
283 that the object passed to C<save()> in this case is still a
284 C<File::MP3> object, even though the method is found in the C<File>
287 We can override a parent's method in a child class. When we do so, we
288 can still call the parent class's method with the C<SUPER>
294 say 'Prepare to rock';
295 $self->SUPER::save();
298 The C<SUPER> modifier can I<only> be used for method calls. You can't
299 use it for regular subroutine calls or class methods:
301 SUPER::save($thing); # FAIL: looks for save() sub in package SUPER
303 SUPER->save($thing); # FAIL: looks for save() method in class
306 $thing->SUPER::save(); # Okay: looks for save() method in parent
310 =head3 How SUPER is Resolved
313 The C<SUPER> pseudo-class is resolved from the package where the call
314 is made. It is I<not> resolved based on the object's class. This is
315 important, because it lets methods at different levels within a deep
316 inheritance hierarchy each correctly call their respective parent
322 return bless {}, shift;
333 use parent -norequire, 'A';
338 $self->SUPER::speak();
345 use parent -norequire, 'B';
350 $self->SUPER::speak();
358 In this example, we will get the following output:
364 This demonstrates how C<SUPER> is resolved. Even though the object is
365 blessed into the C<C> class, the C<speak()> method in the C<B> class
366 can still call C<SUPER::speak()> and expect it to correctly look in the
367 parent class of C<B> (i.e the class the method call is in), not in the
368 parent class of C<C> (i.e. the class the object belongs to).
370 There are rare cases where this package-based resolution can be a
371 problem. If you copy a subroutine from one package to another, C<SUPER>
372 resolution will be done based on the original package.
374 =head3 Multiple Inheritance
375 X<multiple inheritance>
377 Multiple inheritance often indicates a design problem, but Perl always
378 gives you enough rope to hang yourself with if you ask for it.
380 To declare multiple parents, you simply need to pass multiple class
381 names to C<use parent>:
385 use parent 'Parent1', 'Parent2';
387 =head3 Method Resolution Order
388 X<method resolution order> X<mro>
390 Method resolution order only matters in the case of multiple
391 inheritance. In the case of single inheritance, Perl simply looks up
392 the inheritance chain to find a method:
400 If we call a method on a C<Child> object and that method is not defined
401 in the C<Child> class, Perl will look for that method in the C<Parent>
402 class and then, if necessary, in the C<Grandparent> class.
404 If Perl cannot find the method in any of these classes, it will die
405 with an error message.
407 When a class has multiple parents, the method lookup order becomes more
410 By default, Perl does a depth-first left-to-right search for a method.
411 That means it starts with the first parent in the C<@ISA> array, and
412 then searches all of its parents, grandparents, etc. If it fails to
413 find the method, it then goes to the next parent in the original
414 class's C<@ISA> array and searches from there.
416 SharedGreatGrandParent
418 PaternalGrandparent MaternalGrandparent
424 So given the diagram above, Perl will search C<Child>, C<Father>,
425 C<PaternalGrandparent>, C<SharedGreatGrandParent>, C<Mother>, and
426 finally C<MaternalGrandparent>. This may be a problem because now we're
427 looking in C<SharedGreatGrandParent> I<before> we've checked all its
428 derived classes (i.e. before we tried C<Mother> and
429 C<MaternalGrandparent>).
431 It is possible to ask for a different method resolution order with the
437 use parent 'Father', 'Mother';
439 This pragma lets you switch to the "C3" resolution order. In simple
440 terms, "C3" order ensures that shared parent classes are never searched
441 before child classes, so Perl will now search: C<Child>, C<Father>,
442 C<PaternalGrandparent>, C<Mother> C<MaternalGrandparent>, and finally
443 C<SharedGreatGrandParent>. Note however that this is not
444 "breadth-first" searching: All the C<Father> ancestors (except the
445 common ancestor) are searched before any of the C<Mother> ancestors are
448 The C3 order also lets you call methods in sibling classes with the
449 C<next> pseudo-class. See the L<mro> documentation for more details on
452 =head3 Method Resolution Caching
454 When Perl searches for a method, it caches the lookup so that future
455 calls to the method do not need to search for it again. Changing a
456 class's parent class or adding subroutines to a class will invalidate
457 the cache for that class.
459 The L<mro> pragma provides some functions for manipulating the method
462 =head2 Writing Constructors
465 As we mentioned earlier, Perl provides no special constructor syntax.
466 This means that a class must implement its own constructor. A
467 constructor is simply a class method that returns a reference to a new
470 The constructor can also accept additional parameters that define the
471 object. Let's write a real constructor for the C<File> class we used
478 my ( $path, $data ) = @_;
488 As you can see, we've stored the path and file data in the object
489 itself. Remember, under the hood, this object is still just a hash.
490 Later, we'll write accessors to manipulate this data.
492 For our C<File::MP3> class, we can check to make sure that the path
493 we're given ends with ".mp3":
499 my ( $path, $data ) = @_;
501 die "You cannot create a File::MP3 without an mp3 extension\n"
502 unless $path =~ /\.mp3\z/;
504 return $class->SUPER::new(@_);
507 This constructor lets its parent class do the actual object
513 An attribute is a piece of data belonging to a particular object.
514 Unlike most object-oriented languages, Perl provides no special syntax
515 or support for declaring and manipulating attributes.
517 Attributes are often stored in the object itself. For example, if the
518 object is an anonymous hash, we can store the attribute values in the
519 hash using the attribute name as the key.
521 While it's possible to refer directly to these hash keys outside of the
522 class, it's considered a best practice to wrap all access to the
523 attribute with accessor methods.
525 This has several advantages. Accessors make it easier to change the
526 implementation of an object later while still preserving the original
529 An accessor lets you add additional code around attribute access. For
530 example, you could apply a default to an attribute that wasn't set in
531 the constructor, or you could validate that a new value for the
532 attribute is acceptable.
534 Finally, using accessors makes inheritance much simpler. Subclasses can
535 use the accessors rather than having to know how a parent class is
536 implemented internally.
538 =head3 Writing Accessors
541 As with constructors, Perl provides no special accessor declaration
542 syntax, so classes must provide explicitly written accessor methods.
543 There are two common types of accessors, read-only and read-write.
545 A simple read-only accessor simply gets the value of a single
551 return $self->{path};
554 A read-write accessor will allow the caller to set the value as well as
561 $self->{path} = shift;
564 return $self->{path};
567 =head2 An Aside About Smarter and Safer Code
569 Our constructor and accessors are not very smart. They don't check that
570 a C<$path> is defined, nor do they check that a C<$path> is a valid
573 Doing these checks by hand can quickly become tedious. Writing a bunch
574 of accessors by hand is also incredibly tedious. There are a lot of
575 modules on CPAN that can help you write safer and more concise code,
576 including the modules we recommend in L<perlootut>.
578 =head2 Method Call Variations
581 Perl supports several other ways to call methods besides the C<<
582 $object->method() >> usage we've seen so far.
584 =head3 Method Names with a Fully Qualified Name
586 Perl allows you to call methods using their fully qualified name (the
587 package and method name):
589 my $mp3 = File::MP3->new( 'Regin.mp3', $data );
592 When you call a fully qualified method name like C<File::save>, the method
593 resolution search for the C<save> method starts in the C<File> class,
594 skipping any C<save> method the C<File::MP3> class may have defined. It
595 still searches the C<File> class's parents if necessary.
597 While this feature is most commonly used to explicitly call methods
598 inherited from an ancestor class, there is no technical restriction
601 my $obj = Tree->new();
604 This calls the C<bark> method from class C<Dog> on an object of class
605 C<Tree>, even if the two classes are completely unrelated. Use this
608 The C<SUPER> pseudo-class that was described earlier is I<not> the same
609 as calling a method with a fully-qualified name. See the earlier
610 L</Inheritance> section for details.
612 =head3 Method Names as Strings
614 Perl lets you use a scalar variable containing a string as a method
617 my $file = File->new( $path, $data );
622 This works exactly like calling C<< $file->save() >>. This can be very
623 useful for writing dynamic code. For example, it allows you to pass a
624 method name to be called as a parameter to another method.
626 =head3 Class Names as Strings
628 Perl also lets you use a scalar containing a string as a class name:
632 my $file = $class->new( $path, $data );
634 Again, this allows for very dynamic code.
636 =head3 Subroutine References as Methods
638 You can also use a subroutine reference as a method:
648 This is exactly equivalent to writing C<< $sub->($file) >>. You may see
649 this idiom in the wild combined with a call to C<can>:
651 if ( my $meth = $object->can('foo') ) {
655 =head3 Dereferencing Method Call
657 Perl also lets you use a dereferenced scalar reference in a method
658 call. That's a mouthful, so let's look at some code:
661 $file->${ returns_scalar_ref() };
662 $file->${ \( returns_scalar() ) };
663 $file->${ returns_ref_to_sub_ref() };
665 This works if the dereference produces a string I<or> a subroutine
668 =head3 Method Calls on Filehandles
670 Under the hood, Perl filehandles are instances of the C<IO::Handle> or
671 C<IO::File> class. Once you have an open filehandle, you can call
672 methods on it. Additionally, you can call methods on the C<STDIN>,
673 C<STDOUT>, and C<STDERR> filehandles.
675 open my $fh, '>', 'path/to/file';
677 $fh->print('content');
681 =head2 Invoking Class Methods
684 Because Perl allows you to use barewords for package names and
685 subroutine names, it sometimes interprets a bareword's meaning
686 incorrectly. For example, the construct C<< Class->new() >> can be
687 interpreted as either C<< 'Class'->new() >> or C<< Class()->new() >>.
688 In English, that second interpretation reads as "call a subroutine
689 named Class(), then call new() as a method on the return value of
690 Class()". If there is a subroutine named C<Class()> in the current
691 namespace, Perl will always interpret C<< Class->new() >> as the second
692 alternative: a call to C<new()> on the object returned by a call to
695 You can force Perl to use the first interpretation (i.e. as a method
696 call on the class named "Class") in two ways. First, you can append a
697 C<::> to the class name:
701 Perl will always interpret this as a method call.
703 Alternatively, you can quote the class name:
707 Of course, if the class name is in a scalar Perl will do the right
713 =head3 Indirect Object Syntax
716 B<Outside of the file handle case, use of this syntax is discouraged as
717 it can confuse the Perl interpreter. See below for more details.>
719 Perl supports another method invocation syntax called "indirect object"
720 notation. This syntax is called "indirect" because the method comes
721 before the object it is being invoked on.
723 This syntax can be used with any class or object method:
725 my $file = new File $path, $data;
728 We recommend that you avoid this syntax, for several reasons.
730 First, it can be confusing to read. In the above example, it's not
731 clear if C<save> is a method provided by the C<File> class or simply a
732 subroutine that expects a file object as its first argument.
734 When used with class methods, the problem is even worse. Because Perl
735 allows subroutine names to be written as barewords, Perl has to guess
736 whether the bareword after the method is a class name or subroutine
737 name. In other words, Perl can resolve the syntax as either C<<
738 File->new( $path, $data ) >> B<or> C<< new( File( $path, $data ) ) >>.
740 To parse this code, Perl uses a heuristic based on what package names
741 it has seen, what subroutines exist in the current package, what
742 barewords it has previously seen, and other input. Needless to say,
743 heuristics can produce very surprising results!
745 Older documentation (and some CPAN modules) encouraged this syntax,
746 particularly for constructors, so you may still find it in the wild.
747 However, we encourage you to avoid using it in new code.
749 You can force Perl to interpret the bareword as a class name by
750 appending "::" to it, like we saw earlier:
752 my $file = new File:: $path, $data;
754 =head2 C<bless>, C<blessed>, and C<ref>
756 As we saw earlier, an object is simply a data structure that has been
757 blessed into a class via the C<bless> function. The C<bless> function
758 can take either one or two arguments:
760 my $object = bless {}, $class;
761 my $object = bless {};
763 In the first form, the anonymous hash is being blessed into the class
764 in C<$class>. In the second form, the anonymous hash is blessed into
767 The second form is strongly discouraged, because it breaks the ability
768 of a subclass to reuse the parent's constructor, but you may still run
769 across it in existing code.
771 If you want to know whether a particular scalar refers to an object,
772 you can use the C<blessed> function exported by L<Scalar::Util>, which
773 is shipped with the Perl core.
775 use Scalar::Util 'blessed';
777 if ( defined blessed($thing) ) { ... }
779 If C<$thing> refers to an object, then this function returns the name
780 of the package the object has been blessed into. If C<$thing> doesn't
781 contain a reference to a blessed object, the C<blessed> function
784 Note that C<blessed($thing)> will also return false if C<$thing> has
785 been blessed into a class named "0". This is a possible, but quite
786 pathological. Don't create a class named "0" unless you know what
789 Similarly, Perl's built-in C<ref> function treats a reference to a
790 blessed object specially. If you call C<ref($thing)> and C<$thing>
791 holds a reference to an object, it will return the name of the class
792 that the object has been blessed into.
794 If you simply want to check that a variable contains an object
795 reference, we recommend that you use C<defined blessed($object)>, since
796 C<ref> returns true values for all references, not just objects.
798 =head2 The UNIVERSAL Class
801 All classes automatically inherit from the L<UNIVERSAL> class, which is
802 built-in to the Perl core. This class provides a number of methods, all
803 of which can be called on either a class or an object. You can also
804 choose to override some of these methods in your class. If you do so,
805 we recommend that you follow the built-in semantics described below.
812 The C<isa> method returns I<true> if the object is a member of the
813 class in C<$class>, or a member of a subclass of C<$class>.
815 If you override this method, it should never throw an exception.
820 The C<DOES> method returns I<true> if its object claims to perform the
821 role C<$role>. By default, this is equivalent to C<isa>. This method is
822 provided for use by object system extensions that implement roles, like
823 C<Moose> and C<Role::Tiny>.
825 You can also override C<DOES> directly in your own classes. If you
826 override this method, it should never throw an exception.
831 The C<can> method checks to see if the class or object it was called on
832 has a method named C<$method>. This checks for the method in the class
833 and all of its parents. If the method exists, then a reference to the
834 subroutine is returned. If it does not then C<undef> is returned.
836 If your class responds to method calls via C<AUTOLOAD>, you may want to
837 overload C<can> to return a subroutine reference for methods which your
838 C<AUTOLOAD> method handles.
840 If you override this method, it should never throw an exception.
845 The C<VERSION> method returns the version number of the class
848 If the C<$need> argument is given then it will check that the current
849 version (as defined by the $VERSION variable in the package) is greater
850 than or equal to C<$need>; it will die if this is not the case. This
851 method is called automatically by the C<VERSION> form of C<use>.
853 use Package 1.2 qw(some imported subs);
855 Package->VERSION(1.2);
857 We recommend that you use this method to access another package's
858 version, rather than looking directly at C<$Package::VERSION>. The
859 package you are looking at could have overridden the C<VERSION> method.
861 We also recommend using this method to check whether a module has a
862 sufficient version. The internal implementation uses the L<version>
863 module to make sure that different types of version numbers are
871 If you call a method that doesn't exist in a class, Perl will throw an
872 error. However, if that class or any of its parent classes defines an
873 C<AUTOLOAD> method, that C<AUTOLOAD> method is called instead.
875 C<AUTOLOAD> is called as a regular method, and the caller will not know
876 the difference. Whatever value your C<AUTOLOAD> method returns is
877 returned to the caller.
879 The fully qualified method name that was called is available in the
880 C<$AUTOLOAD> package global for your class. Since this is a global, if
881 you want to refer to do it without a package name prefix under C<strict
882 'vars'>, you need to declare it.
884 # XXX - this is a terrible way to implement accessors, but it makes
885 # for a simple example.
890 # Remove qualifier from original method name...
891 my $called = $AUTOLOAD =~ s/.*:://r;
893 # Is there an attribute of that name?
894 die "No such attribute: $called"
895 unless exists $self->{$called};
897 # If so, return it...
898 return $self->{$called};
901 sub DESTROY { } # see below
903 Without the C<our $AUTOLOAD> declaration, this code will not compile
904 under the L<strict> pragma.
906 As the comment says, this is not a good way to implement accessors.
907 It's slow and too clever by far. However, you may see this as a way to
908 provide accessors in older Perl code. See L<perlootut> for
909 recommendations on OO coding in Perl.
911 If your class does have an C<AUTOLOAD> method, we strongly recommend
912 that you override C<can> in your class as well. Your overridden C<can>
913 method should return a subroutine reference for any method that your
914 C<AUTOLOAD> responds to.
917 X<destructor> X<DESTROY>
919 When the last reference to an object goes away, the object is
920 destroyed. If you only have one reference to an object stored in a
921 lexical scalar, the object is destroyed when that scalar goes out of
922 scope. If you store the object in a package global, that object may not
923 go out of scope until the program exits.
925 If you want to do something when the object is destroyed, you can
926 define a C<DESTROY> method in your class. This method will always be
927 called by Perl at the appropriate time, unless the method is empty.
929 This is called just like any other method, with the object as the first
930 argument. It does not receive any additional arguments. However, the
931 C<$_[0]> variable will be read-only in the destructor, so you cannot
932 assign a value to it.
934 If your C<DESTROY> method throws an exception, this will not cause
935 any control transfer beyond exiting the method. The exception will be
936 reported to C<STDERR> as a warning, marked "(in cleanup)", and Perl will
937 continue with whatever it was doing before.
939 Because C<DESTROY> methods can be called at any time, you should localize
940 any global status variables that might be set by anything you do in
941 your C<DESTROY> method. If you are in doubt about a particular status
942 variable, it doesn't hurt to localize it. There are five global status
943 variables, and the safest way is to localize all five of them:
946 local($., $@, $!, $^E, $?);
951 If you define an C<AUTOLOAD> in your class, then Perl will call your
952 C<AUTOLOAD> to handle the C<DESTROY> method. You can prevent this by
953 defining an empty C<DESTROY>, like we did in the autoloading example.
954 You can also check the value of C<$AUTOLOAD> and return without doing
955 anything when called to handle C<DESTROY>.
957 =head3 Global Destruction
959 The order in which objects are destroyed during the global destruction
960 before the program exits is unpredictable. This means that any objects
961 contained by your object may already have been destroyed. You should
962 check that a contained object is defined before calling a method on it:
967 $self->{handle}->close() if $self->{handle};
970 You can use the C<${^GLOBAL_PHASE}> variable to detect if you are
971 currently in the global destruction phase:
976 return if ${^GLOBAL_PHASE} eq 'DESTRUCT';
978 $self->{handle}->close();
981 Note that this variable was added in Perl 5.14.0. If you want to detect
982 the global destruction phase on older versions of Perl, you can use the
983 C<Devel::GlobalDestruction> module on CPAN.
985 If your C<DESTROY> method issues a warning during global destruction,
986 the Perl interpreter will append the string " during global
987 destruction" to the warning.
989 During global destruction, Perl will always garbage collect objects
990 before unblessed references. See L<perlhacktips/PERL_DESTRUCT_LEVEL>
991 for more information about global destruction.
993 =head2 Non-Hash Objects
995 All the examples so far have shown objects based on a blessed hash.
996 However, it's possible to bless any type of data structure or referent,
997 including scalars, globs, and subroutines. You may see this sort of
998 thing when looking at code in the wild.
1000 Here's an example of a module as a blessed scalar:
1011 return bless \$time, $class;
1019 my $time = Time->new();
1020 print $time->epoch();
1022 =head2 Inside-Out objects
1024 In the past, the Perl community experimented with a technique called
1025 "inside-out objects". An inside-out object stores its data outside of
1026 the object's reference, indexed on a unique property of the object,
1027 such as its memory address, rather than in the object itself. This has
1028 the advantage of enforcing the encapsulation of object attributes,
1029 since their data is not stored in the object itself.
1031 This technique was popular for a while (and was recommended in Damian
1032 Conway's I<Perl Best Practices>), but never achieved universal
1033 adoption. The L<Object::InsideOut> module on CPAN provides a
1034 comprehensive implementation of this technique, and you may see it or
1035 other inside-out modules in the wild.
1037 Here is a simple example of the technique, using the
1038 L<Hash::Util::FieldHash> core module. This module was added to the core
1039 to support inside-out object implementations.
1046 use Hash::Util::FieldHash 'fieldhash';
1048 fieldhash my %time_for;
1053 my $self = bless \( my $object ), $class;
1055 $time_for{$self} = time;
1063 return $time_for{$self};
1066 my $time = Time->new;
1069 =head2 Pseudo-hashes
1071 The pseudo-hash feature was an experimental feature introduced in
1072 earlier versions of Perl and removed in 5.10.0. A pseudo-hash is an
1073 array reference which can be accessed using named keys like a hash. You
1074 may run in to some code in the wild which uses it. See the L<fields>
1075 pragma for more information.
1079 A kinder, gentler tutorial on object-oriented programming in Perl can
1080 be found in L<perlootut>. You should also check out L<perlmodlib> for
1081 some style guides on constructing both modules and classes.