


PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


NAME
       perlobj - Perl objects

DESCRIPTION
       First you need to understand what references are in Perl.
       See perlref for that.  Second, if you still find the
       following reference work too complicated, a tutorial on
       object-oriented programming in Perl can be found in
       perltoot and perltooc.

       If you're still with us, then here are three very simple
       definitions that you should find reassuring.

       1.  An object is simply a reference that happens to know
           which class it belongs to.

       2.  A class is simply a package that happens to provide
           methods to deal with object references.

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

       We'll cover these points now in more depth.

   An Object is Simply a Reference
       Unlike say C++, Perl doesn't provide any special syntax
       for constructors.  A constructor is merely a subroutine
       that returns a reference to something "blessed" into a
       class, generally the class that the subroutine is defined
       in.  Here is a typical constructor:

           package Critter;
           sub new { bless {} }

       That word "new" isn't special.  You could have written a
       construct this way, too:

           package Critter;
           sub spawn { bless {} }

       This might even be preferable, because the C++ programmers
       won't be tricked into thinking that "new" works in Perl as
       it does in C++.  It doesn't.  We recommend that you name
       your constructors whatever makes sense in the context of
       the problem you're solving.  For example, constructors in
       the Tk extension to Perl are named after the widgets they
       create.

       One thing that's different about Perl constructors
       compared with those in C++ is that in Perl, they have to
       allocate their own memory.  (The other things is that they
       don't automatically call overridden base-class
       constructors.)  The "{}" allocates an anonymous hash



perl v5.10.1                2009-10-12                          1





PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


       containing no key/value pairs, and returns it  The bless()
       takes that reference and tells the object it references
       that it's now a Critter, and returns the reference.  This
       is for convenience, because the referenced object itself
       knows that it has been blessed, and the reference to it
       could have been returned directly, like this:

           sub new {
               my $self = {};
               bless $self;
               return $self;
           }

       You often see such a thing in more complicated
       constructors that wish to call methods in the class as
       part of the construction:

           sub new {
               my $self = {};
               bless $self;
               $self->initialize();
               return $self;
           }

       If you care about inheritance (and you should; see
       "Modules: Creation, Use, and Abuse" in perlmodlib), then
       you want to use the two-arg form of bless so that your
       constructors may be inherited:

           sub new {
               my $class = shift;
               my $self = {};
               bless $self, $class;
               $self->initialize();
               return $self;
           }

       Or if you expect people to call not just "CLASS->new()"
       but also "$obj->new()", then use something like the
       following.  (Note that using this to call new() on an
       instance does not automatically perform any copying.  If
       you want a shallow or deep copy of an object, you'll have
       to specifically allow for that.)  The initialize() method
       used will be of whatever $class we blessed the object
       into:

           sub new {
               my $this = shift;
               my $class = ref($this) || $this;
               my $self = {};
               bless $self, $class;
               $self->initialize();
               return $self;
           }



perl v5.10.1                2009-10-12                          2





PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


       Within the class package, the methods will typically deal
       with the reference as an ordinary reference.  Outside the
       class package, the reference is generally treated as an
       opaque value that may be accessed only through the class's
       methods.

       Although a constructor can in theory re-bless a referenced
       object currently belonging to another class, this is
       almost certainly going to get you into trouble.  The new
       class is responsible for all cleanup later.  The previous
       blessing is forgotten, as an object may belong to only one
       class at a time.  (Although of course it's free to inherit
       methods from many classes.)  If you find yourself having
       to do this, the parent class is probably misbehaving,
       though.

       A clarification:  Perl objects are blessed.  References
       are not.  Objects know which package they belong to.
       References do not.  The bless() function uses the
       reference to find the object.  Consider the following
       example:

           $a = {};
           $b = $a;
           bless $a, BLAH;
           print "\$b is a ", ref($b), "\n";

       This reports $b as being a BLAH, so obviously bless()
       operated on the object and not on the reference.

   A Class is Simply a Package
       Unlike say C++, Perl doesn't provide any special syntax
       for class definitions.  You use a package as a class by
       putting method definitions into the class.

       There is a special array within each package called @ISA,
       which says where else to look for a method if you can't
       find it in the current package.  This is how Perl
       implements inheritance.  Each element of the @ISA array is
       just the name of another package that happens to be a
       class package.  The classes are searched for missing
       methods in depth-first, left-to-right order by default
       (see mro for alternative search order and other in-depth
       information).  The classes accessible through @ISA are
       known as base classes of the current class.

       All classes implicitly inherit from class "UNIVERSAL" as
       their last base class.  Several commonly used methods are
       automatically supplied in the UNIVERSAL class; see
       "Default UNIVERSAL methods" or UNIVERSAL for more details.

       If a missing method is found in a base class, it is cached
       in the current class for efficiency.  Changing @ISA or
       defining new subroutines invalidates the cache and causes



perl v5.10.1                2009-10-12                          3





PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


       Perl to do the lookup again.

       If neither the current class, its named base classes, nor
       the UNIVERSAL class contains the requested method, these
       three places are searched all over again, this time
       looking for a method named AUTOLOAD().  If an AUTOLOAD is
       found, this method is called on behalf of the missing
       method, setting the package global $AUTOLOAD to be the
       fully qualified name of the method that was intended to be
       called.

       If none of that works, Perl finally gives up and
       complains.

       If you want to stop the AUTOLOAD inheritance say simply

               sub AUTOLOAD;

       and the call will die using the name of the sub being
       called.

       Perl classes do method inheritance only.  Data inheritance
       is left up to the class itself.  By and large, this is not
       a problem in Perl, because most classes model the
       attributes of their object using an anonymous hash, which
       serves as its own little namespace to be carved up by the
       various classes that might want to do something with the
       object.  The only problem with this is that you can't sure
       that you aren't using a piece of the hash that isn't
       already used.  A reasonable workaround is to prepend your
       fieldname in the hash with the package name.

           sub bump {
               my $self = shift;
               $self->{ __PACKAGE__ . ".count"}++;
           }

   A Method is Simply a Subroutine
       Unlike say C++, Perl doesn't provide any special syntax
       for method definition.  (It does provide a little syntax
       for method invocation though.  More on that later.)  A
       method expects its first argument to be the object
       (reference) or package (string) it is being invoked on.
       There are two ways of calling methods, which we'll call
       class methods and instance methods.

       A class method expects a class name as the first argument.
       It provides functionality for the class as a whole, not
       for any individual object belonging to the class.
       Constructors are often class methods, but see perltoot and
       perltooc for alternatives.  Many class methods simply
       ignore their first argument, because they already know
       what package they're in and don't care what package they
       were invoked via.  (These aren't necessarily the same,



perl v5.10.1                2009-10-12                          4





PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


       because class methods follow the inheritance tree just
       like ordinary instance methods.)  Another typical use for
       class methods is to look up an object by name:

           sub find {
               my ($class, $name) = @_;
               $objtable{$name};
           }

       An instance method expects an object reference as its
       first argument.  Typically it shifts the first argument
       into a "self" or "this" variable, and then uses that as an
       ordinary reference.

           sub display {
               my $self = shift;
               my @keys = @_ ? @_ : sort keys %$self;
               foreach $key (@keys) {
                   print "\t$key => $self->{$key}\n";
               }
           }

   Method Invocation
       For various historical and other reasons, Perl offers two
       equivalent ways to write a method call.  The simpler and
       more common way is to use the arrow notation:

           my $fred = Critter->find("Fred");
           $fred->display("Height", "Weight");

       You should already be familiar with the use of the "->"
       operator with references.  In fact, since $fred above is a
       reference to an object, you could think of the method call
       as just another form of dereferencing.

       Whatever is on the left side of the arrow, whether a
       reference or a class name, is passed to the method
       subroutine as its first argument.  So the above code is
       mostly equivalent to:

           my $fred = Critter::find("Critter", "Fred");
           Critter::display($fred, "Height", "Weight");

       How does Perl know which package the subroutine is in?  By
       looking at the left side of the arrow, which must be
       either a package name or a reference to an object, i.e.
       something that has been blessed to a package.  Either way,
       that's the package where Perl starts looking.  If that
       package has no subroutine with that name, Perl starts
       looking for it in any base classes of that package, and so
       on.

       If you need to, you can force Perl to start looking in
       some other package:



perl v5.10.1                2009-10-12                          5





PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


           my $barney = MyCritter->Critter::find("Barney");
           $barney->Critter::display("Height", "Weight");

       Here "MyCritter" is presumably a subclass of "Critter"
       that defines its own versions of find() and display().  We
       haven't specified what those methods do, but that doesn't
       matter above since we've forced Perl to start looking for
       the subroutines in "Critter".

       As a special case of the above, you may use the "SUPER"
       pseudo-class to tell Perl to start looking for the method
       in the packages named in the current class's @ISA list.

           package MyCritter;
           use base 'Critter';    # sets @MyCritter::ISA = ('Critter');

           sub display {
               my ($self, @args) = @_;
               $self->SUPER::display("Name", @args);
           }

       It is important to note that "SUPER" refers to the
       superclass(es) of the current package and not to the
       superclass(es) of the object. Also, the "SUPER" pseudo-
       class can only currently be used as a modifier to a method
       name, but not in any of the other ways that class names
       are normally used, eg:

           something->SUPER::method(...);      # OK
           SUPER::method(...);                 # WRONG
           SUPER->method(...);                 # WRONG

       Instead of a class name or an object reference, you can
       also use any expression that returns either of those on
       the left side of the arrow.  So the following statement is
       valid:

           Critter->find("Fred")->display("Height", "Weight");

       and so is the following:

           my $fred = (reverse "rettirC")->find(reverse "derF");

       The right side of the arrow typically is the method name,
       but a simple scalar variable containing either the method
       name or a subroutine reference can also be used.

       If the right side of the arrow is a scalar containing a
       reference to a subroutine, then this is equivalent to
       calling the referenced subroutine directly with the class
       name or object on the left side of the arrow as its first
       argument. No lookup is done and there is no requirement
       that the subroutine be defined in any package related to
       the class name or object on the left side of the arrow.



perl v5.10.1                2009-10-12                          6





PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


       For example, the following calls to $display are
       equivalent:

           my $display = sub { my $self = shift; ... };
           $fred->$display("Height", "Weight");
           $display->($fred, "Height", "Weight");

   Indirect Object Syntax
       The other way to invoke a method is by using the so-called
       "indirect object" notation.  This syntax was available in
       Perl 4 long before objects were introduced, and is still
       used with filehandles like this:

          print STDERR "help!!!\n";

       The same syntax can be used to call either object or class
       methods.

          my $fred = find Critter "Fred";
          display $fred "Height", "Weight";

       Notice that there is no comma between the object or class
       name and the parameters.  This is how Perl can tell you
       want an indirect method call instead of an ordinary
       subroutine call.

       But what if there are no arguments?  In that case, Perl
       must guess what you want.  Even worse, it must make that
       guess at compile time.  Usually Perl gets it right, but
       when it doesn't you get a function call compiled as a
       method, or vice versa.  This can introduce subtle bugs
       that are hard to detect.

       For example, a call to a method "new" in indirect notation
       -- as C++ programmers are wont to make -- can be
       miscompiled into a subroutine call if there's already a
       "new" function in scope.  You'd end up calling the current
       package's "new" as a subroutine, rather than the desired
       class's method.  The compiler tries to cheat by
       remembering bareword "require"s, but the grief when it
       messes up just isn't worth the years of debugging it will
       take you to track down such subtle bugs.

       There is another problem with this syntax: the indirect
       object is limited to a name, a scalar variable, or a
       block, because it would have to do too much lookahead
       otherwise, just like any other postfix dereference in the
       language.  (These are the same quirky rules as are used
       for the filehandle slot in functions like "print" and
       "printf".)  This can lead to horribly confusing precedence
       problems, as in these next two lines:

           move $obj->{FIELD};                 # probably wrong!
           move $ary[$i];                      # probably wrong!



perl v5.10.1                2009-10-12                          7





PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


       Those actually parse as the very surprising:

           $obj->move->{FIELD};                # Well, lookee here
           $ary->move([$i]);                   # Didn't expect this one, eh?

       Rather than what you might have expected:

           $obj->{FIELD}->move();              # You should be so lucky.
           $ary[$i]->move;                     # Yeah, sure.

       To get the correct behavior with indirect object syntax,
       you would have to use a block around the indirect object:

           move {$obj->{FIELD}};
           move {$ary[$i]};

       Even then, you still have the same potential problem if
       there happens to be a function named "move" in the current
       package.  The "->" notation suffers from neither of these
       disturbing ambiguities, so we recommend you use it
       exclusively.  However, you may still end up having to read
       code using the indirect object notation, so it's important
       to be familiar with it.

   Default UNIVERSAL methods
       The "UNIVERSAL" package automatically contains the
       following methods that are inherited by all other classes:

       isa(CLASS)
           "isa" returns true if its object is blessed into a
           subclass of "CLASS"

       DOES(ROLE)
           "DOES" returns true if its object claims to perform
           the role "ROLE".  By default, this is equivalent to
           "isa".

       can(METHOD)
           "can" checks to see if its object has a method called
           "METHOD", if it does then a reference to the sub is
           returned, if it does not then "undef" is returned.

       VERSION( [NEED] )
           "VERSION" 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 given package) not less than
           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);



perl v5.10.1                2009-10-12                          8





PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


   Destructors
       When the last reference to an object goes away, the object
       is automatically destroyed.  (This may even be after you
       exit, if you've stored references in global variables.)
       If you want to capture control just before the object is
       freed, you may define a DESTROY method in your class.  It
       will automatically be called at the appropriate moment,
       and you can do any extra cleanup you need to do.  Perl
       passes a reference to the object under destruction as the
       first (and only) argument.  Beware that the reference is a
       read-only value, and cannot be modified by manipulating
       $_[0] within the destructor.  The object itself (i.e.  the
       thingy the reference points to, namely "${$_[0]}",
       "@{$_[0]}", "%{$_[0]}" etc.) is not similarly constrained.

       Since DESTROY methods can be called at unpredictable
       times, it is important that you localise any global
       variables that the method may update.  In particular,
       localise $@ if you use "eval {}" and localise $? if you
       use "system" or backticks.

       If you arrange to re-bless the reference before the
       destructor returns, perl will again call the DESTROY
       method for the re-blessed object after the current one
       returns.  This can be used for clean delegation of object
       destruction, or for ensuring that destructors in the base
       classes of your choosing get called.  Explicitly calling
       DESTROY is also possible, but is usually never needed.

       Do not confuse the previous discussion with how objects
       CONTAINED in the current one are destroyed.  Such objects
       will be freed and destroyed automatically when the current
       object is freed, provided no other references to them
       exist elsewhere.

   Summary
       That's about all there is to it.  Now you need just to go
       off and buy a book about object-oriented design
       methodology, and bang your forehead with it for the next
       six months or so.

   Two-Phased Garbage Collection
       For most purposes, Perl uses a fast and simple, reference-
       based garbage collection system.  That means there's an
       extra dereference going on at some level, so if you
       haven't built your Perl executable using your C compiler's
       "-O" flag, performance will suffer.  If you have built
       Perl with "cc -O", then this probably won't matter.

       A more serious concern is that unreachable memory with a
       non-zero reference count will not normally get freed.
       Therefore, this is a bad idea:





perl v5.10.1                2009-10-12                          9





PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


           {
               my $a;
               $a = \$a;
           }

       Even thought $a should go away, it can't.  When building
       recursive data structures, you'll have to break the self-
       reference yourself explicitly if you don't care to leak.
       For example, here's a self-referential node such as one
       might use in a sophisticated tree structure:

           sub new_node {
               my $class = shift;
               my $node  = {};
               $node->{LEFT} = $node->{RIGHT} = $node;
               $node->{DATA} = [ @_ ];
               return bless $node => $class;
           }

       If you create nodes like that, they (currently) won't go
       away unless you break their self reference yourself.  (In
       other words, this is not to be construed as a feature, and
       you shouldn't depend on it.)

       Almost.

       When an interpreter thread finally shuts down (usually
       when your program exits), then a rather costly but
       complete mark-and-sweep style of garbage collection is
       performed, and everything allocated by that thread gets
       destroyed.  This is essential to support Perl as an
       embedded or a multithreadable language.  For example, this
       program demonstrates Perl's two-phased garbage collection:

           #!/usr/bin/perl
           package Subtle;

           sub new {
               my $test;
               $test = \$test;
               warn "CREATING " . \$test;
               return bless \$test;
           }

           sub DESTROY {
               my $self = shift;
               warn "DESTROYING $self";
           }

           package main;

           warn "starting program";
           {
               my $a = Subtle->new;



perl v5.10.1                2009-10-12                         10





PERLOBJ(1)       Perl Programmers Reference Guide      PERLOBJ(1)


               my $b = Subtle->new;
               $$a = 0;  # break selfref
               warn "leaving block";
           }

           warn "just exited block";
           warn "time to die...";
           exit;

       When run as /foo/test, the following output is produced:

           starting program at /foo/test line 18.
           CREATING SCALAR(0x8e5b8) at /foo/test line 7.
           CREATING SCALAR(0x8e57c) at /foo/test line 7.
           leaving block at /foo/test line 23.
           DESTROYING Subtle=SCALAR(0x8e5b8) at /foo/test line 13.
           just exited block at /foo/test line 26.
           time to die... at /foo/test line 27.
           DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.

       Notice that "global destruction" bit there?  That's the
       thread garbage collector reaching the unreachable.

       Objects are always destructed, even when regular refs
       aren't.  Objects are destructed in a separate pass before
       ordinary refs just to prevent object destructors from
       using refs that have been themselves destructed.  Plain
       refs are only garbage-collected if the destruct level is
       greater than 0.  You can test the higher levels of global
       destruction by setting the PERL_DESTRUCT_LEVEL environment
       variable, presuming "-DDEBUGGING" was enabled during perl
       build time.  See "PERL_DESTRUCT_LEVEL" in perlhack for
       more information.

       A more complete garbage collection strategy will be
       implemented at a future date.

       In the meantime, the best solution is to create a non-
       recursive container class that holds a pointer to the
       self-referential data structure.  Define a DESTROY method
       for the containing object's class that manually breaks the
       circularities in the self-referential structure.

SEE ALSO
       A kinder, gentler tutorial on object-oriented programming
       in Perl can be found in perltoot, perlboot and perltooc.
       You should also check out perlbot for other object tricks,
       traps, and tips, as well as perlmodlib for some style
       guides on constructing both modules and classes.








perl v5.10.1                2009-10-12                         11


