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


NAME
       perlobj - Perl objects



DESCRIPTION
       First of all, you need to understand what references are
       in Perl.  See the perlref manpage 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 the perltoot manpage.

       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 {} }

       In fact, 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



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       constructors.)  The {} allocates an anonymous hash
       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;
           }

       In fact, 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 the
       section on Modules: Creation, Use, and Abuse in the
       perlmodlib manpage), 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 this.  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;
           }

       Within the class package, the methods will typically deal
       with the reference as an ordinary reference.  Outside the



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       class package, the reference is generally treated as an
       opaque value that may be accessed only through the class's
       methods.

       A constructor may re-bless a referenced object currently
       belonging to another class, but then 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 (depth first) for
       missing methods in the order that they occur in @ISA.  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 the
       section on Default UNIVERSAL methods for more details.

       If a missing method is found in one of the base classes,
       it is cached in the current class for efficiency.
       Changing @ISA or defining new subroutines invalidates the
       cache and causes Perl to do the lookup again.

       If neither the current class, its named base classes, nor
       the UNIVERSAL class contains the requested method, these



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       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.

       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 just two types of methods, which we'll call
       class and instance.  (Sometimes you'll hear these called
       static and virtual, in honor of the two C++ method types
       they most closely resemble.)

       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 typically class methods.  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, 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:







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           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

       There are two ways to invoke a method, one of which you're
       already familiar with, and the other of which will look
       familiar.  Perl 4 already had an "indirect object" syntax
       that you use when you say

           print STDERR "help!!!\n";

       This same syntax can be used to call either class or
       instance methods.  We'll use the two methods defined
       above, the class method to lookup an object reference and
       the instance method to print out its attributes.

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

       These could be combined into one statement by using a
       BLOCK in the indirect object slot:

           display {find Critter "Fred"} 'Height', 'Weight';

       For C++ fans, there's also a syntax using -> notation that
       does exactly the same thing.  The parentheses are required
       if there are any arguments.

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

       or in one statement,

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

       There are times when one syntax is more readable, and
       times when the other syntax is more readable.  The



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       indirect object syntax is less cluttered, but it has the
       same ambiguity as ordinary list operators.  Indirect
       object method calls are usually parsed using the same rule
       as list operators: "If it looks like a function, it is a
       function".  (Presuming for the moment that you think two
       words in a row can look like a function name.  C++
       programmers seem to think so with some regularity,
       especially when the first word is "new".)  Thus, the
       parentheses of

           new Critter ('Barney', 1.5, 70)

       are assumed to surround ALL the arguments of the method
       call, regardless of what comes after.  Saying

           new Critter ('Bam' x 2), 1.4, 45

       would be equivalent to

           Critter->new('Bam' x 2), 1.4, 45

       which is unlikely to do what you want.  Confusingly,
       however, this rule applies only when the indirect object
       is a bareword package name, not when it's a scalar, a
       BLOCK, or a Package:: qualified package name.  In those
       cases, the arguments are parsed in the same way as an
       indirect object list operator like print, so

           new Critter:: ('Bam' x 2), 1.4, 45

       is the same as

          Critter::->new(('Bam' x 2), 1.4, 45)

       For more reasons why the indirect object syntax is
       ambiguous, see the section on WARNING below.

       There are times when you wish to specify which class's
       method to use.  In this case, you can call your method as
       an ordinary subroutine call, being sure to pass the
       requisite first argument explicitly:

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

       Note however, that this does not do any inheritance.  If
       you wish merely to specify that Perl should START looking
       for a method in a particular package, use an ordinary
       method call, but qualify the method name with the package
       like this:

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




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       If you're trying to control where the method search begins
       and you're executing in the class itself, then you may use
       the SUPER pseudo class, which says to start looking in
       your base class's @ISA list without having to name it
       explicitly:

           $self->SUPER::display('Height', 'Weight');

       Please note that the SUPER:: construct is meaningful only
       within the class.

       Sometimes you want to call a method when you don't know
       the method name ahead of time.  You can use the arrow
       form, replacing the method name with a simple scalar
       variable containing the method name:

           $method = $fast ? "findfirst" : "findbest";
           $fred->$method(@args);


       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

           isa is also exportable and can be called as a sub with
           two arguments. This allows the ability to check what a
           reference points to. Example

               use UNIVERSAL qw(isa);

               if(isa($ref, 'ARRAY')) {
                   #...
               }


       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 normally called as a class method.  This
           method is called automatically by the VERSION form of
           use.



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               use A 1.2 qw(some imported subs);
               # implies:
               A->VERSION(1.2);


       NOTE: can directly uses Perl's internal code for method
       lookup, and isa uses a very similar method and cache-ing
       strategy. This may cause strange effects if the Perl code
       dynamically changes @ISA in any package.

       You may add other methods to the UNIVERSAL class via Perl
       or XS code.  You do not need to use UNIVERSAL in order to
       make these methods available to your program.  This is
       necessary only if you wish to have isa available as a
       plain subroutine in the current package.

       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.

       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 foregoing 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.

       WARNING

       While indirect object syntax may well be appealing to
       English speakers and to C++ programmers, be not seduced!
       It suffers from two grave problems.

       The first problem is that an indirect object is limited to
       a name, a scalar variable, or a block, because it would



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       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!

       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.

       The left side of ``->'' is not so limited, because it's an
       infix operator, not a postfix operator.

       As if that weren't bad enough, think about this: Perl must
       guess at compile time whether name and move above are
       functions or methods.  Usually Perl gets it right, but
       when it doesn't it, you get a function call compiled as a
       method, or vice versa.  This can introduce subtle bugs
       that are hard to unravel.  For example, calling a method
       new in indirect notation--as C++ programmers are so wont
       to do--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 requires, but the grief if
       it messes up just isn't worth the years of debugging it
       would likely take you to to track such subtle bugs down.

       The infix arrow notation using ``->'' doesn't suffer from
       either of these disturbing ambiguities, so we recommend
       you use it exclusively.

       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.  For this reason, there's
       an extra dereference going on at some level, so if you



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       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:

           {
               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 $self = shift;
               my $class = ref($self) || $self;
               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;
           }





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

           package main;

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

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

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

           starting program at /tmp/test line 18.
           CREATING SCALAR(0x8e5b8) at /tmp/test line 7.
           CREATING SCALAR(0x8e57c) at /tmp/test line 7.
           leaving block at /tmp/test line 23.
           DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13.
           just exited block at /tmp/test line 26.
           time to die... at /tmp/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 and in fact are destructed in a separate pass
       before ordinary refs just to try 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.

       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



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       in Perl can be found in the perltoot manpage.  You should
       also check out the perlbot manpage for other object
       tricks, traps, and tips, as well as the perlmodlib manpage
       for some style guides on constructing both modules and
       classes.




















































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