


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


NAME
       perlobj - Perl objects

DESCRIPTION
       First 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 tuto-
       rial on object-oriented programming in Perl can be found
       in the perltoot manpage and the perltootc 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 {} }

       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 com-
       pared 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 con-
       taining 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 construc-
       tors 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
       Modules: Creation, Use, and Abuse entry 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.

       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.  Ref-
       erences 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 imple-
       ments 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 miss-
       ing 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 a base class, 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



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       the UNIVERSAL class contains the requested method, these
       three places are searched all over again, this time look-
       ing 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 com-
       plains.

       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 (refer-
       ence) 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.  Con-
       structors are often class methods, but see the perltoot
       manpage and the perltootc manpage 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 neces-
       sarily the same, because class methods follow the



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       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

       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,




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           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 indi-
       rect 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++ pro-
       grammers 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, how-
       ever, 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 ambigu-
       ous, see the section on "WARNING" below.

       There are times when you wish to specify which class's
       method to use.  Here you can call your method as an ordi-
       nary subroutine call, being sure to pass the requisite
       first argument explicitly:

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

       Unlike method calls, function calls don't consider inheri-
       tance.  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:




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

       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 vari-
       able containing the method name or a reference to the
       function.

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

           if ($coderef = $fred->can($parent . "::findbest")) {
               $self->$coderef(@args);     # call by coderef
           }


       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
       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:





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           $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 "require"s, but the
       grief if it messes up just isn't worth the years of debug-
       ging it would likely take you to track such subtle bugs
       down.

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

       Default UNIVERSAL methods

       The "UNIVERSAL" package automatically contains the follow-
       ing 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



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           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".

               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" to make
       these methods available to your program.  This is neces-
       sary 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 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.





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       Summary

       That's about all there is to it.  Now you need just to go
       off and buy a book about object-oriented design methodol-
       ogy, 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:

           {
               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 com-
       plete mark-and-sweep style of garbage collection is per-
       formed, and everything allocated by that thread gets
       destroyed.  This is essential to support Perl as an embed-
       ded or a multithreadable language.  For example, this pro-
       gram demonstrates Perl's two-phased garbage collection:



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           #!/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;
               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.  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.

       A more complete garbage collection strategy will be



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       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 the perltoot manpage, the perl-
       bootc manpage and the perltootc 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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