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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 tutorial 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 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 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 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
     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 (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 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
     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.

     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 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 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:


         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 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.  Here 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');

     Unlike method calls, function calls don't consider
     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');

     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 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:


         $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 debugging 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 recommend you
     use it exclusively.

     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'.

             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 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 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:

         {
             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;
         }

         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
     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 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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