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perlobj



DESCRIPTION

       First you need to understand what references are in Perl.
       See perlref for that.  Second, if you still find the fol­
       lowing reference work too complicated, a tutorial on
       object-oriented programming in Perl can be found in perl­
       toot 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 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 construc­
       tors.)  The "{}" allocates an anonymous hash containing no
       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 "Mod­
       ules: Creation, Use, and Abuse" in perlmodlib), then you
       want to use the two-arg form of bless so that your con­
       structors 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 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
       "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 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
       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 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,
       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.

       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:

           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);
           }

       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");

       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";

       -- as C++ programmers are wont to make -- can be miscom­
       piled into a subroutine call if there's already a "new"
       function in scope.  You'd end up calling the current pack­
       age's "new" as a subroutine, rather than the desired
       class's method.  The compiler tries to cheat by remember­
       ing 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 oth­
       erwise, 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.

       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 exclu­
       sively.  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 follow­
       ing methods that are inherited by all other classes:

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

               print "It's an object\n" if UNIVERSAL::isa($val, 'UNIVERSAL');

       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.

           "UNIVERSAL::can" can also be called as a subroutine
           with two arguments.  It'll always return undef if its
           first argument isn't an object or a class name.    So
           here's another way to check if a reference is a
           blessed object

               print "It's still an object\n" if UNIVERSAL::can($val, 'can');

           You can also use the "blessed" function of
           Scalar::Util:

               use Scalar::Util 'blessed';

               my $blessing = blessed $suspected_object;

           "blessed" returns the name of the package the argument
           has been blessed into, or "undef".

       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 (and you should
       not do so).

       Destructors

       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 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;

       ded or a multithreadable language.  For example, this pro­
       gram 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


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 v5.8.1                 2003-09-02                 PERLOBJ(1)
  




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