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RedHat 9 (Linux i386) - man page for perlobj (redhat section 1)

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

NAME
       perlobj - Perl objects

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

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

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

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

       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, gen-
       erally 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 exam-
       ple, 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 auto-
       matically 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 "Modules: Creation, Use, and Abuse" in
       perlmodlib), then you want to use the two-arg form of bless so that your constructors may
       be inherited:

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

       Or if you expect people to call not just "CLASS->new()" but also "$obj->new()", then use
       something like 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 ordi-
       nary 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 mis-
       behaving, though.

       A clarification:  Perl objects are blessed.  References are not.  Objects know which pack-
       age 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 hap-
       pens 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 "Default UNI-
       VERSAL methods" for more details.

       If a missing method is found in a base class, it is cached in the current class for effi-
       ciency.	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.

       If you want to stop the AUTOLOAD inheritance say simply

	       sub AUTOLOAD;

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

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

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

       A Method is Simply a Subroutine

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

       A class method expects a class name as the first argument.  It provides functionality for
       the class as a whole, not for any individual object belonging to the class.  Constructors
       are often class methods, but see perltoot and perltooc for alternatives.  Many class meth-
       ods 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.

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

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

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

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

	   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";

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

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

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

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

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

       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 dis-
       turbing ambiguities, so we recommend you use it exclusively.  However, you may still end
       up having to read code using the indirect object notation, so it's important to be famil-
       iar with it.

       Default UNIVERSAL methods

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

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

	   You can also call "UNIVERSAL::isa" as a subroutine with two arguments.  The first does
	   not need to be an object or even a reference.  This allows you to check what a refer-
	   ence points to, or whether something is a reference of a given type. Example

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

	   To determine if a reference is a blessed object, you can write

	       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 ref-
	   erence 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

       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 possi-
       ble, 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 exam-
       ple, here's a self-referential node such as one might use in a sophisticated tree struc-
       ture:

	   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.  See "PERL_DESTRUCT_LEVEL" in perlhack for more information.

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

       In the meantime, the best solution is to create a non-recursive container class that holds
       a pointer to the self-referential data structure.  Define a DESTROY method for the con-
       taining 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 perl-
       toot, 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 v5.8.0				    2003-02-18				       PERLOBJ(1)


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