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PERLBOOT(1)	       Perl Programmers	Reference Guide		   PERLBOOT(1)

NAME
       perlboot	- Beginner's Object-Oriented Tutorial

DESCRIPTION
       If you're not familiar with objects from	other languages, some of the
       other Perl object documentation may be a	little daunting, such as
       perlobj,	a basic	reference in using objects, and	perltoot, which
       introduces readers to the peculiarities of Perl's object	system in a
       tutorial	way.

       So, let's take a	different approach, presuming no prior object
       experience. It helps if you know	about subroutines (perlsub),
       references (perlref et. seq.), and packages (perlmod), so become
       familiar	with those first if you	haven't	already.

   If we could talk to the animals...
       Let's let the animals talk for a	moment:

	   sub Cow::speak {
	     print "a Cow goes moooo!\n";
	   }
	   sub Horse::speak {
	     print "a Horse goes neigh!\n";
	   }
	   sub Sheep::speak {
	     print "a Sheep goes baaaah!\n";
	   }

	   Cow::speak;
	   Horse::speak;
	   Sheep::speak;

       This results in:

	   a Cow goes moooo!
	   a Horse goes	neigh!
	   a Sheep goes	baaaah!

       Nothing spectacular here.  Simple subroutines, albeit from separate
       packages, and called using the full package name.  So let's create an
       entire pasture:

	   # Cow::speak, Horse::speak, Sheep::speak as before
	   @pasture = qw(Cow Cow Horse Sheep Sheep);
	   foreach $animal (@pasture) {
	     &{$animal."::speak"};
	   }

       This results in:

	   a Cow goes moooo!
	   a Cow goes moooo!
	   a Horse goes	neigh!
	   a Sheep goes	baaaah!
	   a Sheep goes	baaaah!

       Wow.  That symbolic coderef de-referencing there	is pretty nasty.
       We're counting on "no strict refs" mode,	certainly not recommended for
       larger programs.	 And why was that necessary?  Because the name of the
       package seems to	be inseparable from the	name of	the subroutine we want
       to invoke within	that package.

       Or is it?

   Introducing the method invocation arrow
       For now,	let's say that "Class->method" invokes subroutine "method" in
       package "Class".	 (Here,	"Class"	is used	in its "category" meaning, not
       its "scholastic"	meaning.) That's not completely	accurate, but we'll do
       this one	step at	a time.	 Now let's use it like so:

	   # Cow::speak, Horse::speak, Sheep::speak as before
	   Cow->speak;
	   Horse->speak;
	   Sheep->speak;

       And once	again, this results in:

	   a Cow goes moooo!
	   a Horse goes	neigh!
	   a Sheep goes	baaaah!

       That's not fun yet.  Same number	of characters, all constant, no
       variables.  But yet, the	parts are separable now.  Watch:

	   $a =	"Cow";
	   $a->speak; #	invokes	Cow->speak

       Ahh!  Now that the package name has been	parted from the	subroutine
       name, we	can use	a variable package name.  And this time, we've got
       something that works even when "use strict refs"	is enabled.

   Invoking a barnyard
       Let's take that new arrow invocation and	put it back in the barnyard
       example:

	   sub Cow::speak {
	     print "a Cow goes moooo!\n";
	   }
	   sub Horse::speak {
	     print "a Horse goes neigh!\n";
	   }
	   sub Sheep::speak {
	     print "a Sheep goes baaaah!\n";
	   }

	   @pasture = qw(Cow Cow Horse Sheep Sheep);
	   foreach $animal (@pasture) {
	     $animal->speak;
	   }

       There!  Now we have the animals all talking, and	safely at that,
       without the use of symbolic coderefs.

       But look	at all that common code.  Each of the "speak" routines has a
       similar structure: a "print" operator and a string that contains	common
       text, except for	two of the words.  It'd	be nice	if we could factor out
       the commonality,	in case	we decide later	to change it all to "says"
       instead of "goes".

       And we actually have a way of doing that	without	much fuss, but we have
       to hear a bit more about	what the method	invocation arrow is actually
       doing for us.

   The extra parameter of method invocation
       The invocation of:

	   Class->method(@args)

       attempts	to invoke subroutine "Class::method" as:

	   Class::method("Class", @args);

       (If the subroutine can't	be found, "inheritance"	kicks in, but we'll
       get to that later.)  This means that we get the class name as the first
       parameter (the only parameter, if no arguments are given).  So we can
       rewrite the "Sheep" speaking subroutine as:

	   sub Sheep::speak {
	     my	$class = shift;
	     print "a $class goes baaaah!\n";
	   }

       And the other two animals come out similarly:

	   sub Cow::speak {
	     my	$class = shift;
	     print "a $class goes moooo!\n";
	   }
	   sub Horse::speak {
	     my	$class = shift;
	     print "a $class goes neigh!\n";
	   }

       In each case, $class will get the value appropriate for that
       subroutine.  But	once again, we have a lot of similar structure.	 Can
       we factor that out even further?	 Yes, by calling another method	in the
       same class.

   Calling a second method to simplify things
       Let's call out from "speak" to a	helper method called "sound".  This
       method provides the constant text for the sound itself.

	   { package Cow;
	     sub sound { "moooo" }
	     sub speak {
	       my $class = shift;
	       print "a	$class goes ", $class->sound, "!\n";
	     }
	   }

       Now, when we call "Cow->speak", we get a	$class of "Cow"	in "speak".
       This in turn selects the	"Cow->sound" method, which returns "moooo".
       But how different would this be for the "Horse"?

	   { package Horse;
	     sub sound { "neigh" }
	     sub speak {
	       my $class = shift;
	       print "a	$class goes ", $class->sound, "!\n";
	     }
	   }

       Only the	name of	the package and	the specific sound change.  So can we
       somehow share the definition for	"speak"	between	the Cow	and the	Horse?
       Yes, with inheritance!

   Inheriting the windpipes
       We'll define a common subroutine	package	called "Animal", with the
       definition for "speak":

	   { package Animal;
	     sub speak {
	     my	$class = shift;
	     print "a $class goes ", $class->sound, "!\n";
	     }
	   }

       Then, for each animal, we say it	"inherits" from	"Animal", along	with
       the animal-specific sound:

	   { package Cow;
	     @ISA = qw(Animal);
	     sub sound { "moooo" }
	   }

       Note the	added @ISA array (pronounced "is a").  We'll get to that in a
       minute.

       But what	happens	when we	invoke "Cow->speak" now?

       First, Perl constructs the argument list.  In this case,	it's just
       "Cow".  Then Perl looks for "Cow::speak".  But that's not there,	so
       Perl checks for the inheritance array @Cow::ISA.	 It's there, and
       contains	the single name	"Animal".

       Perl next checks	for "speak" inside "Animal" instead, as	in
       "Animal::speak".	 And that's found, so Perl invokes that	subroutine
       with the	already	frozen argument	list.

       Inside the "Animal::speak" subroutine, $class becomes "Cow" (the	first
       argument).  So when we get to the step of invoking "$class->sound",
       it'll be	looking	for "Cow->sound", which	gets it	on the first try
       without looking at @ISA.	 Success!

   A few notes about @ISA
       This magical @ISA variable has declared that "Cow" "is a" "Animal".
       Note that it's an array,	not a simple single value, because on rare
       occasions, it makes sense to have more than one parent class searched
       for the missing methods.

       If "Animal" also	had an @ISA, then we'd check there too.	 The search is
       recursive, depth-first, left-to-right in	each @ISA by default (see mro
       for alternatives).  Typically, each @ISA	has only one element (multiple
       elements	means multiple inheritance and multiple	headaches), so we get
       a nice tree of inheritance.

       When we turn on "use strict", we'll get complaints on @ISA, since it's
       not a variable containing an explicit package name, nor is it a lexical
       ("my") variable.	 We can't make it a lexical variable though (it	has to
       belong to the package to	be found by the	inheritance mechanism),	so
       there's a couple	of straightforward ways	to handle that.

       The easiest is to just spell the	package	name out:

	   @Cow::ISA = qw(Animal);

       Or declare it as	package	global variable:

	   package Cow;
	   our @ISA = qw(Animal);

       Or allow	it as an implicitly named package variable:

	   package Cow;
	   use vars qw(@ISA);
	   @ISA	= qw(Animal);

       If the "Animal" class comes from	another	(object-oriented) module, then
       just employ "use	base" to specify that "Animal" should serve as the
       basis for the "Cow" class:

	   package Cow;
	   use base qw(Animal);

       Now that's pretty darn simple!

   Overriding the methods
       Let's add a mouse, which	can barely be heard:

	   # Animal package from before
	   { package Mouse;
	     @ISA = qw(Animal);
	     sub sound { "squeak" }
	     sub speak {
	       my $class = shift;
	       print "a	$class goes ", $class->sound, "!\n";
	       print "[but you can barely hear it!]\n";
	     }
	   }

	   Mouse->speak;

       which results in:

	   a Mouse goes	squeak!
	   [but	you can	barely hear it!]

       Here, "Mouse" has its own speaking routine, so "Mouse->speak" doesn't
       immediately invoke "Animal->speak". This	is known as "overriding". In
       fact, we	don't even need	to say that a "Mouse" is an "Animal" at	all,
       because all of the methods needed for "speak" are completely defined
       for "Mouse"; this is known as "duck typing": "If	it walks like a	duck
       and quacks like a duck, I would call it a duck" (James Whitcomb).
       However,	it would probably be beneficial	to allow a closer examination
       to conclude that	a "Mouse" is indeed an "Animal", so it is actually
       better to define	"Mouse"	with "Animal" as its base (that	is, it is
       better to "derive "Mouse" from "Animal"").

       Moreover, this duplication of code could	become a maintenance headache
       (though code-reuse is not actually a good reason	for inheritance; good
       design practices	dictate	that a derived class should be usable wherever
       its base	class is usable, which might not be the	outcome	if code-reuse
       is the sole criterion for inheritance. Just remember that a "Mouse"
       should always act like an "Animal").

       So, let's make "Mouse" an "Animal"!

       The obvious solution is to invoke "Animal::speak" directly:

	   # Animal package from before
	   { package Mouse;
	     @ISA = qw(Animal);
	     sub sound { "squeak" }
	     sub speak {
	       my $class = shift;
	       Animal::speak($class);
	       print "[but you can barely hear it!]\n";
	     }
	   }

       Note that we're using "Animal::speak". If we were to invoke
       "Animal->speak" instead,	the first parameter to "Animal::speak" would
       automatically be	"Animal" rather	than "Mouse", so that the call to
       "$class->sound" in "Animal::speak" would	become "Animal->sound" rather
       than "Mouse->sound".

       Also, without the method	arrow "->", it becomes necessary to specify
       the first parameter to "Animal::speak" ourselves, which is why $class
       is explicitly passed: "Animal::speak($class)".

       However,	invoking "Animal::speak" directly is a mess: Firstly, it
       assumes that the	"speak"	method is a member of the "Animal" class; what
       if "Animal" actually inherits "speak" from its own base?	Because	we are
       no longer using "->" to access "speak", the special method look up
       mechanism wouldn't be used, so "speak" wouldn't even be found!

       The second problem is more subtle: "Animal" is now hardwired into the
       subroutine selection. Let's assume that "Animal::speak" does exist.
       What happens when, at a later time, someone expands the class hierarchy
       by having "Mouse" inherit from "Mus" instead of "Animal". Unless	the
       invocation of "Animal::speak" is	also changed to	an invocation of
       "Mus::speak", centuries worth of	taxonomical classification could be
       obliterated!

       What we have here is a fragile or leaky abstraction; it is the
       beginning of a maintenance nightmare. What we need is the ability to
       search for the right method wih as few assumptions as possible.

   Starting the	search from a different	place
       A better	solution is to tell Perl where in the inheritance chain	to
       begin searching for "speak". This can be	achieved with a	modified
       version of the method arrow "->":

	   ClassName->FirstPlaceToLook::method

       So, the improved	"Mouse"	class is:

	   # same Animal as before
	   { package Mouse;
	     # same @ISA, &sound as before
	     sub speak {
	       my $class = shift;
	       $class->Animal::speak;
	       print "[but you can barely hear it!]\n";
	     }
	   }

       Using this syntax, we start with	"Animal" to find "speak", and then use
       all of "Animal"'s inheritance chain if it is not	found immediately.  As
       usual, the first	parameter to "speak" would be $class, so we no longer
       need to pass $class explicitly to "speak".

       But what	about the second problem? We're	still hardwiring "Animal" into
       the method lookup.

   The SUPER way of doing things
       If "Animal" is replaced with the	special	placeholder "SUPER" in that
       invocation, then	the contents of	"Mouse"'s @ISA are used	for the
       search, beginning with $ISA[0]. So, all of the problems can be fixed as
       follows:

	   # same Animal as before
	   { package Mouse;
	     # same @ISA, &sound as before
	     sub speak {
	       my $class = shift;
	       $class->SUPER::speak;
	       print "[but you can barely hear it!]\n";
	     }
	   }

       In general, "SUPER::speak" means	look in	the current package's @ISA for
       a class that implements "speak",	and invoke the first one found.	 The
       placeholder is called "SUPER", because many other languages refer to
       base classes as "superclasses", and Perl	likes to be eclectic.

       Note that a call	such as

	   $class->SUPER::method;

       does not	look in	the @ISA of $class unless $class happens to be the
       current package.

   Let's review...
       So far, we've seen the method arrow syntax:

	 Class->method(@args);

       or the equivalent:

	 $a = "Class";
	 $a->method(@args);

       which constructs	an argument list of:

	 ("Class", @args)

       and attempts to invoke:

	 Class::method("Class",	@args);

       However,	if "Class::method" is not found, then @Class::ISA is examined
       (recursively) to	locate a class (a package) that	does indeed contain
       "method", and that subroutine is	invoked	instead.

       Using this simple syntax, we have class methods,	(multiple)
       inheritance, overriding,	and extending. Using just what we've seen so
       far, we've been able to factor out common code (though that's never a
       good reason for inheritance!), and provide a nice way to	reuse
       implementations with variations.

       Now, what about data?

   A horse is a	horse, of course of course, or is it?
       Let's start with	the code for the "Animal" class	and the	"Horse"	class:

	 { package Animal;
	   sub speak {
	     my	$class = shift;
	     print "a $class goes ", $class->sound, "!\n";
	   }
	 }
	 { package Horse;
	   @ISA	= qw(Animal);
	   sub sound { "neigh" }
	 }

       This lets us invoke "Horse->speak" to ripple upward to "Animal::speak",
       calling back to "Horse::sound" to get the specific sound, and the
       output of:

	 a Horse goes neigh!

       But all of our Horse objects would have to be absolutely	identical.  If
       we add a	subroutine, all	horses automatically share it. That's great
       for making horses the same, but how do we capture the distinctions of
       an individual horse?  For example, suppose we want to give our first
       horse a name. There's got to be a way to	keep its name separate from
       the other horses.

       That is to say, we want particular instances of "Horse" to have
       different names.

       In Perl,	any reference can be an	"instance", so let's start with	the
       simplest	reference that can hold	a horse's name:	a scalar reference.

	 my $name = "Mr. Ed";
	 my $horse = \$name;

       So, now $horse is a reference to	what will be the instance-specific
       data (the name).	The final step is to turn this reference into a	real
       instance	of a "Horse" by	using the special operator "bless":

	 bless $horse, Horse;

       This operator stores information	about the package named	"Horse"	into
       the thing pointed at by the reference.  At this point, we say $horse is
       an instance of "Horse".	That is, it's a	specific horse.	 The reference
       is otherwise unchanged, and can still be	used with traditional
       dereferencing operators.

   Invoking an instance	method
       The method arrow	can be used on instances, as well as classes (the
       names of	packages). So, let's get the sound that	$horse makes:

	 my $noise = $horse->sound("some", "unnecessary", "args");

       To invoke "sound", Perl first notes that	$horse is a blessed reference
       (and thus an instance).	It then	constructs an argument list, as	per
       usual.

       Now for the fun part: Perl takes	the class in which the instance	was
       blessed,	in this	case "Horse", and uses that class to locate the
       subroutine. In this case, "Horse::sound"	is found directly (without
       using inheritance). In the end, it is as	though our initial line	were
       written as follows:

	 my $noise = Horse::sound($horse, "some", "unnecessary", "args");

       Note that the first parameter here is still the instance, not the name
       of the class as before.	We'll get "neigh" as the return	value, and
       that'll end up as the $noise variable above.

       If Horse::sound had not been found, we'd	be wandering up	the
       @Horse::ISA array, trying to find the method in one of the
       superclasses. The only difference between a class method	and an
       instance	method is whether the first parameter is an instance (a
       blessed reference) or a class name (a string).

   Accessing the instance data
       Because we get the instance as the first	parameter, we can now access
       the instance-specific data.  In this case, let's	add a way to get at
       the name:

	 { package Horse;
	   @ISA	= qw(Animal);
	   sub sound { "neigh" }
	   sub name {
	     my	$self =	shift;
	     $$self;
	   }
	 }

       Inside "Horse::name", the @_ array contains:

	   ($horse, "some", "unnecessary", "args")

       so the "shift" stores $horse into $self.	Then, $self gets de-referenced
       with $$self as normal, yielding "Mr. Ed".

       It's traditional	to "shift" the first parameter into a variable named
       $self for instance methods and into a variable named $class for class
       methods.

       Then, the following line:

	 print $horse->name, " says ", $horse->sound, "\n";

       outputs:

	 Mr. Ed	says neigh.

   How to build	a horse
       Of course, if we	constructed all	of our horses by hand, we'd most
       likely make mistakes from time to time.	We're also violating one of
       the properties of object-oriented programming, in that the "inside
       guts" of	a Horse	are visible.  That's good if you're a veterinarian,
       but not if you just like	to own horses.	So, let's have the Horse class
       handle the details inside a class method:

	 { package Horse;
	   @ISA	= qw(Animal);
	   sub sound { "neigh" }
	   sub name {
	     my	$self =	shift;	   # instance method, so use $self
	     $$self;
	   }
	   sub named {
	     my	$class = shift;	   # class method, so use $class
	     my	$name =	shift;
	     bless \$name, $class;
	   }
	 }

       Now with	the new	"named"	method,	we can build a horse as	follows:

	 my $horse = Horse->named("Mr. Ed");

       Notice we're back to a class method, so the two arguments to
       "Horse::named" are "Horse" and "Mr. Ed".	 The "bless" operator not only
       blesses "\$name", it also returns that reference.

       This "Horse::named" method is called a "constructor".

       We've called the	constructor "named" here, so that it quickly denotes
       the constructor's argument as the name for this particular "Horse".
       You can use different constructors with different names for different
       ways of "giving birth" to the object (like maybe	recording its pedigree
       or date of birth).  However, you'll find	that most people coming	to
       Perl from more limited languages	use a single constructor named "new",
       with various ways of interpreting the arguments to "new".  Either style
       is fine,	as long	as you document	your particular	way of giving birth to
       an object.  (And	you were going to do that, right?)

   Inheriting the constructor
       But was there anything specific to "Horse" in that method?  No.
       Therefore, it's also the	same recipe for	building anything else that
       inherited from "Animal",	so let's put "name" and	"named"	there:

	 { package Animal;
	   sub speak {
	     my	$class = shift;
	     print "a $class goes ", $class->sound, "!\n";
	   }
	   sub name {
	     my	$self =	shift;
	     $$self;
	   }
	   sub named {
	     my	$class = shift;
	     my	$name =	shift;
	     bless \$name, $class;
	   }
	 }
	 { package Horse;
	   @ISA	= qw(Animal);
	   sub sound { "neigh" }
	 }

       Ahh, but	what happens if	we invoke "speak" on an	instance?

	 my $horse = Horse->named("Mr. Ed");
	 $horse->speak;

       We get a	debugging value:

	 a Horse=SCALAR(0xaca42ac) goes	neigh!

       Why?  Because the "Animal::speak" routine is expecting a	classname as
       its first parameter, not	an instance.  When the instance	is passed in,
       we'll end up using a blessed scalar reference as	a string, and that
       shows up	as we saw it just now.

   Making a method work	with either classes or instances
       All we need is for a method to detect if	it is being called on a	class
       or called on an instance.  The most straightforward way is with the
       "ref" operator.	This returns a string (the classname) when used	on a
       blessed reference, and an empty string when used	on a string (like a
       classname).  Let's modify the "name" method first to notice the change:

	 sub name {
	   my $either =	shift;
	   ref $either ? $$either : "Any $either";
	 }

       Here, the "?:" operator comes in	handy to select	either the dereference
       or a derived string.  Now we can	use this with either an	instance or a
       class.  Note that I've changed the first	parameter holder to $either to
       show that this is intended:

	 my $horse = Horse->named("Mr. Ed");
	 print Horse->name, "\n"; # prints "Any	Horse\n"
	 print $horse->name, "\n"; # prints "Mr	Ed.\n"

       and now we'll fix "speak" to use	this:

	 sub speak {
	   my $either =	shift;
	   print $either->name,	" goes ", $either->sound, "\n";
	 }

       And since "sound" already worked	with either a class or an instance,
       we're done!

   Adding parameters to	a method
       Let's train our animals to eat:

	 { package Animal;
	   sub named {
	     my	$class = shift;
	     my	$name =	shift;
	     bless \$name, $class;
	   }
	   sub name {
	     my	$either	= shift;
	     ref $either ? $$either : "Any $either";
	   }
	   sub speak {
	     my	$either	= shift;
	     print $either->name, " goes ", $either->sound, "\n";
	   }
	   sub eat {
	     my	$either	= shift;
	     my	$food =	shift;
	     print $either->name, " eats $food.\n";
	   }
	 }
	 { package Horse;
	   @ISA	= qw(Animal);
	   sub sound { "neigh" }
	 }
	 { package Sheep;
	   @ISA	= qw(Animal);
	   sub sound { "baaaah"	}
	 }

       And now try it out:

	 my $horse = Horse->named("Mr. Ed");
	 $horse->eat("hay");
	 Sheep->eat("grass");

       which prints:

	 Mr. Ed	eats hay.
	 Any Sheep eats	grass.

       An instance method with parameters gets invoked with the	instance, and
       then the	list of	parameters.  So	that first invocation is like:

	 Animal::eat($horse, "hay");

   More	interesting instances
       What if an instance needs more data?  Most interesting instances	are
       made of many items, each	of which can in	turn be	a reference or even
       another object.	The easiest way	to store these is often	in a hash.
       The keys	of the hash serve as the names of parts	of the object (often
       called "instance	variables" or "member variables"), and the
       corresponding values are, well, the values.

       But how do we turn the horse into a hash?  Recall that an object	was
       any blessed reference.  We can just as easily make it a blessed hash
       reference as a blessed scalar reference,	as long	as everything that
       looks at	the reference is changed accordingly.

       Let's make a sheep that has a name and a	color:

	 my $bad = bless { Name	=> "Evil", Color => "black" }, Sheep;

       so "$bad->{Name}" has "Evil", and "$bad->{Color}" has "black".  But we
       want to make "$bad->name" access	the name, and that's now messed	up
       because it's expecting a	scalar reference.  Not to worry, because
       that's pretty easy to fix up.

       One solution is to override "Animal::name" and "Animal::named" by
       defining	them anew in "Sheep", but then any methods added later to
       "Animal"	might still mess up, and we'd have to override all of those
       too. Therefore, it's never a good idea to define	the data layout	in a
       way that's different from the data layout of the	base classes. In fact,
       it's a good idea	to use blessed hash references in all cases. Also,
       this is why it's	important to have constructors do the low-level	work.
       So, let's redefine "Animal":

	 ## in Animal
	 sub name {
	   my $either =	shift;
	   ref $either ? $either->{Name} : "Any	$either";
	 }
	 sub named {
	   my $class = shift;
	   my $name = shift;
	   my $self = {	Name =>	$name };
	   bless $self,	$class;
	 }

       Of course, we still need	to override "named" in order to	handle
       constructing a "Sheep" with a certain color:

	 ## in Sheep
	 sub named {
	   my ($class, $name) =	@_;
	   my $self = $class->SUPER::named(@_);
	   $$self{Color} = $class->default_color;
	   $self
	 }

       (Note that @_ contains the parameters to	"named".)

       What's this "default_color"?  Well, if "named" has only the name, we
       still need to set a color, so we'll have	a class-specific default
       color.  For a sheep, we might define it as white:

	 ## in Sheep
	 sub default_color { "white" }

       Now:

	 my $sheep = Sheep->named("Bad");
	 print $sheep->{Color},	"\n";

       outputs:

	 white

       Now, there's nothing particularly specific to "Sheep" when it comes to
       color, so let's remove "Sheep::named" and implement "Animal::named" to
       handle color instead:

	 ## in Animal
	 sub named {
	   my ($class, $name) =	@_;
	   my $self = {	Name =>	$name, Color =>	$class->default_color };
	   bless $self,	$class;
	 }

       And then	to keep	from having to define "default_color" for each
       additional class, we'll define a	method that serves as the "default
       default"	directly in "Animal":

	 ## in Animal
	 sub default_color { "brown" }

       Of course, because "name" and "named" were the only methods that
       referenced the "structure" of the object, the rest of the methods can
       remain the same,	so "speak" still works as before.

   A horse of a	different color
       But having all our horses be brown would	be boring.  So let's add a
       method or two to	get and	set the	color.

	 ## in Animal
	 sub color {
	   $_[0]->{Color}
	 }
	 sub set_color {
	   $_[0]->{Color} = $_[1];
	 }

       Note the	alternate way of accessing the arguments: $_[0]	is used	in-
       place, rather than with a "shift".  (This saves us a bit	of time	for
       something that may be invoked frequently.)  And now we can fix that
       color for Mr. Ed:

	 my $horse = Horse->named("Mr. Ed");
	 $horse->set_color("black-and-white");
	 print $horse->name, " is colored ", $horse->color, "\n";

       which results in:

	 Mr. Ed	is colored black-and-white

   Summary
       So, now we have class methods, constructors, instance methods, instance
       data, and even accessors. But that's still just the beginning of	what
       Perl has	to offer. We haven't even begun	to talk	about accessors	that
       double as getters and setters, destructors, indirect object notation,
       overloading, "isa" and "can" tests, the "UNIVERSAL" class, and so on.
       That's for the rest of the Perl documentation to	cover. Hopefully, this
       gets you	started, though.

SEE ALSO
       For more	information, see perlobj (for all the gritty details about
       Perl objects, now that you've seen the basics), perltoot	(the tutorial
       for those who already know objects), perltooc (dealing with class
       data), perlbot (for some	more tricks), and books	such as	Damian
       Conway's	excellent Object Oriented Perl.

       Some modules which might	prove interesting are Class::Accessor,
       Class::Class, Class::Contract, Class::Data::Inheritable,
       Class::MethodMaker and Tie::SecureHash

COPYRIGHT
       Copyright (c) 1999, 2000	by Randal L. Schwartz and Stonehenge
       Consulting Services, Inc.

       Copyright (c) 2009 by Michael F.	Witten.

       Permission is hereby granted to distribute this document	intact with
       the Perl	distribution, and in accordance	with the licenses of the Perl
       distribution; derived documents must include this copyright notice
       intact.

       Portions	of this	text have been derived from Perl Training materials
       originally appearing in the Packages, References, Objects, and Modules
       course taught by	instructors for	Stonehenge Consulting Services,	Inc.
       and used	with permission.

       Portions	of this	text have been derived from materials originally
       appearing in Linux Magazine and used with permission.

perl v5.12.1			  2010-04-26			   PERLBOOT(1)

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