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PERLBOOT
!!!PERLBOOT
NAME
DESCRIPTION
SEE ALSO
COPYRIGHT
----
!!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
    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) {
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 subs 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- 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-
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 =
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
    @pasture = qw(Cow Cow Horse Sheep Sheep);
foreach $animal (@pasture) {
$animal-
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-
attempts to invoke subroutine Class::method as:


    Class::method(
(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
And the other two animals come out similarly:


    sub Cow::speak {
my $class = shift;
print
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 {
Now, when we call Cow-, we get a $class of Cow in speak. This in turn selects the Cow- method, which returns moooo. But how different would this be for the Horse?


    { package Horse;
sub sound {
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
Then, for each animal, we say it ``inherits'' from Animal, along with the animal-specific sound:


    { package Cow;
@ISA = qw(Animal);
sub sound {
Note the added @ISA array. We'll get to that in a minute.


But what happens when we invoke Cow-
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-, it'll be looking for
Cow-, which gets it on the first try
without looking at @ISA. Success!


__A few notes about__ @ISA


This magical @ISA variable (pronounced ``is a'' not
``ice-uh''), 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. 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 allow it as an implicitly named package variable:


    package Cow;
use vars qw(@ISA);
@ISA = qw(Animal);
If you're bringing in the class from outside, via an object-oriented module, you change:


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


    package Cow;
use base qw(Animal);
And that's pretty darn compact.


__Overriding the methods__


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


    # Animal package from before
{ package Mouse;
@ISA = qw(Animal);
sub sound {
    Mouse-
which results in:


    a Mouse goes squeak!
[[but you can barely hear it!]
Here, Mouse has its own speaking routine, so Mouse- doesn't immediately invoke Animal-. This is known as ``overriding''. In fact, we didn't even need to say that a Mouse was an Animal at all, since all of the methods needed for speak are completely defined with Mouse.


But we've now duplicated some of the code from
Animal-, and this can once again be a
maintenance headache. So, can we avoid that? Can we say
somehow that a Mouse does everything any other
Animal does, but add in the extra comment?
Sure!


First, we can invoke the Animal::speak method
directly:


    # Animal package from before
{ package Mouse;
@ISA = qw(Animal);
sub sound {
Note that we have to include the $class parameter (almost surely the value of ) as the first parameter to Animal::speak, since we've stopped using the method arrow. Why did we stop? Well, if we invoke Animal- there, the first parameter to the method will be  not , and when time comes for it to call for the sound, it won't have the right class to come back to this package.


Invoking Animal::speak directly is a mess, however.
What if Animal::speak didn't exist before, and was
being inherited from a class mentioned in
@Animal::ISA? Because we are no longer using the
method arrow, we get one and only one chance to hit the
right subroutine.


Also note that the Animal classname is now
hardwired into the subroutine selection. This is a mess if
someone maintains the code, changing @ISA for
Animal there in
speak. So, this is probably not the right way to
go.


__Starting the search from a different
place__


A better solution is to tell Perl to search from a higher
place in the inheritance chain:


    # same Animal as before
{ package Mouse;
# same @ISA,
Ahh. This works. Using this syntax, we start with Animal to find speak, and use all of Animal's inheritance chain if not found immediately. And yet the first parameter will be $class, so the found speak method will get Mouse as its first entry, and eventually work its way back to Mouse::sound for the details.


But this isn't the best solution. We still have to keep the
@ISA and the initial search package coordinated.
Worse, if Mouse had multiple entries in
@ISA, we wouldn't necessarily know which one had
actually defined speak. So, is there an even better
way?


__The SUPER way of doing
things__


By changing the Animal class to the SUPER
class in that invocation, we get a search of all of our
super classes (classes listed in @ISA)
automatically:


    # same Animal as before
{ package Mouse;
# same @ISA,
So, SUPER::speak means look in the current package's @ISA for speak, invoking the first one found.


__Where we're at so far...__


So far, we've seen the method arrow syntax:


  Class-
or the equivalent:


  $a =
which constructs an argument list of:


  (
and attempts to invoke


  Class::method(
However, if Class::method is not found, then @Class::ISA is examined (recursively) to locate 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, and provide a nice way to reuse implementations with
variations. This is at the core of what objects provide, but
objects also provide instance data, which we haven't even
begun to cover.


__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
This lets us invoke Horse- 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 I add a subroutine, all horses automatically share it. That's great for making horses the same, but how do we capture the distinctions about an individual horse? For example, suppose I want to give my first horse a name. There's got to be a way to keep its name separate from the other horses.


We can do that by drawing a new distinction, called an
``instance''. An ``instance'' is generally created by a
class. 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 =
So now $talking is a reference to what will be the instance-specific data (the name). The final step in turning this into a real instance is with a special operator called bless:


  bless $talking, Horse;
This operator stores information about the package named Horse into the thing pointed at by the reference. At this point, we say $talking 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 names
of packages (classes). So, let's get the sound that
$talking makes:


  my $noise = $talking-
To invoke sound, Perl first notes that $talking is a blessed reference (and thus an instance). It then constructs an argument list, in this case from just ($talking). (Later we'll see that arguments will take their place following the instance variable, just like with classes.)


Now for the fun part: Perl takes the class in which the
instance was blessed, in this case Horse, and uses
that to locate the subroutine to invoke the method. In this
case, Horse::sound is found directly (without using
inheritance), yielding the final subroutine
invocation:


  Horse::sound($talking)
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 list to try to find the method in one
of the superclasses, just as for a class method. 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 {
Now we call for the name:


  print $talking-
Inside Horse::name, the @_ array contains just $talking, which the shift stores into $self. (It's traditional to shift the first parameter off into a variable named $self for instance methods, so stay with that unless you have strong reasons otherwise.) Then, $self gets de-referenced as a scalar ref, yielding Mr. Ed, and we're done with that. The result is:


  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 let the Horse class
build a new horse:


  { package Horse;
@ISA = qw(Animal);
sub sound {
Now with the new named method, we can build a horse:


  my $talking = Horse-
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 the reference to $name, so that's fine as a return value. And that's how to build a horse.


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 it there:


  { package Animal;
sub speak {
my $class = shift;
print
Ahh, but what happens if we invoke speak on an instance?


  my $talking = Horse-
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 undef 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 # it's an instance, return name
:
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 $talking = Horse-
and now we'll fix speak to use this:


  sub speak {
my $either = shift;
print $either-
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 # it's an instance, return name
:
And now try it out:


  my $talking = Horse-
which prints:


  Mr. Ed eats hay.
an unnamed 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($talking,


__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 =
so $bad- has Evil, and $bad- has black. But we want to make $bad- 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:


  ## in Animal
sub name {
my $either = shift;
ref $either ?
$either-
And of course named still builds a scalar sheep, so let's fix that as well:


  ## in Animal
sub named {
my $class = shift;
my $name = shift;
my $self = { Name =
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 initial color. For a sheep, we might define it as white:


  ## in Sheep
sub default_color {
And then to keep from having to define one for each additional class, we'll define a ``backstop'' method that serves as the ``default default'', directly in Animal:


  ## in Animal
sub default_color {
Now, 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]-
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 $talking = Horse-
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,
subclasses that add instance data, per-class data,
overloading, ``isa'' and ``can'' tests, 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), perltootc (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. 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.
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