perlboot - Beginner's Object-Oriented Tutorial
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... [Toc] [Back]
Let's let the animals talk for a moment:
sub Cow::speak {
print "a Cow goes moooo!0;
}
sub Horse::speak {
print "a Horse goes neigh!0;
}
sub Sheep::speak {
print "a Sheep goes baaaah!0
}
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 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 [Toc] [Back]
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 [Toc] [Back]
Let's take that new arrow invocation and put it back in
the barnyard example:
sub Cow::speak {
print "a Cow goes moooo!0;
}
sub Horse::speak {
print "a Horse goes neigh!0;
}
sub Sheep::speak {
print "a Sheep goes baaaah!0
}
@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 [Toc] [Back]
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!0;
}
And the other two animals come out similarly:
sub Cow::speak {
my $class = shift;
print "a $class goes moooo!0;
}
sub Horse::speak {
my $class = shift;
print "a $class goes neigh!0;
}
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 [Toc] [Back]
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, "!0
}
}
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, "!0
}
}
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 [Toc] [Back]
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, "!0
}
}
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. 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 (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 [Toc] [Back]
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, "!0;
print "[but you can barely hear it!]0;
}
}
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 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->speak",
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 { "squeak" }
sub speak {
my $class = shift;
Animal::speak($class);
print "[but you can barely hear it!]0;
}
}
Note that we have to include the $class parameter (almost
surely the value of "Mouse") as the first parameter to
"Animal::speak", since we've stopped using the method
arrow. Why did we stop? Well, if we invoke "Animal->speak"
there, the first parameter to the method will
be "Animal" not "Mouse", 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 <Mouse> and didn't
notice "Animal" there in "speak". So, this is probably
not the right way to go.
Starting the search from a different place [Toc] [Back]
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, &sound as before
sub speak {
my $class = shift;
$class->Animal::speak;
print "[but you can barely hear it!]0;
}
}
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 [Toc] [Back]
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, &sound as before
sub speak {
my $class = shift;
$class->SUPER::speak;
print "[but you can barely hear it!]0;
}
}
So, "SUPER::speak" means look in the current package's
@ISA for "speak", invoking the first one found. Note that
it does not look in the @ISA of $class.
Where we're at so far...
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 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 "a $class goes ", $class->sound, "!0
}
}
{ 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 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 = "Mr. Ed";
my $talking = ame;
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 [Toc] [Back]
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->sound;
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 { "neigh" }
sub name {
my $self = shift;
$$self;
}
}
Now we call for the name:
print $talking->name, " says ", $talking->sound, "0;
Inside "Horse::name", the @_ array contains just $talking,
which the "shift" stores into $self. (It's traditional to
shift the first parameter off into a variable named $self
for instance methods, so stay with that unless you have
strong reasons otherwise.) Then, $self gets de-referenced
as a scalar ref, yielding "Mr. Ed", and we're done with
that. The result is:
Mr. Ed says neigh.
How to build a horse [Toc] [Back]
Of course, if we constructed all of our horses by hand,
we'd most likely make mistakes from time to time. We're
also violating one of the properties of object-oriented
programming, in that the "inside guts" of a Horse are visible.
That's good if you're a veterinarian, but not if
you just like to own horses. So, let's let the Horse
class build a new horse:
{ package Horse;
@ISA = qw(Animal);
sub sound { "neigh" }
sub name {
my $self = shift;
$$self;
}
sub named {
my $class = shift;
my $name = shift;
bless ame, $class;
}
}
Now with the new "named" method, we can build a horse:
my $talking = 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 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 [Toc] [Back]
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 "a $class goes ", $class->sound, "!0
}
sub name {
my $self = shift;
$$self;
}
sub named {
my $class = shift;
my $name = shift;
bless ame, $class;
}
}
{ package Horse;
@ISA = qw(Animal);
sub sound { "neigh" }
}
Ahh, but what happens if we invoke "speak" on an instance?
my $talking = Horse->named("Mr. Ed");
$talking->speak;
We get a debugging value:
a Horse=SCALAR(0xaca42ac) goes neigh!
Why? Because the "Animal::speak" routine is expecting a
classname as its first parameter, not an instance. When
the instance is passed in, we'll end up using a blessed
scalar reference as a string, and that shows up as we saw
it just now.
Making a method work with either classes or instances [Toc] [Back]
All we need is for a method to detect if it is being
called on a class or called on an instance. The most
straightforward way is with the "ref" operator. This
returns a string (the classname) when used on a blessed
reference, and "undef" when used on a string (like a
classname). Let's modify the "name" method first to
notice the change:
sub name {
my $either = shift;
ref $either
? $$either # it's an instance, return name
: "an unnamed $either"; # it's a class, return
generic
}
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->named("Mr. Ed");
print Horse->name, "0; # prints "an unnamed Horse0
print $talking->name, "0; # prints "Mr Ed.0
and now we'll fix "speak" to use this:
sub speak {
my $either = shift;
print $either->name, " goes ", $either->sound, "0;
}
And since "sound" already worked with either a class or an
instance, we're done!
Adding parameters to a method [Toc] [Back]
Let's train our animals to eat:
{ package Animal;
sub named {
my $class = shift;
my $name = shift;
bless ame, $class;
}
sub name {
my $either = shift;
ref $either
? $$either # it's an instance, return name
: "an unnamed $either"; # it's a class, return
generic
}
sub speak {
my $either = shift;
print $either->name, " goes ", $either->sound, "0;
}
sub eat {
my $either = shift;
my $food = shift;
print $either->name, " eats $food.0;
}
}
{ package Horse;
@ISA = qw(Animal);
sub sound { "neigh" }
}
{ package Sheep;
@ISA = qw(Animal);
sub sound { "baaaah" }
}
And now try it out:
my $talking = Horse->named("Mr. Ed");
$talking->eat("hay");
Sheep->eat("grass");
which prints:
Mr. Ed eats hay.
an unnamed Sheep eats grass.
An instance method with parameters gets invoked with the
instance, and then the list of parameters. So that first
invocation is like:
Animal::eat($talking, "hay");
More interesting instances [Toc] [Back]
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:
## in Animal
sub name {
my $either = shift;
ref $either ?
$either->{Name} :
"an unnamed $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 => $name, Color => $class->default_color };
bless $self, $class;
}
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 { "white" }
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 { "brown" }
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 [Toc] [Back]
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 $talking = Horse->named("Mr. Ed");
$talking->set_color("black-and-white");
print $talking->name, " is colored ", $talking->color,
"0;
which results in:
Mr. Ed is colored black-and-white
Summary [Toc] [Back]
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.
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 (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, Refer-
ences, 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 v5.8.5 2002-11-06 17 [ Back ] |
