| |
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 per-
lobj, a basic reference in using objects, and perltoot, which intro-
duces readers to the peculiarities of Perl's object system in a tuto-
rial way.
So, let's take a different approach, presuming no prior object experi-
ence. 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 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->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 vari-
ables. 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, with-
out 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 subrou-
tine. But once again, we have a lot of similar structure. Can we fac-
tor 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 def-
inition 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. 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 con-
tains the single name "Animal".
Perl next checks for "speak" inside "Animal" instead, as in "Ani-
mal::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 with-
out 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 { "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 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!]\n";
}
}
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 "Ani-
mal::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 subrou-
tine selection. This is a mess if someone maintains the code, changing
@ISA for 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
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!]\n";
}
}
Ahh. This works. Using this syntax, we start with "Animal" to find
"speak", and use all of "Animal"'s inheritance chain if not found imme-
diately. 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, &sound as before
sub speak {
my $class = shift;
$class->SUPER::speak;
print "[but you can barely hear it!]\n";
}
}
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) inheri-
tance, 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, "!\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 out-
put 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 = \$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 refer-
ence 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->sound;
To invoke "sound", Perl first notes that $talking is a blessed refer-
ence (and thus an instance). It then constructs an argument list, in
this case from just "($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;
$$self;
}
}
Now we call for the name:
print $talking->name, " says ", $talking->sound, "\n";
Inside "Horse::name", the @_ array contains just $talking, which the
"shift" stores into $self. (It's traditional to shift the first param-
eter off into a variable named $self for instance methods, so stay with
that unless you have strong reasons otherwise.) Then, $self gets de-
referenced as a scalar ref, yielding "Mr. Ed", and we're done with
that. The result is:
Mr. Ed says neigh.
How to build a horse
Of course, if we constructed all of our horses by hand, we'd most
likely make mistakes from time to time. We're also violating one of
the properties of object-oriented programming, in that the "inside
guts" of a Horse are visible. That's good if you're a veterinarian,
but not if you just like to own horses. So, let's let the Horse class
build a new horse:
{ package Horse;
@ISA = qw(Animal);
sub sound { "neigh" }
sub name {
my $self = shift;
$$self;
}
sub named {
my $class = shift;
my $name = shift;
bless \$name, $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
But was there anything specific to "Horse" in that method? No. There-
fore, it's also the same recipe for building anything else that inher-
ited from "Animal", so let's put it there:
{ package Animal;
sub speak {
my $class = shift;
print "a $class goes ", $class->sound, "!\n"
}
sub name {
my $self = shift;
$$self;
}
sub named {
my $class = shift;
my $name = shift;
bless \$name, $class;
}
}
{ package Horse;
@ISA = qw(Animal);
sub sound { "neigh" }
}
Ahh, but what happens if we invoke "speak" on an instance?
my $talking = Horse->named("Mr. Ed");
$talking->speak;
We get a debugging value:
a Horse=SCALAR(0xaca42ac) goes neigh!
Why? Because the "Animal::speak" routine is expecting a classname as
its first parameter, not an instance. When the instance is passed in,
we'll end up using a blessed scalar reference as a string, and that
shows up as we saw it just now.
Making a method work with either classes or instances
All we need is for a method to detect if it is being called on a class
or called on an instance. The most straightforward way is with the
"ref" operator. This returns a string (the classname) when used on a
blessed reference, and "undef" when used on a string (like a class-
name). Let's modify the "name" method first to notice the change:
sub name {
my $either = shift;
ref $either
? $$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, "\n"; # prints "an unnamed Horse\n"
print $talking->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 # 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, "\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 $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
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 correspond-
ing 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
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, "\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,
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), perltooc (dealing with class
data), perlbot (for some more tricks), and books such as Damian Con-
way's excellent Object Oriented Perl.
Some modules which might prove interesting are Class::Accessor,
Class::Class, Class::Contract, Class::Data::Inheritable, Class::Method-
Maker and Tie::SecureHash
COPYRIGHT
Copyright (c) 1999, 2000 by Randal L. Schwartz and Stonehenge Consult-
ing 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.
perl v5.8.8 2006-01-07 PERLBOOT(1)
|