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!
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.
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
{ 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;
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";
}
}
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::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!
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 {
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
$$self;
}
sub named {
my $class = shift; # class method, so use $class
my $name = shift;
bless \$name, $class;
}
}
Now with the new "named" method, we can build a horse as follows:
my $horse = Horse->named("Mr. Ed");
Notice we're back to a class method, so the two arguments to
"Horse::named" are "Horse" and "Mr. Ed". The "bless" operator not only
blesses "\$name", it also returns that reference.
This "Horse::named" method is called a "constructor".
We've called the constructor "named" here, so that it quickly denotes
the constructor's argument as the name for this particular "Horse".
You can use different constructors with different names for different
ways of "giving birth" to the object (like maybe recording its pedigree
or date of birth). However, you'll find that most people coming to
Perl from more limited languages use a single constructor named "new",
with various ways of interpreting the arguments to "new". Either style
is fine, as long as you document your particular way of giving birth to
an object. (And you were going to do that, right?)
$$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 ? $$either : "Any $either";
}
Here, the "?:" operator comes in handy to select either the dereference
or a derived string. Now we can use this with either an instance or a
class. Note that I've changed the first parameter holder to $either to
show that this is intended:
my $horse = Horse->named("Mr. Ed");
print Horse->name, "\n"; # prints "Any Horse\n"
print $horse->name, "\n"; # prints "Mr Ed.\n"
and now we'll fix "speak" to use this:
sub speak {
my $either = shift;
print $either->name, " goes ", $either->sound, "\n";
}
}
sub name {
my $either = shift;
ref $either ? $$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:
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 };
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
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