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 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 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. 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 \f(CW@ISA\fP

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
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 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 
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

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 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 \s-1SUPER\s0 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) 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, "!\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 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 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->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, "\n";

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 { "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. 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, "!\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 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 # its an instance, return name
      : "an unnamed $either"; # its 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 # its an instance, return name
        : "an unnamed $either"; # its 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 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

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 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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