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perlboot - Beginner’s Object-Oriented Tutorial

CONTENTS

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.

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?

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.

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 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.

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!

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!

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.

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.

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?

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.

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.

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.

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).

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.

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?)

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.

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, "\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!

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");

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.

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

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, 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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