perlboot.pod

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=head1 NAME

perlboot - Beginner's Object-Oriented Tutorial

=head1 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
L<perlobj>, a basic reference in using objects, and L<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 (L<perlsub>),
references (L<perlref> et. seq.), and packages (L<perlmod>), so become
familiar with those first if you haven't already.

=head2 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 C<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?

=head2 Introducing the method invocation arrow

For now, let's say that C<< Class->method >> invokes subroutine
C<method> in package C<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 C<use strict refs> is enabled.

=head2 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 C<speak> routines has a
similar structure: a C<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 C<says> instead of C<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.

=head2 The extra parameter of method invocation

The invocation of:

    Class->method(@args)

attempts to invoke subroutine C<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 C<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, C<$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.

=head2 Calling a second method to simplify things

Let's call out from C<speak> to a helper method called C<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 C<< Cow->speak >>, we get a C<$class> of C<Cow> in
C<speak>.  This in turn selects the C<< Cow->sound >> method, which
returns C<moooo>.  But how different would this be for the C<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 C<speak> between the Cow and the
Horse?  Yes, with inheritance!

=head2 Inheriting the windpipes

We'll define a common subroutine package called C<Animal>, with the
definition for C<speak>:

    { package Animal;
      sub speak {
	my $class = shift;
	print "a $class goes ", $class->sound, "!\n";
      }
    }

Then, for each animal, we say it "inherits" from C<Animal>, along
with the animal-specific sound:

    { package Cow;
      @ISA = qw(Animal);
      sub sound { "moooo" }
    }

Note the added C<@ISA> array.  We'll get to that in a minute.

But what happens when we invoke C<< Cow->speak >> now?

First, Perl constructs the argument list.  In this case, it's just
C<Cow>.  Then Perl looks for C<Cow::speak>.  But that's not there, so
Perl checks for the inheritance array C<@Cow::ISA>.  It's there,
and contains the single name C<Animal>.

Perl next checks for C<speak> inside C<Animal> instead, as in
C<Animal::speak>.  And that's found, so Perl invokes that subroutine
with the already frozen argument list.

Inside the C<Animal::speak> subroutine, C<$class> becomes C<Cow> (the
first argument).  So when we get to the step of invoking
C<< $class->sound >>, it'll be looking for C<< Cow->sound >>, which
gets it on the first try without looking at C<@ISA>.  Success!

=head2 A few notes about @ISA

This magical C<@ISA> variable (pronounced "is a" not "ice-uh"), has
declared that C<Cow> "is a" C<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 C<Animal> also had an C<@ISA>, then we'd check there too.  The
search is recursive, depth-first, left-to-right in each C<@ISA> by
default (see L<mro> for alternatives).  Typically, each C<@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 C<use strict>, we'll get complaints on C<@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.

=head2 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, C<Mouse> has its own speaking routine, so C<< Mouse->speak >>
doesn't immediately invoke C<< Animal->speak >>.  This is known as
"overriding".  In fact, we didn't even need to say that a C<Mouse> was
an C<Animal> at all, since all of the methods needed for C<speak> are
completely defined with C<Mouse>.

But we've now duplicated some of the code from C<< Animal->speak >>,
and this can once again be a maintenance headache.  So, can we avoid
that?  Can we say somehow that a C<Mouse> does everything any other
C<Animal> does, but add in the extra comment?  Sure!

First, we can invoke the C<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 C<$class> parameter (almost surely
the value of C<"Mouse">) as the first parameter to C<Animal::speak>,
since we've stopped using the method arrow.  Why did we stop?  Well,
if we invoke C<< Animal->speak >> there, the first parameter to the
method will be C<"Animal"> not C<"Mouse">, and when time comes for it
to call for the C<sound>, it won't have the right class to come back
to this package.

Invoking C<Animal::speak> directly is a mess, however.  What if
C<Animal::speak> didn't exist before, and was being inherited from a
class mentioned in C<@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 C<Animal> classname is now hardwired into the
subroutine selection.  This is a mess if someone maintains the code,
changing C<@ISA> for C<Mouse> and didn't notice C<Animal> there in
C<speak>.  So, this is probably not the right way to go.

=head2 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 C<Animal> to find
C<speak>, and use all of C<Animal>'s inheritance chain if not found
immediately.  And yet the first parameter will be C<$class>, so the
found C<speak> method will get C<Mouse> as its first entry, and
eventually work its way back to C<Mouse::sound> for the details.

But this isn't the best solution.  We still have to keep the C<@ISA>
and the initial search package coordinated.  Worse, if C<Mouse> had
multiple entries in C<@ISA>, we wouldn't necessarily know which one
had actually defined C<speak>.  So, is there an even better way?

=head2 The SUPER way of doing things

By changing the C<Animal> class to the C<SUPER> class in that
invocation, we get a search of all of our super classes (classes
listed in C<@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, C<SUPER::speak> means look in the current package's C<@ISA> for
C<speak>, invoking the first one found. Note that it does I<not> look in
the C<@ISA> of C<$class>.

=head2 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 C<Class::method> is not found, then C<@Class::ISA> is examined
(recursively) to locate a package that does indeed contain C<method>,
and that subroutine is invoked instead.