perltoot.pod

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=head1 NAME

perltoot - Tom's object-oriented tutorial for perl

=head1 DESCRIPTION

Object-oriented programming is a big seller these days.  Some managers
would rather have objects than sliced bread.  Why is that?  What's so
special about an object?  Just what I<is> an object anyway?

An object is nothing but a way of tucking away complex behaviours into
a neat little easy-to-use bundle.  (This is what professors call
abstraction.) Smart people who have nothing to do but sit around for
weeks on end figuring out really hard problems make these nifty
objects that even regular people can use. (This is what professors call
software reuse.)  Users (well, programmers) can play with this little
bundle all they want, but they aren't to open it up and mess with the
insides.  Just like an expensive piece of hardware, the contract says
that you void the warranty if you muck with the cover.  So don't do that.

The heart of objects is the class, a protected little private namespace
full of data and functions.  A class is a set of related routines that
addresses some problem area.  You can think of it as a user-defined type.
The Perl package mechanism, also used for more traditional modules,
is used for class modules as well.  Objects "live" in a class, meaning
that they belong to some package.

More often than not, the class provides the user with little bundles.
These bundles are objects.  They know whose class they belong to,
and how to behave.  Users ask the class to do something, like "give
me an object."  Or they can ask one of these objects to do something.
Asking a class to do something for you is calling a I<class method>.
Asking an object to do something for you is calling an I<object method>.
Asking either a class (usually) or an object (sometimes) to give you
back an object is calling a I<constructor>, which is just a
kind of method.

That's all well and good, but how is an object different from any other
Perl data type?  Just what is an object I<really>; that is, what's its
fundamental type?  The answer to the first question is easy.  An object
is different from any other data type in Perl in one and only one way:
you may dereference it using not merely string or numeric subscripts
as with simple arrays and hashes, but with named subroutine calls.
In a word, with I<methods>.

The answer to the second question is that it's a reference, and not just
any reference, mind you, but one whose referent has been I<bless>()ed
into a particular class (read: package).  What kind of reference?  Well,
the answer to that one is a bit less concrete.  That's because in Perl
the designer of the class can employ any sort of reference they'd like
as the underlying intrinsic data type.  It could be a scalar, an array,
or a hash reference.  It could even be a code reference.  But because
of its inherent flexibility, an object is usually a hash reference.

=head1 Creating a Class

Before you create a class, you need to decide what to name it.  That's
because the class (package) name governs the name of the file used to
house it, just as with regular modules.  Then, that class (package)
should provide one or more ways to generate objects.  Finally, it should
provide mechanisms to allow users of its objects to indirectly manipulate
these objects from a distance.

For example, let's make a simple Person class module.  It gets stored in
the file Person.pm.  If it were called a Happy::Person class, it would
be stored in the file Happy/Person.pm, and its package would become
Happy::Person instead of just Person.  (On a personal computer not
running Unix or Plan 9, but something like MacOS or VMS, the directory
separator may be different, but the principle is the same.)  Do not assume
any formal relationship between modules based on their directory names.
This is merely a grouping convenience, and has no effect on inheritance,
variable accessibility, or anything else.

For this module we aren't going to use Exporter, because we're
a well-behaved class module that doesn't export anything at all.
In order to manufacture objects, a class needs to have a I<constructor
method>.  A constructor gives you back not just a regular data type,
but a brand-new object in that class.  This magic is taken care of by
the bless() function, whose sole purpose is to enable its referent to
be used as an object.  Remember: being an object really means nothing
more than that methods may now be called against it.

While a constructor may be named anything you'd like, most Perl
programmers seem to like to call theirs new().  However, new() is not
a reserved word, and a class is under no obligation to supply such.
Some programmers have also been known to use a function with
the same name as the class as the constructor.

=head2 Object Representation

By far the most common mechanism used in Perl to represent a Pascal
record, a C struct, or a C++ class is an anonymous hash.  That's because a
hash has an arbitrary number of data fields, each conveniently accessed by
an arbitrary name of your own devising.

If you were just doing a simple
struct-like emulation, you would likely go about it something like this:

    $rec = {
        name  => "Jason",
        age   => 23,
        peers => [ "Norbert", "Rhys", "Phineas"],
    };

If you felt like it, you could add a bit of visual distinction
by up-casing the hash keys:

    $rec = {
        NAME  => "Jason",
        AGE   => 23,
        PEERS => [ "Norbert", "Rhys", "Phineas"],
    };

And so you could get at C<< $rec->{NAME} >> to find "Jason", or
C<< @{ $rec->{PEERS} } >> to get at "Norbert", "Rhys", and "Phineas".
(Have you ever noticed how many 23-year-old programmers seem to
be named "Jason" these days? :-)

This same model is often used for classes, although it is not considered
the pinnacle of programming propriety for folks from outside the
class to come waltzing into an object, brazenly accessing its data
members directly.  Generally speaking, an object should be considered
an opaque cookie that you use I<object methods> to access.  Visually,
methods look like you're dereffing a reference using a function name
instead of brackets or braces.

=head2 Class Interface

Some languages provide a formal syntactic interface to a class's methods,
but Perl does not.  It relies on you to read the documentation of each
class.  If you try to call an undefined method on an object, Perl won't
complain, but the program will trigger an exception while it's running.
Likewise, if you call a method expecting a prime number as its argument
with a non-prime one instead, you can't expect the compiler to catch this.
(Well, you can expect it all you like, but it's not going to happen.)

Let's suppose you have a well-educated user of your Person class,
someone who has read the docs that explain the prescribed
interface.  Here's how they might use the Person class:

    use Person;

    $him = Person->new();
    $him->name("Jason");
    $him->age(23);
    $him->peers( "Norbert", "Rhys", "Phineas" );

    push @All_Recs, $him;  # save object in array for later

    printf "%s is %d years old.\n", $him->name, $him->age;
    print "His peers are: ", join(", ", $him->peers), "\n";

    printf "Last rec's name is %s\n", $All_Recs[-1]->name;

As you can see, the user of the class doesn't know (or at least, has no
business paying attention to the fact) that the object has one particular
implementation or another.  The interface to the class and its objects
is exclusively via methods, and that's all the user of the class should
ever play with.

=head2 Constructors and Instance Methods

Still, I<someone> has to know what's in the object.  And that someone is
the class.  It implements methods that the programmer uses to access
the object.  Here's how to implement the Person class using the standard
hash-ref-as-an-object idiom.  We'll make a class method called new() to
act as the constructor, and three object methods called name(), age(), and
peers() to get at per-object data hidden away in our anonymous hash.

    package Person;
    use strict;

    ##################################################
    ## the object constructor (simplistic version)  ##
    ##################################################
    sub new {
        my $self  = {};
        $self->{NAME}   = undef;
        $self->{AGE}    = undef;
        $self->{PEERS}  = [];
        bless($self);           # but see below
        return $self;
    }

    ##############################################
    ## methods to access per-object data        ##
    ##                                          ##
    ## With args, they set the value.  Without  ##
    ## any, they only retrieve it/them.         ##
    ##############################################

    sub name {
        my $self = shift;
        if (@_) { $self->{NAME} = shift }
        return $self->{NAME};
    }

    sub age {
        my $self = shift;
        if (@_) { $self->{AGE} = shift }
        return $self->{AGE};
    }

    sub peers {
        my $self = shift;
        if (@_) { @{ $self->{PEERS} } = @_ }
        return @{ $self->{PEERS} };
    }

    1;  # so the require or use succeeds

We've created three methods to access an object's data, name(), age(),
and peers().  These are all substantially similar.  If called with an
argument, they set the appropriate field; otherwise they return the
value held by that field, meaning the value of that hash key.

=head2 Planning for the Future: Better Constructors

Even though at this point you may not even know what it means, someday
you're going to worry about inheritance.  (You can safely ignore this
for now and worry about it later if you'd like.)  To ensure that this
all works out smoothly, you must use the double-argument form of bless().
The second argument is the class into which the referent will be blessed.
By not assuming our own class as the default second argument and instead
using the class passed into us, we make our constructor inheritable.

While we're at it, let's make our constructor a bit more flexible.
Rather than being uniquely a class method, we'll set it up so that
it can be called as either a class method I<or> an object
method.  That way you can say:

    $me  = Person->new();
    $him = $me->new();

To do this, all we have to do is check whether what was passed in
was a reference or not.  If so, we were invoked as an object method,
and we need to extract the package (class) using the ref() function.
If not, we just use the string passed in as the package name
for blessing our referent.

    sub new {
        my $proto = shift;
        my $class = ref($proto) || $proto;
        my $self  = {};
        $self->{NAME}   = undef;
        $self->{AGE}    = undef;
        $self->{PEERS}  = [];
        bless ($self, $class);
        return $self;
    }

That's about all there is for constructors.  These methods bring objects
to life, returning neat little opaque bundles to the user to be used in
subsequent method calls.

=head2 Destructors

Every story has a beginning and an end.  The beginning of the object's
story is its constructor, explicitly called when the object comes into
existence.  But the ending of its story is the I<destructor>, a method
implicitly called when an object leaves this life.  Any per-object
clean-up code is placed in the destructor, which must (in Perl) be called
DESTROY.

If constructors can have arbitrary names, then why not destructors?
Because while a constructor is explicitly called, a destructor is not.
Destruction happens automatically via Perl's garbage collection (GC)
system, which is a quick but somewhat lazy reference-based GC system.
To know what to call, Perl insists that the destructor be named DESTROY.
Perl's notion of the right time to call a destructor is not well-defined
currently, which is why your destructors should not rely on when they are
called.

Why is DESTROY in all caps?  Perl on occasion uses purely uppercase
function names as a convention to indicate that the function will
be automatically called by Perl in some way.  Others that are called
implicitly include BEGIN, END, AUTOLOAD, plus all methods used by
tied objects, described in L<perltie>.

In really good object-oriented programming languages, the user doesn't
care when the destructor is called.  It just happens when it's supposed
to.  In low-level languages without any GC at all, there's no way to
depend on this happening at the right time, so the programmer must
explicitly call the destructor to clean up memory and state, crossing
their fingers that it's the right time to do so.   Unlike C++, an
object destructor is nearly never needed in Perl, and even when it is,
explicit invocation is uncalled for.  In the case of our Person class,
we don't need a destructor because Perl takes care of simple matters
like memory deallocation.

The only situation where Perl's reference-based GC won't work is
when there's a circularity in the data structure, such as:

    $this->{WHATEVER} = $this;

In that case, you must delete the self-reference manually if you expect
your program not to leak memory.  While admittedly error-prone, this is
the best we can do right now.  Nonetheless, rest assured that when your
program is finished, its objects' destructors are all duly called.
So you are guaranteed that an object I<eventually> gets properly
destroyed, except in the unique case of a program that never exits.
(If you're running Perl embedded in another application, this full GC
pass happens a bit more frequently--whenever a thread shuts down.)

=head2 Other Object Methods

The methods we've talked about so far have either been constructors or
else simple "data methods", interfaces to data stored in the object.
These are a bit like an object's data members in the C++ world, except
that strangers don't access them as data.  Instead, they should only
access the object's data indirectly via its methods.  This is an
important rule: in Perl, access to an object's data should I<only>
be made through methods.

Perl doesn't impose restrictions on who gets to use which methods.
The public-versus-private distinction is by convention, not syntax.
(Well, unless you use the Alias module described below in
L<Data Members as Variables>.)  Occasionally you'll see method names beginning or ending
with an underscore or two.  This marking is a convention indicating
that the methods are private to that class alone and sometimes to its
closest acquaintances, its immediate subclasses.  But this distinction
is not enforced by Perl itself.  It's up to the programmer to behave.

There's no reason to limit methods to those that simply access data.
Methods can do anything at all.  The key point is that they're invoked
against an object or a class.  Let's say we'd like object methods that
do more than fetch or set one particular field.

    sub exclaim {
        my $self = shift;
        return sprintf "Hi, I'm %s, age %d, working with %s",
            $self->{NAME}, $self->{AGE}, join(", ", @{$self->{PEERS}});
    }

Or maybe even one like this:

    sub happy_birthday {
        my $self = shift;
        return ++$self->{AGE};
    }

Some might argue that one should go at these this way:

    sub exclaim {
        my $self = shift;
        return sprintf "Hi, I'm %s, age %d, working with %s",
            $self->name, $self->age, join(", ", $self->peers);