perlobj.pod

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=head1 NAME

perlobj - Perl objects

=head1 DESCRIPTION

First you need to understand what references are in Perl.
See L<perlref> for that.  Second, if you still find the following
reference work too complicated, a tutorial on object-oriented programming
in Perl can be found in L<perltoot> and L<perltootc>.

If you're still with us, then
here are three very simple definitions that you should find reassuring.

=over 4

=item 1.

An object is simply a reference that happens to know which class it
belongs to.

=item 2.

A class is simply a package that happens to provide methods to deal
with object references.

=item 3.

A method is simply a subroutine that expects an object reference (or
a package name, for class methods) as the first argument.

=back

We'll cover these points now in more depth.

=head2 An Object is Simply a Reference

Unlike say C++, Perl doesn't provide any special syntax for
constructors.  A constructor is merely a subroutine that returns a
reference to something "blessed" into a class, generally the
class that the subroutine is defined in.  Here is a typical
constructor:

    package Critter;
    sub new { bless {} }

That word C<new> isn't special.  You could have written
a construct this way, too:

    package Critter;
    sub spawn { bless {} }

This might even be preferable, because the C++ programmers won't
be tricked into thinking that C<new> works in Perl as it does in C++.
It doesn't.  We recommend that you name your constructors whatever
makes sense in the context of the problem you're solving.  For example,
constructors in the Tk extension to Perl are named after the widgets
they create.

One thing that's different about Perl constructors compared with those in
C++ is that in Perl, they have to allocate their own memory.  (The other
things is that they don't automatically call overridden base-class
constructors.)  The C<{}> allocates an anonymous hash containing no
key/value pairs, and returns it  The bless() takes that reference and
tells the object it references that it's now a Critter, and returns
the reference.  This is for convenience, because the referenced object
itself knows that it has been blessed, and the reference to it could
have been returned directly, like this:

    sub new {
	my $self = {};
	bless $self;
	return $self;
    }

You often see such a thing in more complicated constructors
that wish to call methods in the class as part of the construction:

    sub new {
	my $self = {};
	bless $self;
	$self->initialize();
	return $self;
    }

If you care about inheritance (and you should; see
L<perlmodlib/"Modules: Creation, Use, and Abuse">),
then you want to use the two-arg form of bless
so that your constructors may be inherited:

    sub new {
	my $class = shift;
	my $self = {};
	bless $self, $class;
	$self->initialize();
	return $self;
    }

Or if you expect people to call not just C<< CLASS->new() >> but also
C<< $obj->new() >>, then use something like this.  The initialize()
method used will be of whatever $class we blessed the
object into:

    sub new {
	my $this = shift;
	my $class = ref($this) || $this;
	my $self = {};
	bless $self, $class;
	$self->initialize();
	return $self;
    }

Within the class package, the methods will typically deal with the
reference as an ordinary reference.  Outside the class package,
the reference is generally treated as an opaque value that may
be accessed only through the class's methods.

Although a constructor can in theory re-bless a referenced object
currently belonging to another class, this is almost certainly going
to get you into trouble.  The new class is responsible for all
cleanup later.  The previous blessing is forgotten, as an object
may belong to only one class at a time.  (Although of course it's
free to inherit methods from many classes.)  If you find yourself
having to do this, the parent class is probably misbehaving, though.

A clarification:  Perl objects are blessed.  References are not.  Objects
know which package they belong to.  References do not.  The bless()
function uses the reference to find the object.  Consider
the following example:

    $a = {};
    $b = $a;
    bless $a, BLAH;
    print "\$b is a ", ref($b), "\n";

This reports $b as being a BLAH, so obviously bless()
operated on the object and not on the reference.

=head2 A Class is Simply a Package

Unlike say C++, Perl doesn't provide any special syntax for class
definitions.  You use a package as a class by putting method
definitions into the class.

There is a special array within each package called @ISA, which says
where else to look for a method if you can't find it in the current
package.  This is how Perl implements inheritance.  Each element of the
@ISA array is just the name of another package that happens to be a
class package.  The classes are searched (depth first) for missing
methods in the order that they occur in @ISA.  The classes accessible
through @ISA are known as base classes of the current class.

All classes implicitly inherit from class C<UNIVERSAL> as their
last base class.  Several commonly used methods are automatically
supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for
more details.

If a missing method is found in a base class, it is cached
in the current class for efficiency.  Changing @ISA or defining new
subroutines invalidates the cache and causes Perl to do the lookup again.

If neither the current class, its named base classes, nor the UNIVERSAL
class contains the requested method, these three places are searched
all over again, this time looking for a method named AUTOLOAD().  If an
AUTOLOAD is found, this method is called on behalf of the missing method,
setting the package global $AUTOLOAD to be the fully qualified name of
the method that was intended to be called.

If none of that works, Perl finally gives up and complains.

If you want to stop the AUTOLOAD inheritance say simply

	sub AUTOLOAD;

and the call will die using the name of the sub being called.

Perl classes do method inheritance only.  Data inheritance is left up
to the class itself.  By and large, this is not a problem in Perl,
because most classes model the attributes of their object using an
anonymous hash, which serves as its own little namespace to be carved up
by the various classes that might want to do something with the object.
The only problem with this is that you can't sure that you aren't using
a piece of the hash that isn't already used.  A reasonable workaround
is to prepend your fieldname in the hash with the package name.

    sub bump {
	my $self = shift;
	$self->{ __PACKAGE__ . ".count"}++;
    } 

=head2 A Method is Simply a Subroutine

Unlike say C++, Perl doesn't provide any special syntax for method
definition.  (It does provide a little syntax for method invocation
though.  More on that later.)  A method expects its first argument
to be the object (reference) or package (string) it is being invoked
on.  There are two ways of calling methods, which we'll call class
methods and instance methods.  

A class method expects a class name as the first argument.  It
provides functionality for the class as a whole, not for any
individual object belonging to the class.  Constructors are often
class methods, but see L<perltoot> and L<perltootc> for alternatives.
Many class methods simply ignore their first argument, because they
already know what package they're in and don't care what package
they were invoked via.  (These aren't necessarily the same, because
class methods follow the inheritance tree just like ordinary instance
methods.)  Another typical use for class methods is to look up an
object by name:

    sub find {
	my ($class, $name) = @_;
	$objtable{$name};
    }

An instance method expects an object reference as its first argument.
Typically it shifts the first argument into a "self" or "this" variable,
and then uses that as an ordinary reference.

    sub display {
	my $self = shift;
	my @keys = @_ ? @_ : sort keys %$self;
	foreach $key (@keys) {
	    print "\t$key => $self->{$key}\n";
	}
    }

=head2 Method Invocation

There are two ways to invoke a method, one of which you're already
familiar with, and the other of which will look familiar.  Perl 4
already had an "indirect object" syntax that you use when you say

    print STDERR "help!!!\n";

This same syntax can be used to call either class or instance methods.
We'll use the two methods defined above, the class method to lookup
an object reference and the instance method to print out its attributes.

    $fred = find Critter "Fred";
    display $fred 'Height', 'Weight';

These could be combined into one statement by using a BLOCK in the
indirect object slot:

    display {find Critter "Fred"} 'Height', 'Weight';

For C++ fans, there's also a syntax using -> notation that does exactly
the same thing.  The parentheses are required if there are any arguments.

    $fred = Critter->find("Fred");
    $fred->display('Height', 'Weight');

or in one statement,

    Critter->find("Fred")->display('Height', 'Weight');

There are times when one syntax is more readable, and times when the
other syntax is more readable.  The indirect object syntax is less
cluttered, but it has the same ambiguity as ordinary list operators.
Indirect object method calls are usually parsed using the same rule as list
operators: "If it looks like a function, it is a function".  (Presuming
for the moment that you think two words in a row can look like a
function name.  C++ programmers seem to think so with some regularity,
especially when the first word is "new".)  Thus, the parentheses of

    new Critter ('Barney', 1.5, 70)

are assumed to surround ALL the arguments of the method call, regardless
of what comes after.  Saying

    new Critter ('Bam' x 2), 1.4, 45

would be equivalent to

    Critter->new('Bam' x 2), 1.4, 45

which is unlikely to do what you want.  Confusingly, however, this
rule applies only when the indirect object is a bareword package name,
not when it's a scalar, a BLOCK, or a C<Package::> qualified package name.
In those cases, the arguments are parsed in the same way as an
indirect object list operator like print, so

    new Critter:: ('Bam' x 2), 1.4, 45