source: trunk/essentials/dev-lang/perl/pod/perlobj.pod@ 3368

=head1 NAME
X<object> X<OOP>

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<perltooc>.

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
X<object> X<bless> X<constructor> X<new>

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 the following.  (Note that using
this to call new() on an instance does not automatically perform any
copying.  If you want a shallow or deep copy of an object, you'll have to
specifically allow for that.)  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
X<class> X<package> X<@ISA> X<inheritance>

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.
X<UNIVERSAL> X<base class> X<class, base>

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.
X<AUTOLOAD>

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

If you want to stop the AUTOLOAD inheritance say simply
X<AUTOLOAD>

        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.
X<inheritance, method> X<inheritance, data>

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

=head2 A Method is Simply a Subroutine
X<method>

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<perltooc> 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
X<invocation> X<method> X<arrow> X<< -> >>

For various historical and other reasons, Perl offers two equivalent
ways to write a method call.  The simpler and more common way is to use
the arrow notation:

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

You should already be familiar with the use of the C<< -> >> operator with
references.  In fact, since C<$fred> above is a reference to an object,
you could think of the method call as just another form of
dereferencing.

Whatever is on the left side of the arrow, whether a reference or a
class name, is passed to the method subroutine as its first argument.
So the above code is mostly equivalent to:

    my $fred = Critter::find("Critter", "Fred");
    Critter::display($fred, "Height", "Weight");

How does Perl know which package the subroutine is in?  By looking at
the left side of the arrow, which must be either a package name or a
reference to an object, i.e. something that has been blessed to a
package.  Either way, that's the package where Perl starts looking.  If
that package has no subroutine with that name, Perl starts looking for
it in any base classes of that package, and so on.

If you need to, you I<can> force Perl to start looking in some other package:

    my $barney = MyCritter->Critter::find("Barney");
    $barney->Critter::display("Height", "Weight");

Here C<MyCritter> is presumably a subclass of C<Critter> that defines
its own versions of find() and display().  We haven't specified what
those methods do, but that doesn't matter above since we've forced Perl
to start looking for the subroutines in C<Critter>.

As a special case of the above, you may use the C<SUPER> pseudo-class to
tell Perl to start looking for the method in the packages named in the
current class's C<@ISA> list.  
X<SUPER>

    package MyCritter;
    use base 'Critter';    # sets @MyCritter::ISA = ('Critter');

    sub display { 
        my ($self, @args) = @_;
        $self->SUPER::display("Name", @args);
    }

It is important to note that C<SUPER> refers to the superclass(es) of the
I<current package> and not to the superclass(es) of the object. Also, the
C<SUPER> pseudo-class can only currently be used as a modifier to a method
name, but not in any of the other ways that class names are normally used,
eg:
X<SUPER>

    something->SUPER::method(...);      # OK
    SUPER::method(...);                 # WRONG
    SUPER->method(...);                 # WRONG

Instead of a class name or an object reference, you can also use any
expression that returns either of those on the left side of the arrow.
So the following statement is valid:

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

and so is the following:

    my $fred = (reverse "rettirC")->find(reverse "derF");

The right side of the arrow typically is the method name, but a simple 
scalar variable containing either the method name or a subroutine 
reference can also be used.

=head2 Indirect Object Syntax
X<indirect object syntax> X<invocation, indirect> X<indirect>

The other way to invoke a method is by using the so-called "indirect
object" notation.  This syntax was available in Perl 4 long before
objects were introduced, and is still used with filehandles like this:

   print STDERR "help!!!\n";

The same syntax can be used to call either object or class methods.

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

Notice that there is no comma between the object or class name and the
parameters.  This is how Perl can tell you want an indirect method call
instead of an ordinary subroutine call.

But what if there are no arguments?  In that case, Perl must guess what
you want.  Even worse, it must make that guess I<at compile time>.
Usually Perl gets it right, but when it doesn't you get a function
call compiled as a method, or vice versa.  This can introduce subtle bugs
that are hard to detect.

For example, a call to a method C<new> in indirect notation -- as C++
programmers are wont to make -- can be miscompiled into a subroutine
call if there's already a C<new> function in scope.  You'd end up
calling the current package's C<new> as a subroutine, rather than the
desired class's method.  The compiler tries to cheat by remembering
bareword C<require>s, but the grief when it messes up just isn't worth the
years of debugging it will take you to track down such subtle bugs.

There is another problem with this syntax: the indirect object is
limited to a name, a scalar variable, or a block, because it would have
to do too much lookahead otherwise, just like any other postfix
dereference in the language.  (These are the same quirky rules as are
used for the filehandle slot in functions like C<print> and C<printf>.)
This can lead to horribly confusing precedence problems, as in these
next two lines:

    move $obj->{FIELD};                 # probably wrong!
    move $ary[$i];                      # probably wrong!

Those actually parse as the very surprising:

    $obj->move->{FIELD};                # Well, lookee here
    $ary->move([$i]);                   # Didn't expect this one, eh?

Rather than what you might have expected:

    $obj->{FIELD}->move();              # You should be so lucky.
    $ary[$i]->move;                     # Yeah, sure.

To get the correct behavior with indirect object syntax, you would have
to use a block around the indirect object:

    move {$obj->{FIELD}};
    move {$ary[$i]};

Even then, you still have the same potential problem if there happens to
be a function named C<move> in the current package.  B<The C<< -> >>
notation suffers from neither of these disturbing ambiguities, so we
recommend you use it exclusively.>  However, you may still end up having
to read code using the indirect object notation, so it's important to be
familiar with it.

=head2 Default UNIVERSAL methods
X<UNIVERSAL>

The C<UNIVERSAL> package automatically contains the following methods that
are inherited by all other classes:

=over 4

=item isa(CLASS)
X<isa>

C<isa> returns I<true> if its object is blessed into a subclass of C<CLASS>

You can also call C<UNIVERSAL::isa> as a subroutine with two arguments.  Of
course, this will do the wrong thing if someone has overridden C<isa> in a
class, so don't do it.

If you need to determine whether you've received a valid invocant, use the
C<blessed> function from L<Scalar::Util>:
X<invocant> X<blessed>

    if (blessed($ref) && $ref->isa( 'Some::Class')) {
        # ...
    }

C<blessed> returns the name of the package the argument has been