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>.
+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.
}
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:
+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;
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.
+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
my $fred = Critter->find("Fred");
$fred->display("Height", "Weight");
-You should already be familiar with the C<< -> >> operator from using
+You should already be familiar with the use of the C<< -> >> operator with
references. In fact, since C<$fred> above is a reference to an object,
you could think of the method call as just another form of
dereferencing.
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 Perl starts looking in. If
+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 want, you I<can> force Perl to start looking in some other
-package:
+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");
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 current class's C<@ISA>
-list.
+tell Perl to start looking for the method in the packages named in the
+current class's C<@ISA> list.
package MyCritter;
use base 'Critter'; # sets @MyCritter::ISA = ('Critter');
$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:
+
+ 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 even the following:
+and so is the following:
my $fred = (reverse "rettirC")->find(reverse "derF");
=head2 Indirect Object Syntax
-The other way to invoke a method is by using the so-called indirect
-object notation. Already in Perl 4, long before objects were
-introduced, this syntax was used with filehandles like this:
+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";
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 the guess I<at compile time>.
-Usually Perl gets it right, but when it doesn't it, you get a function
-call compiled as a method, or vice versa. This can introduce subtle bugs
-that are hard to unravel.
+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, calling a method C<new> in indirect notation -- as C++
-programmers are so wont to do -- can be miscompiled into a subroutine
+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 if it messes up just isn't worth the
-years of debugging it would likely take you to track such subtle bugs
-down.
+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
C<isa> returns I<true> if its object is blessed into a subclass of C<CLASS>
-C<isa> is also exportable and can be called as a sub with two arguments. This
-allows the ability to check what a reference points to. Example
+You can also call C<UNIVERSAL::isa> as a subroutine with two arguments.
+The first does not need to be an object or even a reference. This
+allows you to check what a reference points to, or whether
+something is a reference of a given type. Example
- use UNIVERSAL qw(isa);
-
- if(isa($ref, 'ARRAY')) {
+ if(UNIVERSAL::isa($ref, 'ARRAY')) {
#...
}
+To determine if a reference is a blessed object, you can write
+
+ print "It's an object\n" if UNIVERSAL::isa($val, 'UNIVERSAL');
+
=item can(METHOD)
C<can> checks to see if its object has a method called C<METHOD>,
if it does then a reference to the sub is returned, if it does not then
I<undef> is returned.
+C<UNIVERSAL::can> can also be called as a subroutine with two arguments.
+It'll always return I<undef> if its first argument isn't an object or a
+class name. So here's another way to check if a reference is a
+blessed object
+
+ print "It's still an object\n" if UNIVERSAL::can($val, 'can');
+
+You can also use the C<blessed> function of Scalar::Util:
+
+ use Scalar::Util 'blessed';
+
+ my $blessing = blessed $suspected_object;
+
+C<blessed> returns the name of the package the argument has been
+blessed into, or C<undef>.
+
=item VERSION( [NEED] )
C<VERSION> returns the version number of the class (package). If the
You may add other methods to the UNIVERSAL class via Perl or XS code.
You do not need to C<use UNIVERSAL> to make these methods
-available to your program. This is necessary only if you wish to
-have C<isa> available as a plain subroutine in the current package.
+available to your program (and you should not do so).
=head2 Destructors
node such as one might use in a sophisticated tree structure:
sub new_node {
- my $self = shift;
- my $class = ref($self) || $self;
- my $node = {};
+ my $class = shift;
+ my $node = {};
$node->{LEFT} = $node->{RIGHT} = $node;
$node->{DATA} = [ @_ ];
return bless $node => $class;
warn "time to die...";
exit;
-When run as F</tmp/test>, the following output is produced:
+When run as F</foo/test>, the following output is produced:
- starting program at /tmp/test line 18.
- CREATING SCALAR(0x8e5b8) at /tmp/test line 7.
- CREATING SCALAR(0x8e57c) at /tmp/test line 7.
- leaving block at /tmp/test line 23.
- DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13.
- just exited block at /tmp/test line 26.
- time to die... at /tmp/test line 27.
+ starting program at /foo/test line 18.
+ CREATING SCALAR(0x8e5b8) at /foo/test line 7.
+ CREATING SCALAR(0x8e57c) at /foo/test line 7.
+ leaving block at /foo/test line 23.
+ DESTROYING Subtle=SCALAR(0x8e5b8) at /foo/test line 13.
+ just exited block at /foo/test line 26.
+ time to die... at /foo/test line 27.
DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.
Notice that "global destruction" bit there? That's the thread
=head1 SEE ALSO
A kinder, gentler tutorial on object-oriented programming in Perl can
-be found in L<perltoot>, L<perlbootc> and L<perltootc>. You should
+be found in L<perltoot>, L<perlboot> and L<perltooc>. You should
also check out L<perlbot> for other object tricks, traps, and tips, as
well as L<perlmodlib> for some style guides on constructing both
modules and classes.
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