=head1 DESCRIPTION
-First of all, you need to understand what references are in Perl. See
-L<perlref> for that.
+First of all, 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>.
-Here are three very simple definitions that you should find reassuring.
+If you're still with us, then
+here are three very simple definitions that you should find reassuring.
=over 4
The C<{}> constructs a reference to an anonymous hash containing no
key/value pairs. 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, since the referenced object itself knows that
-it has been blessed, and its reference to it could have been returned
+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 {
}
If you care about inheritance (and you should; see
-L<perlmod/"Modules: Creation, Use and Abuse">),
+L<perlmod/"Modules: Creation, Use, and Abuse">),
then you want to use the two-arg form of bless
so that your constructors may be inherited:
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
-only be accessed through the class's methods.
+be accessed only through the class's methods.
A constructor may re-bless a referenced object currently belonging to
another class, but then the new class is responsible for all cleanup
-later. The previous blessing is forgotten, as an object may only
-belong to one class at a time. (Although of course it's free to
+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.)
A clarification: Perl objects are blessed. References are not. Objects
know which package they belong to. References do not. The bless()
-function simply uses the reference in order to find the object. Consider
+function uses the reference to find the object. Consider
the following example:
$a = {};
=head2 A Class is Simply a Package
Unlike say C++, Perl doesn't provide any special syntax for class
-definitions. You just use a package as a class by putting method
+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
more details.) If that doesn't work, Perl finally gives up and
complains.
-Perl classes only do method inheritance. Data inheritance is left
+Perl classes do only method inheritance. 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
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 typically class
-methods. Many class methods simply ignore their first argument, since
+methods. 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, since
+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:
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 parens of
+especially when the first word is "new".) Thus, the parentheses of
new Critter ('Barney', 1.5, 70)
$fred = MyCritter::find("Critter", "Fred");
MyCritter::display($fred, 'Height', 'Weight');
-Note however, that this does not do any inheritance. If you merely
-wish to specify that Perl should I<START> looking for a method in a
+Note however, that this does not do any inheritance. If you wish
+merely to specify that Perl should I<START> looking for a method in a
particular package, use an ordinary method call, but qualify the method
name with the package like this:
$fred->MyCritter::display('Height', 'Weight');
If you're trying to control where the method search begins I<and> you're
-executing in the class itself, then you may use the SUPER pseudoclass,
+executing in the class itself, then you may use the SUPER pseudo class,
which says to start looking in your base class's @ISA list without having
-to explicitly name it:
+to name it explicitly:
$self->SUPER::display('Height', 'Weight');
-Please note that the C<SUPER::> construct is I<only> meaningful within the
+Please note that the C<SUPER::> construct is meaningful I<only> within the
class.
Sometimes you want to call a method when you don't know the method name
=over 4
-=item isa ( CLASS )
+=item isa(CLASS)
-C<isa> returns I<true> if its object is blessed into a sub-class of C<CLASS>
+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
...
}
-=item can ( METHOD )
+=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.
-=item VERSION ( [ VERSION ] )
-
-C<VERSION> returns the VERSION number of the class (package). If
-an argument is given then it will check that the current version is not
-less that the given argument. This method is normally called as a class
-method. This method is also called when the C<VERSION> form of C<use> is
-used.
+=item VERSION( [NEED] )
+C<VERSION> returns the version number of the class (package). If the
+NEED argument is given then it will check that the current version (as
+defined by the $VERSION variable in the given package) not less than
+NEED; it will die if this is not the case. This method is normally
+called as a class method. This method is called automatically by the
+C<VERSION> form of C<use>.
use A 1.2 qw(some imported subs);
-
- A->require_version( 1.2 );
-
-=item class ()
-
-C<class> returns the class name of its object.
-
-=item is_instance ()
-
-C<is_instance> returns true if its object is an instance of some
-class, false if its object is the class (package) itself. Example
-
- A->is_instance(); # False
-
- $var = 'A';
- $var->is_instance(); # False
-
- $ref = bless [], 'A';
- $ref->is_instance(); # True
+ # implies:
+ A->VERSION(1.2);
=back
strange effects if the Perl code dynamically changes @ISA in any package.
You may add other methods to the UNIVERSAL class via Perl or XS code.
+You do not need to C<use UNIVERSAL> in order 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.
=head2 Destructors
and you can do any extra cleanup you need to do.
Perl doesn't do nested destruction for you. If your constructor
-reblessed a reference from one of your base classes, your DESTROY may
-need to call DESTROY for any base classes that need it. But this only
-applies to reblessed objects--an object reference that is merely
+re-blessed a reference from one of your base classes, your DESTROY may
+need to call DESTROY for any base classes that need it. But this applies
+to only re-blessed objects--an object reference that is merely
I<CONTAINED> in the current object will be freed and destroyed
automatically when the current object is freed.
other postfix dereference in the language. The left side of -E<gt> is not so
limited, because it's an infix operator, not a postfix operator.
-That means that below, A and B are equivalent to each other, and C and D
-are equivalent, but AB and CD are different:
+That means that in the following, A and B are equivalent to each other, and
+C and D are equivalent, but A/B and C/D are different:
A: method $obref->{"fieldname"}
B: (method $obref)->{"fieldname"}
=head2 Summary
-That's about all there is to it. Now you just need to go off and buy a
+That's about all there is to it. Now you need just to go off and buy a
book about object-oriented design methodology, and bang your forehead
with it for the next six months or so.
exits), then a rather costly but complete mark-and-sweep style of garbage
collection is performed, and everything allocated by that thread gets
destroyed. This is essential to support Perl as an embedded or a
-multithreadable language. For example, this program demonstrates Perl's
+multi-threadable language. For example, this program demonstrates Perl's
two-phased garbage collection:
#!/usr/bin/perl
Objects are always destructed, even when regular refs aren't and in fact
are destructed in a separate pass before ordinary refs just to try to
prevent object destructors from using refs that have been themselves
-destructed. Plain refs are only garbage collected if the destruct level
+destructed. Plain refs are only garbage-collected if the destruct level
is greater than 0. You can test the higher levels of global destruction
by setting the PERL_DESTRUCT_LEVEL environment variable, presuming
C<-DDEBUGGING> was enabled during perl build time.
=head1 SEE ALSO
+A kinder, gentler tutorial on object-oriented programming in Perl can
+be found in L<perltoot>.
You should also check out L<perlbot> for other object tricks, traps, and tips,
as well as L<perlmod> for some style guides on constructing both modules
and classes.
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