=head1 NAME
+X<object> X<OOP>
perlobj - Perl objects
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
}
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;
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
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
+class package. The classes are searched for missing methods in
+depth-first, left-to-right order by default (see L<mro> for alternative
+search order and other in-depth information). 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.
+supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> or
+L<UNIVERSAL|UNIVERSAL> 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
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;
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;
}
=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
}
=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
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');
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
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.
+
+If the right side of the arrow is a scalar containing a reference
+to a subroutine, then this is equivalent to calling the referenced
+subroutine directly with the class name or object on the left side
+of the arrow as its first argument. No lookup is done and there is
+no requirement that the subroutine be defined in any package related
+to the class name or object on the left side of the arrow.
+
+For example, the following calls to $display are equivalent:
+
+ my $display = sub { my $self = shift; ... };
+ $fred->$display("Height", "Weight");
+ $display->($fred, "Height", "Weight");
+
=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
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.
-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
-
- if(UNIVERSAL::isa($ref, 'ARRAY')) {
- #...
- }
-
-To determine if a reference is a blessed object, you can write
+=item DOES(ROLE)
+X<DOES>
- print "It's an object\n" if UNIVERSAL::isa($val, 'UNIVERSAL');
+C<DOES> returns I<true> if its object claims to perform the role C<ROLE>. By
+default, this is equivalent to C<isa>.
=item can(METHOD)
+X<can>
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>.
+C<undef> is returned.
=item VERSION( [NEED] )
+X<VERSION>
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 $VERSION variable in the given package) not less than
-NEED; it will die if this is not the case. This method is normally
-called as a class method. This method is called automatically by the
-C<VERSION> form of C<use>.
+NEED; it will die if this is not the case. This method is called automatically
+by the C<VERSION> form of C<use>.
- use A 1.2 qw(some imported subs);
+ use Package 1.2 qw(some imported subs);
# implies:
- A->VERSION(1.2);
+ Package->VERSION(1.2);
=back
-B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
-C<isa> uses a very similar method and cache-ing strategy. This may cause
-strange effects if the Perl code dynamically changes @ISA in any package.
-
-You may add other methods to the UNIVERSAL class via Perl or XS code.
-You do not need to C<use UNIVERSAL> to make these methods
-available to your program (and you should not do so).
-
=head2 Destructors
+X<destructor> X<DESTROY>
When the last reference to an object goes away, the object is
automatically destroyed. (This may even be after you exit, if you've
the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>,
C<%{$_[0]}> etc.) is not similarly constrained.
+Since DESTROY methods can be called at unpredictable times, it is
+important that you localise any global variables that the method may
+update. In particular, localise C<$@> if you use C<eval {}> and
+localise C<$?> if you use C<system> or backticks.
+
If you arrange to re-bless the reference before the destructor returns,
perl will again call the DESTROY method for the re-blessed object after
the current one returns. This can be used for clean delegation of
of your choosing get called. Explicitly calling DESTROY is also possible,
but is usually never needed.
+DESTROY is subject to AUTOLOAD lookup, just like any other method. Hence, if
+your class has an AUTOLOAD method, but does not need any DESTROY actions,
+you probably want to provide a DESTROY method anyway, to prevent an
+expensive call to AUTOLOAD each time an object is freed. As this technique
+makes empty DESTROY methods common, the implementation is optimised so that
+a DESTROY method that is an empty or constant subroutine, and hence could
+have no side effects anyway, is not actually called.
+X<AUTOLOAD> X<DESTROY>
+
Do not confuse the previous discussion with how objects I<CONTAINED> in the current
one are destroyed. Such objects will be freed and destroyed automatically
when the current object is freed, provided no other references to them exist
with it for the next six months or so.
=head2 Two-Phased Garbage Collection
+X<garbage collection> X<GC> X<circular reference>
+X<reference, circular> X<DESTROY> X<destructor>
For most purposes, Perl uses a fast and simple, reference-based
garbage collection system. That means there's an extra
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;
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