| NEXT.pm - Provide a pseudo-class NEXT that allows method redispatch |
EVERY methods
NEXT.pm - Provide a pseudo-class NEXT (et al) that allows method redispatch
use NEXT;
package A;
sub A::method { print "$_[0]: A method\n"; $_[0]->NEXT::method() }
sub A::DESTROY { print "$_[0]: A dtor\n"; $_[0]->NEXT::DESTROY() }
package B;
use base qw( A );
sub B::AUTOLOAD { print "$_[0]: B AUTOLOAD\n"; $_[0]->NEXT::AUTOLOAD() }
sub B::DESTROY { print "$_[0]: B dtor\n"; $_[0]->NEXT::DESTROY() }
package C;
sub C::method { print "$_[0]: C method\n"; $_[0]->NEXT::method() }
sub C::AUTOLOAD { print "$_[0]: C AUTOLOAD\n"; $_[0]->NEXT::AUTOLOAD() }
sub C::DESTROY { print "$_[0]: C dtor\n"; $_[0]->NEXT::DESTROY() }
package D;
use base qw( B C );
sub D::method { print "$_[0]: D method\n"; $_[0]->NEXT::method() }
sub D::AUTOLOAD { print "$_[0]: D AUTOLOAD\n"; $_[0]->NEXT::AUTOLOAD() }
sub D::DESTROY { print "$_[0]: D dtor\n"; $_[0]->NEXT::DESTROY() }
package main;
my $obj = bless {}, "D";
$obj->method(); # Calls D::method, A::method, C::method
$obj->missing_method(); # Calls D::AUTOLOAD, B::AUTOLOAD, C::AUTOLOAD
# Clean-up calls D::DESTROY, B::DESTROY, A::DESTROY, C::DESTROY
NEXT.pm adds a pseudoclass named NEXT to any program
that uses it. If a method m calls $self->NEXT::m(), the call to
m is redispatched as if the calling method had not originally been found.
In other words, a call to $self->NEXT::m() resumes the depth-first,
left-to-right search of $self's class hierarchy that resulted in the
original call to m.
Note that this is not the same thing as $self->SUPER::m(), which
begins a new dispatch that is restricted to searching the ancestors
of the current class. $self->NEXT::m() can backtrack
past the current class -- to look for a suitable method in other
ancestors of $self -- whereas $self->SUPER::m() cannot.
A typical use would be in the destructors of a class hierarchy, as illustrated in the synopsis above. Each class in the hierarchy has a DESTROY method that performs some class-specific action and then redispatches the call up the hierarchy. As a result, when an object of class D is destroyed, the destructors of all its parent classes are called (in depth-first, left-to-right order).
Another typical use of redispatch would be in AUTOLOAD'ed methods.
If such a method determined that it was not able to handle a
particular call, it might choose to redispatch that call, in the
hope that some other AUTOLOAD (above it, or to its left) might
do better.
By default, if a redispatch attempt fails to find another method
elsewhere in the objects class hierarchy, it quietly gives up and does
nothing (but see Enforcing redispatch). This gracious acquiesence
is also unlike the (generally annoying) behaviour of SUPER, which
throws an exception if it cannot redispatch.
Note that it is a fatal error for any method (including AUTOLOAD)
to attempt to redispatch any method that does not have the
same name. For example:
sub D::oops { print "oops!\n"; $_[0]->NEXT::other_method() }
It is possible to make NEXT redispatch more demandingly (i.e. like
SUPER does), so that the redispatch throws an exception if it cannot
find a ``next'' method to call.
To do this, simple invoke the redispatch as:
$self->NEXT::ACTUAL::method();
rather than:
$self->NEXT::method();
The ACTUAL tells NEXT that there must actually be a next method to call,
or it should throw an exception.
NEXT::ACTUAL is most commonly used in AUTOLOAD methods, as a means to
decline an AUTOLOAD request, but preserve the normal exception-on-failure
semantics:
sub AUTOLOAD {
if ($AUTOLOAD =~ /foo|bar/) {
# handle here
}
else { # try elsewhere
shift()->NEXT::ACTUAL::AUTOLOAD(@_);
}
}
By using NEXT::ACTUAL, if there is no other AUTOLOAD to handle the
method call, an exception will be thrown (as usually happens in the absence of
a suitable AUTOLOAD).
If NEXT redispatching is used in the methods of a ``diamond'' class hierarchy:
# A B
# / \ /
# C D
# \ /
# E
use NEXT;
package A;
sub foo { print "called A::foo\n"; shift->NEXT::foo() }
package B;
sub foo { print "called B::foo\n"; shift->NEXT::foo() }
package C; @ISA = qw( A );
sub foo { print "called C::foo\n"; shift->NEXT::foo() }
package D; @ISA = qw(A B);
sub foo { print "called D::foo\n"; shift->NEXT::foo() }
package E; @ISA = qw(C D);
sub foo { print "called E::foo\n"; shift->NEXT::foo() }
E->foo();
then derived classes may (re-)inherit base-class methods through two or
more distinct paths (e.g. in the way E inherits A::foo twice --
through C and D). In such cases, a sequence of NEXT redispatches
will invoke the multiply inherited method as many times as it is
inherited. For example, the above code prints:
called E::foo
called C::foo
called A::foo
called D::foo
called A::foo
called B::foo
(i.e. A::foo is called twice).
In some cases this may be the desired effect within a diamond hierarchy, but in others (e.g. for destructors) it may be more appropriate to call each method only once during a sequence of redispatches.
To cover such cases, you can redispatch methods via:
$self->NEXT::DISTINCT::method();
rather than:
$self->NEXT::method();
This causes the redispatcher to only visit each distinct method method
once. That is, to skip any classes in the hierarchy that it has
already visited during redispatch. So, for example, if the
previous example were rewritten:
package A;
sub foo { print "called A::foo\n"; shift->NEXT::DISTINCT::foo() }
package B;
sub foo { print "called B::foo\n"; shift->NEXT::DISTINCT::foo() }
package C; @ISA = qw( A );
sub foo { print "called C::foo\n"; shift->NEXT::DISTINCT::foo() }
package D; @ISA = qw(A B);
sub foo { print "called D::foo\n"; shift->NEXT::DISTINCT::foo() }
package E; @ISA = qw(C D);
sub foo { print "called E::foo\n"; shift->NEXT::DISTINCT::foo() }
E->foo();
then it would print:
called E::foo
called C::foo
called A::foo
called D::foo
called B::foo
and omit the second call to A::foo (since it would not be distinct
from the first call to A::foo).
Note that you can also use:
$self->NEXT::DISTINCT::ACTUAL::method();
or:
$self->NEXT::ACTUAL::DISTINCT::method();
to get both unique invocation and exception-on-failure.
Note that, for historical compatibility, you can also use
NEXT::UNSEEN instead of NEXT::DISTINCT.
Yet another pseudo-class that NEXT.pm provides is EVERY.
Its behaviour is considerably simpler than that of the NEXT family.
A call to:
$obj->EVERY::foo();
calls every method named foo that the object in $obj has inherited.
That is:
use NEXT;
package A; @ISA = qw(B D X);
sub foo { print "A::foo " }
package B; @ISA = qw(D X);
sub foo { print "B::foo " }
package X; @ISA = qw(D);
sub foo { print "X::foo " }
package D;
sub foo { print "D::foo " }
package main;
my $obj = bless {}, 'A';
$obj->EVERY::foo(); # prints" A::foo B::foo X::foo D::foo
Prefixing a method call with EVERY:: causes every method in the
object's hierarchy with that name to be invoked. As the above example
illustrates, they are not called in Perl's usual ``left-most-depth-first''
order. Instead, they are called ``breadth-first-dependency-wise''.
That means that the inheritance tree of the object is traversed breadth-first
and the resulting order of classes is used as the sequence in which methods
are called. However, that sequence is modified by imposing a rule that the
appropritae method of a derived class must be called before the same method of
any ancestral class. That's why, in the above example, X::foo is called
before D::foo, even though D comes before X in @B::ISA.
In general, there's no need to worry about the order of calls. They will be
left-to-right, breadth-first, most-derived-first. This works perfectly for
most inherited methods (including destructors), but is inappropriate for
some kinds of methods (such as constructors, cloners, debuggers, and
initializers) where it's more appropriate that the least-derived methods be
called first (as more-derived methods may rely on the behaviour of their
``ancestors''). In that case, instead of using the EVERY pseudo-class:
$obj->EVERY::foo(); # prints" A::foo B::foo X::foo D::foo
you can use the EVERY::LAST pseudo-class:
$obj->EVERY::LAST::foo(); # prints" D::foo X::foo B::foo A::foo
which reverses the order of method call.
Whichever version is used, the actual methods are called in the same
context (list, scalar, or void) as the original call via EVERY, and return:
EVERY methodsThe typical way to use an EVERY call is to wrap it in another base
method, that all classes inherit. For example, to ensure that every
destructor an object inherits is actually called (as opposed to just the
left-most-depth-first-est one):
package Base;
sub DESTROY { $_[0]->EVERY::Destroy }
package Derived1;
use base 'Base';
sub Destroy {...}
package Derived2;
use base 'Base', 'Derived1';
sub Destroy {...}
et cetera. Every derived class than needs its own clean-up
behaviour simply adds its own Destroy method (not a DESTROY method),
which the call to EVERY::LAST::Destroy in the inherited destructor
then correctly picks up.
Likewise, to create a class hierarchy in which every initializer inherited by a new object is invoked:
package Base;
sub new {
my ($class, %args) = @_;
my $obj = bless {}, $class;
$obj->EVERY::LAST::Init(\%args);
}
package Derived1;
use base 'Base';
sub Init {
my ($argsref) = @_;
...
}
package Derived2;
use base 'Base', 'Derived1';
sub Init {
my ($argsref) = @_;
...
}
et cetera. Every derived class than needs some additional initialization
behaviour simply adds its own Init method (not a new method),
which the call to EVERY::LAST::Init in the inherited constructor
then correctly picks up.
Damian Conway (damian@conway.org)
Because it's a module, not an integral part of the interpreter, NEXT.pm has to guess where the surrounding call was found in the method look-up sequence. In the presence of diamond inheritance patterns it occasionally guesses wrong.
It's also too slow (despite caching).
Comment, suggestions, and patches welcome.
Copyright (c) 2000-2001, Damian Conway. All Rights Reserved.
This module is free software. It may be used, redistributed
and/or modified under the same terms as Perl itself.
| NEXT.pm - Provide a pseudo-class NEXT that allows method redispatch |