NEXT(3perl)
NAME
NEXT.pm - Provide a pseudo-class NEXT (et al) that allows method redispatch
SYNOPSIS
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
$obj->missing_method(); # Calls D::AUTOLOAD, B::AUTOLOAD, C::AUTOLOAD
# Clean-up calls D::DESTROY, B::DESTROY, A::DESTROY, C::DESTROY
DESCRIPTION
NEXT.pm adds a pseudoclass named "NEXT" to any program that uses it. If
a method "m" calls "$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 acquiescence 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() }
Enforcing redispatch
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").
Avoiding repetitions
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".
Invoking all versions of a method with a single call
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 appropriate 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:
* A hash of array references in list context. Each entry of the hash
has the fully qualified method name as its key and a reference to
an array containing the method's list-context return values as its
value.
* A reference to a hash of scalar values in scalar context. Each
entry of the hash has the fully qualified method name as its key
and the method's scalar-context return values as its value.
* Nothing in void context (obviously).
Using "EVERY" methods
The 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.
AUTHOR
Damian Conway (damian@conway.org)
BUGS AND IRRITATIONS
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
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.
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