NEXT(3p) Perl Programmers Reference Guide NEXT(3p)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 ori-
ginally 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.
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A typical use would be in the destructors of a class hierar-
chy, 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 des-
troyed, 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() }
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:
$self->NEXT::method();
The "ACTUAL" tells "NEXT" that there must actually be a next
method to call, or it should throw an exception.
"NEXT::ACTUAL" is most commonly used in "AUTOLOAD" methods,
as a means to decline an "AUTOLOAD" request, but preserve
the normal exception-on-failure semantics:
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sub AUTOLOAD {
if ($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 usu-
ally happens in the absence of a suitable "AUTOLOAD").
Avoiding repetitions
If "NEXT" redispatching is used in the methods of a "dia-
mond" 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:
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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 dia-
mond 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
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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".
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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. How-
ever, 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 "ances-
tors"). 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 con-
text. 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 actu-
ally called (as opposed to just the left-most-depth-first-
est one):
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package Base;
sub DESTROY { $_[0]->EVERY::Destroy }
package Derived1;
use base 'Base';
sub Destroy {...}
package Derived2;
use base 'Base', 'Derived1';
sub Destroy {...}
et cetera. Every derived class than needs its own clean-up
behaviour simply adds its own "Destroy" method (not a "DES-
TROY" method), which the call to "EVERY::LAST::Destroy" in
the inherited destructor then correctly picks up.
Likewise, to create a class hierarchy in which every ini-
tializer inherited by a new object is invoked:
package Base;
sub new {
my ($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 inter-
preter, NEXT.pm has to guess where the surrounding call was
found in the method look-up sequence. In the presence of
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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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