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.