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NAME
       NEXT - 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



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       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:











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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
       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:









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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
       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").



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       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,



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       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:

       o   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.

       o   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.

       o   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';



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               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




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        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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