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NEXT(3pm)              Perl Programmers Reference Guide              NEXT(3pm)




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:

       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';
               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.



perl v5.24.0                      2015-10-14                         NEXT(3pm)

perl 5.24 - Generated Fri Nov 18 18:50:59 CST 2016
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