.\" Automatically generated by Pod::Man v1.34, Pod::Parser v1.13
.\"
.\" Standard preamble:
.\" ========================================================================
.de Sh \" Subsection heading
.br
.if t .Sp
.ne 5
.PP
\fB\\$1\fR
.PP
..
.de Sp \" Vertical space (when we can't use .PP)
.if t .sp .5v
.if n .sp
..
.de Vb \" Begin verbatim text
.ft CW
.nf
.ne \\$1
..
.de Ve \" End verbatim text
.ft R
.fi
..
.\" Set up some character translations and predefined strings.  \*(-- will
.\" give an unbreakable dash, \*(PI will give pi, \*(L" will give a left
.\" double quote, and \*(R" will give a right double quote.  | will give a
.\" real vertical bar.  \*(C+ will give a nicer C++.  Capital omega is used to
.\" do unbreakable dashes and therefore won't be available.  \*(C` and \*(C'
.\" expand to `' in nroff, nothing in troff, for use with C<>.
.tr \(*W-|\(bv\*(Tr
.ds C+ C\v'-.1v'\h'-1p'\s-2+\h'-1p'+\s0\v'.1v'\h'-1p'
.ie n \{\
.    ds -- \(*W-
.    ds PI pi
.    if (\n(.H=4u)&(1m=24u) .ds -- \(*W\h'-12u'\(*W\h'-12u'-\" diablo 10 pitch
.    if (\n(.H=4u)&(1m=20u) .ds -- \(*W\h'-12u'\(*W\h'-8u'-\"  diablo 12 pitch
.    ds L" ""
.    ds R" ""
.    ds C` ""
.    ds C' ""
'br\}
.el\{\
.    ds -- \|\(em\|
.    ds PI \(*p
.    ds L" ``
.    ds R" ''
'br\}
.\"
.\" If the F register is turned on, we'll generate index entries on stderr for
.\" titles (.TH), headers (.SH), subsections (.Sh), items (.Ip), and index
.\" entries marked with X<> in POD.  Of course, you'll have to process the
.\" output yourself in some meaningful fashion.
.if \nF \{\
.    de IX
.    tm Index:\\$1\t\\n%\t"\\$2"
..
.    nr % 0
.    rr F
.\}
.\"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.hy 0
.if n .na
.\"
.\" Accent mark definitions (@(#)ms.acc 1.5 88/02/08 SMI; from UCB 4.2).
.\" Fear.  Run.  Save yourself.  No user-serviceable parts.
.    \" fudge factors for nroff and troff
.if n \{\
.    ds #H 0
.    ds #V .8m
.    ds #F .3m
.    ds #[ \f1
.    ds #] \fP
.\}
.if t \{\
.    ds #H ((1u-(\\\\n(.fu%2u))*.13m)
.    ds #V .6m
.    ds #F 0
.    ds #[ \&
.    ds #] \&
.\}
.    \" simple accents for nroff and troff
.if n \{\
.    ds ' \&
.    ds ` \&
.    ds ^ \&
.    ds , \&
.    ds ~ ~
.    ds /
.\}
.if t \{\
.    ds ' \\k:\h'-(\\n(.wu*8/10-\*(#H)'\'\h"|\\n:u"
.    ds ` \\k:\h'-(\\n(.wu*8/10-\*(#H)'\`\h'|\\n:u'
.    ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'^\h'|\\n:u'
.    ds , \\k:\h'-(\\n(.wu*8/10)',\h'|\\n:u'
.    ds ~ \\k:\h'-(\\n(.wu-\*(#H-.1m)'~\h'|\\n:u'
.    ds / \\k:\h'-(\\n(.wu*8/10-\*(#H)'\z\(sl\h'|\\n:u'
.\}
.    \" troff and (daisy-wheel) nroff accents
.ds : \\k:\h'-(\\n(.wu*8/10-\*(#H+.1m+\*(#F)'\v'-\*(#V'\z.\h'.2m+\*(#F'.\h'|\\n:u'\v'\*(#V'
.ds 8 \h'\*(#H'\(*b\h'-\*(#H'
.ds o \\k:\h'-(\\n(.wu+\w'\(de'u-\*(#H)/2u'\v'-.3n'\*(#[\z\(de\v'.3n'\h'|\\n:u'\*(#]
.ds d- \h'\*(#H'\(pd\h'-\w'~'u'\v'-.25m'\f2\(hy\fP\v'.25m'\h'-\*(#H'
.ds D- D\\k:\h'-\w'D'u'\v'-.11m'\z\(hy\v'.11m'\h'|\\n:u'
.ds th \*(#[\v'.3m'\s+1I\s-1\v'-.3m'\h'-(\w'I'u*2/3)'\s-1o\s+1\*(#]
.ds Th \*(#[\s+2I\s-2\h'-\w'I'u*3/5'\v'-.3m'o\v'.3m'\*(#]
.ds ae a\h'-(\w'a'u*4/10)'e
.ds Ae A\h'-(\w'A'u*4/10)'E
.    \" corrections for vroff
.if v .ds ~ \\k:\h'-(\\n(.wu*9/10-\*(#H)'\s-2\u~\d\s+2\h'|\\n:u'
.if v .ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'\v'-.4m'^\v'.4m'\h'|\\n:u'
.    \" for low resolution devices (crt and lpr)
.if \n(.H>23 .if \n(.V>19 \
\{\
.    ds : e
.    ds 8 ss
.    ds o a
.    ds d- d\h'-1'\(ga
.    ds D- D\h'-1'\(hy
.    ds th \o'bp'
.    ds Th \o'LP'
.    ds ae ae
.    ds Ae AE
.\}
.rm #[ #] #H #V #F C
.\" ========================================================================
.\"
.IX Title "NEXT 3"
.TH NEXT 3 "2002-11-24" "perl v5.8.0" "Perl Programmers Reference Guide"
.SH "NAME"
NEXT.pm \- Provide a pseudo\-class NEXT that allows method redispatch
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use NEXT;
.Ve
.PP
.Vb 3
\&    package A;
\&    sub A::method   { print "$_[0]: A method\en";   $_[0]->NEXT::method() }
\&    sub A::DESTROY  { print "$_[0]: A dtor\en";     $_[0]->NEXT::DESTROY() }
.Ve
.PP
.Vb 4
\&    package B;
\&    use base qw( A );
\&    sub B::AUTOLOAD { print "$_[0]: B AUTOLOAD\en"; $_[0]->NEXT::AUTOLOAD() }
\&    sub B::DESTROY  { print "$_[0]: B dtor\en";     $_[0]->NEXT::DESTROY() }
.Ve
.PP
.Vb 4
\&    package C;
\&    sub C::method   { print "$_[0]: C method\en";   $_[0]->NEXT::method() }
\&    sub C::AUTOLOAD { print "$_[0]: C AUTOLOAD\en"; $_[0]->NEXT::AUTOLOAD() }
\&    sub C::DESTROY  { print "$_[0]: C dtor\en";     $_[0]->NEXT::DESTROY() }
.Ve
.PP
.Vb 5
\&    package D;
\&    use base qw( B C );
\&    sub D::method   { print "$_[0]: D method\en";   $_[0]->NEXT::method() }
\&    sub D::AUTOLOAD { print "$_[0]: D AUTOLOAD\en"; $_[0]->NEXT::AUTOLOAD() }
\&    sub D::DESTROY  { print "$_[0]: D dtor\en";     $_[0]->NEXT::DESTROY() }
.Ve
.PP
.Vb 1
\&    package main;
.Ve
.PP
.Vb 1
\&    my $obj = bless {}, "D";
.Ve
.PP
.Vb 2
\&    $obj->method();             # Calls D::method, A::method, C::method
\&    $obj->missing_method(); # Calls D::AUTOLOAD, B::AUTOLOAD, C::AUTOLOAD
.Ve
.PP
.Vb 1
\&    # Clean-up calls D::DESTROY, B::DESTROY, A::DESTROY, C::DESTROY
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\s-1NEXT\s0.pm adds a pseudoclass named \f(CW\*(C`NEXT\*(C'\fR to any program
that uses it. If a method \f(CW\*(C`m\*(C'\fR calls \f(CW\*(C`$self\-\*(C'\fR\fINEXT::m()\fR>, the call to
\&\f(CW\*(C`m\*(C'\fR is redispatched as if the calling method had not originally been found.
.PP
In other words, a call to \f(CW\*(C`$self\-\*(C'\fR\fINEXT::m()\fR> resumes the depth\-first,
left-to-right search of \f(CW$self\fR's class hierarchy that resulted in the
original call to \f(CW\*(C`m\*(C'\fR.
.PP
Note that this is not the same thing as \f(CW\*(C`$self\-\*(C'\fR\fISUPER::m()\fR>, which 
begins a new dispatch that is restricted to searching the ancestors
of the current class. \f(CW\*(C`$self\-\*(C'\fR\fINEXT::m()\fR> can backtrack
past the current class \*(-- to look for a suitable method in other
ancestors of \f(CW$self\fR \*(-- whereas \f(CW\*(C`$self\-\*(C'\fR\fISUPER::m()\fR> cannot.
.PP
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 \s-1DESTROY\s0 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 \fIall\fR
its parent classes are called (in depth\-first, left-to-right order).
.PP
Another typical use of redispatch would be in \f(CW\*(C`AUTOLOAD\*(C'\fR'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 \f(CW\*(C`AUTOLOAD\*(C'\fR (above it, or to its left) might
do better.
.PP
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 \*(L"Enforcing redispatch\*(R"). This gracious acquiesence
is also unlike the (generally annoying) behaviour of \f(CW\*(C`SUPER\*(C'\fR, which
throws an exception if it cannot redispatch.
.PP
Note that it is a fatal error for any method (including \f(CW\*(C`AUTOLOAD\*(C'\fR)
to attempt to redispatch any method that does not have the
same name. For example:
.PP
.Vb 1
\&        sub D::oops { print "oops!\en"; $_[0]->NEXT::other_method() }
.Ve
.Sh "Enforcing redispatch"
.IX Subsection "Enforcing redispatch"
It is possible to make \f(CW\*(C`NEXT\*(C'\fR redispatch more demandingly (i.e. like
\&\f(CW\*(C`SUPER\*(C'\fR does), so that the redispatch throws an exception if it cannot
find a \*(L"next\*(R" method to call.
.PP
To do this, simple invoke the redispatch as:
.PP
.Vb 1
\&        $self->NEXT::ACTUAL::method();
.Ve
.PP
rather than:
.PP
.Vb 1
\&        $self->NEXT::method();
.Ve
.PP
The \f(CW\*(C`ACTUAL\*(C'\fR tells \f(CW\*(C`NEXT\*(C'\fR that there must actually be a next method to call,
or it should throw an exception.
.PP
\&\f(CW\*(C`NEXT::ACTUAL\*(C'\fR is most commonly used in \f(CW\*(C`AUTOLOAD\*(C'\fR methods, as a means to
decline an \f(CW\*(C`AUTOLOAD\*(C'\fR request, but preserve the normal exception-on-failure 
semantics:
.PP
.Vb 8
\&        sub AUTOLOAD {
\&                if ($AUTOLOAD =~ /foo|bar/) {
\&                        # handle here
\&                }
\&                else {  # try elsewhere
\&                        shift()->NEXT::ACTUAL::AUTOLOAD(@_);
\&                }
\&        }
.Ve
.PP
By using \f(CW\*(C`NEXT::ACTUAL\*(C'\fR, if there is no other \f(CW\*(C`AUTOLOAD\*(C'\fR to handle the
method call, an exception will be thrown (as usually happens in the absence of
a suitable \f(CW\*(C`AUTOLOAD\*(C'\fR).
.Sh "Avoiding repetitions"
.IX Subsection "Avoiding repetitions"
If \f(CW\*(C`NEXT\*(C'\fR redispatching is used in the methods of a \*(L"diamond\*(R" class hierarchy:
.PP
.Vb 5
\&        #     A   B
\&        #    / \e /
\&        #   C   D
\&        #    \e /
\&        #     E
.Ve
.PP
.Vb 1
\&        use NEXT;
.Ve
.PP
.Vb 2
\&        package A;                 
\&        sub foo { print "called A::foo\en"; shift->NEXT::foo() }
.Ve
.PP
.Vb 2
\&        package B;                 
\&        sub foo { print "called B::foo\en"; shift->NEXT::foo() }
.Ve
.PP
.Vb 2
\&        package C; @ISA = qw( A );
\&        sub foo { print "called C::foo\en"; shift->NEXT::foo() }
.Ve
.PP
.Vb 2
\&        package D; @ISA = qw(A B);
\&        sub foo { print "called D::foo\en"; shift->NEXT::foo() }
.Ve
.PP
.Vb 2
\&        package E; @ISA = qw(C D);
\&        sub foo { print "called E::foo\en"; shift->NEXT::foo() }
.Ve
.PP
.Vb 1
\&        E->foo();
.Ve
.PP
then derived classes may (re\-)inherit base-class methods through two or
more distinct paths (e.g. in the way \f(CW\*(C`E\*(C'\fR inherits \f(CW\*(C`A::foo\*(C'\fR twice \*(--
through \f(CW\*(C`C\*(C'\fR and \f(CW\*(C`D\*(C'\fR). In such cases, a sequence of \f(CW\*(C`NEXT\*(C'\fR redispatches
will invoke the multiply inherited method as many times as it is
inherited. For example, the above code prints:
.PP
.Vb 6
\&        called E::foo
\&        called C::foo
\&        called A::foo
\&        called D::foo
\&        called A::foo
\&        called B::foo
.Ve
.PP
(i.e. \f(CW\*(C`A::foo\*(C'\fR is called twice).
.PP
In some cases this \fImay\fR 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.
.PP
To cover such cases, you can redispatch methods via:
.PP
.Vb 1
\&        $self->NEXT::UNSEEN::method();
.Ve
.PP
rather than:
.PP
.Vb 1
\&        $self->NEXT::method();
.Ve
.PP
This causes the redispatcher to skip any classes in the hierarchy that it has
already visited in an earlier redispatch. So, for example, if the
previous example were rewritten:
.PP
.Vb 2
\&        package A;                 
\&        sub foo { print "called A::foo\en"; shift->NEXT::UNSEEN::foo() }
.Ve
.PP
.Vb 2
\&        package B;                 
\&        sub foo { print "called B::foo\en"; shift->NEXT::UNSEEN::foo() }
.Ve
.PP
.Vb 2
\&        package C; @ISA = qw( A );
\&        sub foo { print "called C::foo\en"; shift->NEXT::UNSEEN::foo() }
.Ve
.PP
.Vb 2
\&        package D; @ISA = qw(A B);
\&        sub foo { print "called D::foo\en"; shift->NEXT::UNSEEN::foo() }
.Ve
.PP
.Vb 2
\&        package E; @ISA = qw(C D);
\&        sub foo { print "called E::foo\en"; shift->NEXT::UNSEEN::foo() }
.Ve
.PP
.Vb 1
\&        E->foo();
.Ve
.PP
then it would print:
.PP
.Vb 5
\&        called E::foo
\&        called C::foo
\&        called A::foo
\&        called D::foo
\&        called B::foo
.Ve
.PP
and omit the second call to \f(CW\*(C`A::foo\*(C'\fR.
.PP
Note that you can also use:
.PP
.Vb 1
\&        $self->NEXT::UNSEEN::ACTUAL::method();
.Ve
.PP
or:
.PP
.Vb 1
\&        $self->NEXT::ACTUAL::UNSEEN::method();
.Ve
.PP
to get both unique invocation \fIand\fR exception\-on\-failure.
.SH "AUTHOR"
.IX Header "AUTHOR"
Damian Conway (damian@conway.org)
.SH "BUGS AND IRRITATIONS"
.IX Header "BUGS AND IRRITATIONS"
Because it's a module, not an integral part of the interpreter, \s-1NEXT\s0.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.
.PP
It's also too slow (despite caching).
.PP
Comment, suggestions, and patches welcome.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
.Vb 3
\& 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.
.Ve