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.\" ========================================================================
.\"
.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
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