Manpages - NEXT.3perl

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).

If NEXT redispatching is used in the methods of a diamond class hierarchy:

“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 Ω- 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.

Yet another pseudo-class that NEXT 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).

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.