where else to look for a method if you can't find it in the current
package. This is how Perl implements inheritance. Each element of the
@ISA array is just the name of another package that happens to be a
-class package. The classes are searched (depth first) for missing
-methods in the order that they occur in @ISA. The classes accessible
+class package. The classes are searched for missing methods in
+depth-first, left-to-right order by default (see L<mro> for alternative
+search order and other in-depth information). The classes accessible
through @ISA are known as base classes of the current class.
All classes implicitly inherit from class C<UNIVERSAL> as their
last base class. Several commonly used methods are automatically
-supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for
-more details.
+supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> or
+L<UNIVERSAL|UNIVERSAL> for more details.
X<UNIVERSAL> X<base class> X<class, base>
If a missing method is found in a base class, it is cached
scalar variable containing either the method name or a subroutine
reference can also be used.
+If the right side of the arrow is a scalar containing a reference
+to a subroutine, then this is equivalent to calling the referenced
+subroutine directly with the class name or object on the left side
+of the arrow as its first argument. No lookup is done and there is
+no requirement that the subroutine be defined in any package related
+to the class name or object on the left side of the arrow.
+
+For example, the following calls to $display are equivalent:
+
+ my $display = sub { my $self = shift; ... };
+ $fred->$display("Height", "Weight");
+ $display->($fred, "Height", "Weight");
+
=head2 Indirect Object Syntax
X<indirect object syntax> X<invocation, indirect> X<indirect>
call compiled as a method, or vice versa. This can introduce subtle bugs
that are hard to detect.
-For example, a call to a method C<new> in indirect notation -- as C++
-programmers are wont to make -- can be miscompiled into a subroutine
+For example, a call to a method C<new> in indirect notation (as C++
+programmers are wont to make) can be miscompiled into a subroutine
call if there's already a C<new> function in scope. You'd end up
calling the current package's C<new> as a subroutine, rather than the
desired class's method. The compiler tries to cheat by remembering
C<isa> returns I<true> if its object is blessed into a subclass of C<CLASS>
-You can also call C<UNIVERSAL::isa> as a subroutine with two arguments. Of
-course, this will do the wrong thing if someone has overridden C<isa> in a
-class, so don't do it.
-
-If you need to determine whether you've received a valid invocant, use the
-C<blessed> function from L<Scalar::Util>:
-X<invocant> X<blessed>
-
- if (blessed($ref) && $ref->isa( 'Some::Class')) {
- # ...
- }
+=item DOES(ROLE)
+X<DOES>
-C<blessed> returns the name of the package the argument has been
-blessed into, or C<undef>.
+C<DOES> returns I<true> if its object claims to perform the role C<ROLE>. By
+default, this is equivalent to C<isa>.
=item can(METHOD)
X<can>
C<can> checks to see if its object has a method called C<METHOD>,
if it does then a reference to the sub is returned, if it does not then
-I<undef> is returned.
-
-C<UNIVERSAL::can> can also be called as a subroutine with two arguments. It'll
-always return I<undef> if its first argument isn't an object or a class name.
-The same caveats for calling C<UNIVERSAL::isa> directly apply here, too.
+C<undef> is returned.
=item VERSION( [NEED] )
X<VERSION>
C<VERSION> returns the version number of the class (package). If the
NEED argument is given then it will check that the current version (as
defined by the $VERSION variable in the given package) not less than
-NEED; it will die if this is not the case. This method is normally
-called as a class method. This method is called automatically by the
-C<VERSION> form of C<use>.
+NEED; it will die if this is not the case. This method is called automatically
+by the C<VERSION> form of C<use>.
- use A 1.2 qw(some imported subs);
+ use Package 1.2 qw(some imported subs);
# implies:
- A->VERSION(1.2);
+ Package->VERSION(1.2);
=back
-B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
-C<isa> uses a very similar method and cache-ing strategy. This may cause
-strange effects if the Perl code dynamically changes @ISA in any package.
-
-You may add other methods to the UNIVERSAL class via Perl or XS code.
-You do not need to C<use UNIVERSAL> to make these methods
-available to your program (and you should not do so).
-
=head2 Destructors
X<destructor> X<DESTROY>
of your choosing get called. Explicitly calling DESTROY is also possible,
but is usually never needed.
+DESTROY is subject to AUTOLOAD lookup, just like any other method. Hence, if
+your class has an AUTOLOAD method, but does not need any DESTROY actions,
+you probably want to provide a DESTROY method anyway, to prevent an
+expensive call to AUTOLOAD each time an object is freed. As this technique
+makes empty DESTROY methods common, the implementation is optimised so that
+a DESTROY method that is an empty or constant subroutine, and hence could
+have no side effects anyway, is not actually called.
+X<AUTOLOAD> X<DESTROY>
+
Do not confuse the previous discussion with how objects I<CONTAINED> in the current
one are destroyed. Such objects will be freed and destroyed automatically
when the current object is freed, provided no other references to them exist