package UNIVERSAL;
+our $VERSION = '1.02';
+
# UNIVERSAL should not contain any extra subs/methods beyond those
# that it exists to define. The use of Exporter below is a historical
-# accident that should be fixed sometime.
+# accident that can't be fixed without breaking code. Note that we
+# *don't* set @ISA here, don't want all classes/objects inheriting from
+# Exporter. It's bad enough that all classes have a import() method
+# whenever UNIVERSAL.pm is loaded.
require Exporter;
-*import = \&Exporter::import;
-@EXPORT_OK = qw(isa can);
+@EXPORT_OK = qw(isa can VERSION);
+
+# Make sure that even though the import method is called, it doesn't do
+# anything unless its called on UNIVERSAL
+sub import {
+ return unless $_[0] eq __PACKAGE__;
+ goto &Exporter::import;
+}
1;
__END__
=head1 SYNOPSIS
- $io = $fd->isa("IO::Handle");
- $sub = $obj->can('print');
+ $is_io = $fd->isa("IO::Handle");
+ $is_io = Class->isa("IO::Handle");
+
+ $sub = $obj->can("print");
+ $sub = Class->can("print");
- $yes = UNIVERSAL::isa($ref, "HASH");
+ use UNIVERSAL qw( isa can VERSION );
+ $yes = isa $ref, "HASH" ;
+ $sub = can $ref, "fandango" ;
+ $ver = VERSION $obj ;
=head1 DESCRIPTION
C<UNIVERSAL> is the base class which all bless references will inherit from,
-see L<perlobj>
+see L<perlobj>.
-C<UNIVERSAL> provides the following methods
+C<UNIVERSAL> provides the following methods and functions:
=over 4
-=item isa ( TYPE )
+=item C<< $obj->isa( TYPE ) >>
-C<isa> returns I<true> if C<REF> is blessed into package C<TYPE>
-or inherits from package C<TYPE>.
+=item C<< CLASS->isa( TYPE ) >>
-C<isa> can be called as either a static or object method call.
+=item C<isa( VAL, TYPE )>
-=item can ( METHOD )
+Where
-C<can> checks if the object has a method called C<METHOD>. If it does
-then a reference to the sub is returned. If it does not then I<undef>
-is returned.
+=over 4
-C<can> can be called as either a static or object method call.
+=item C<TYPE>
-=item VERSION ( [ REQUIRE ] )
+is a package name
-C<VERSION> will return the value of the variable C<$VERSION> in the
-package the object is blessed into. If C<REQUIRE> is given then
-it will do a comparison and die if the package version is not
-greater than or equal to C<REQUIRE>.
+=item C<$obj>
+
+is a blessed reference or a string containing a package name
+
+=item C<CLASS>
+
+is a package name
-C<VERSION> can be called as either a static or object method call.
+=item C<VAL>
+
+is any of the above or an unblessed reference
=back
-The C<isa> and C<can> methods can also be called as subroutines
+When used as an instance or class method (C<< $obj->isa( TYPE ) >>),
+C<isa> returns I<true> if $obj is blessed into package C<TYPE> or
+inherits from package C<TYPE>.
-=over 4
+When used as a class method (C<< CLASS->isa( TYPE ) >>: sometimes
+referred to as a static method), C<isa> returns I<true> if C<CLASS>
+inherits from (or is itself) the name of the package C<TYPE> or
+inherits from package C<TYPE>.
-=item UNIVERSAL::isa ( VAL, TYPE )
+When used as a function, like
-C<isa> returns I<true> if one of the following statements is true.
+ use UNIVERSAL qw( isa ) ;
+ $yes = isa $h, "HASH";
+ $yes = isa "Foo", "Bar";
-=over 8
+or
-=item *
+ require UNIVERSAL ;
+ $yes = UNIVERSAL::isa $a, "ARRAY";
-C<VAL> is a reference blessed into either package C<TYPE> or a package
-which inherits from package C<TYPE>.
+C<isa> returns I<true> in the same cases as above and also if C<VAL> is an
+unblessed reference to a perl variable of type C<TYPE>, such as "HASH",
+"ARRAY", or "Regexp".
-=item *
+=item C<< $obj->can( METHOD ) >>
-C<VAL> is a reference to a C<TYPE> of Perl variable (e.g. 'HASH').
+=item C<< CLASS->can( METHOD ) >>
-=item *
+=item C<can( VAL, METHOD )>
-C<VAL> is the name of a package that inherits from (or is itself)
-package C<TYPE>.
+C<can> checks if the object or class has a method called C<METHOD>. If it does
+then a reference to the sub is returned. If it does not then I<undef> is
+returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
+C<VAL>.
-=back
+C<can> cannot know whether an object will be able to provide a method
+through AUTOLOAD, so a return value of I<undef> does not necessarily mean
+the object will not be able to handle the method call. To get around
+this some module authors use a forward declaration (see L<perlsub>)
+for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can>
+will still return a code reference, which, when called, will fall through
+to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef
+will cause an error.
+
+C<can> can be called as a class (static) method, an object method, or a
+function.
+
+When used as a function, if C<VAL> is a blessed reference or package name which
+has a method called C<METHOD>, C<can> returns a reference to the subroutine.
+If C<VAL> is not a blessed reference, or if it does not have a method
+C<METHOD>, I<undef> is returned.
+
+=item C<VERSION ( [ REQUIRE ] )>
+
+C<VERSION> will return the value of the variable C<$VERSION> in the
+package the object is blessed into. If C<REQUIRE> is given then
+it will do a comparison and die if the package version is not
+greater than or equal to C<REQUIRE>.
-=item UNIVERSAL::can ( VAL, METHOD )
+C<VERSION> can be called as either a class (static) method, an object
+method or a function.
-If C<VAL> is a blessed reference which has a method called C<METHOD>,
-C<can> returns a reference to the subroutine. If C<VAL> is not
-a blessed reference, or if it does not have a method C<METHOD>,
-I<undef> is returned.
=back
-These subroutines should I<not> be imported via S<C<use UNIVERSAL qw(...)>>.
-If you want simple local access to them you can do
+=head1 EXPORTS
- *isa = \&UNIVERSAL::isa;
+None by default.
-to import isa into your package.
+You may request the import of all three functions (C<isa>, C<can>, and
+C<VERSION>), however it isn't usually necessary to do so. Perl magically
+makes these functions act as methods on all objects. The one exception is
+C<isa>, which is useful as a function when operating on non-blessed
+references.
=cut
https://perl5.git.perl.org / perl5.git / blobdiff