package UNIVERSAL;
+our $VERSION = '1.11';
+
+# 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 can't be fixed without breaking code. Note that we
+# *don't* set @ISA here, as we 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;
-@ISA = qw(Exporter);
-@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 called on UNIVERSAL.
+sub import {
+ return unless $_[0] eq __PACKAGE__;
+ return unless @_ > 1;
+ require warnings;
+ warnings::warnif(
+ 'deprecated',
+ 'UNIVERSAL->import is deprecated and will be removed in a future perl',
+ );
+ goto &Exporter::import;
+}
1;
__END__
=head1 SYNOPSIS
- use UNIVERSAL qw(isa);
+ $is_io = $fd->isa("IO::Handle");
+ $is_io = Class->isa("IO::Handle");
- $yes = isa($ref, "HASH");
- $io = $fd->isa("IO::Handle");
- $sub = $obj->can('print');
+ $does_log = $obj->DOES("Logger");
+ $does_log = Class->DOES("Logger");
+
+ $sub = $obj->can("print");
+ $sub = Class->can("print");
+
+ $sub = eval { $ref->can("fandango") };
+ $ver = $obj->VERSION;
+
+ # but never do this!
+ $is_io = UNIVERSAL::isa($fd, "IO::Handle");
+ $sub = UNIVERSAL::can($obj, "print");
=head1 DESCRIPTION
-C<UNIVERSAL> is the base class which all bless references will inherit from,
-see L<perlobj>
+C<UNIVERSAL> is the base class from which all blessed references inherit.
+See L<perlobj>.
+
+C<UNIVERSAL> provides the following methods:
+
+=over 4
+
+=item C<< $obj->isa( TYPE ) >>
+
+=item C<< CLASS->isa( TYPE ) >>
-C<UNIVERSAL> provides the following methods
+=item C<< eval { VAL->isa( TYPE ) } >>
+
+Where
=over 4
-=item isa ( TYPE )
+=item C<TYPE>
+
+is a package name
+
+=item C<$obj>
+
+is a blessed reference or a package name
+
+=item C<CLASS>
+
+is a package name
+
+=item C<VAL>
+
+is any of the above or an unblessed reference
+
+=back
+
+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>.
-C<isa> returns I<true> if C<REF> is blessed into package C<TYPE>
-or inherits from package C<TYPE>.
+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>.
-C<isa> can be called as either a static or object method call.
+If you're not sure what you have (the C<VAL> case), wrap the method call in an
+C<eval> block to catch the exception if C<VAL> is undefined.
-=item can ( METHOD )
+If you want to be sure that you're calling C<isa> as a method, not a class,
+check the invocand with C<blessed> from L<Scalar::Util> first:
-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.
+ use Scalar::Util 'blessed';
-C<can> can be called as either a static or object method call.
+ if ( blessed( $obj ) && $obj->isa("Some::Class") ) {
+ ...
+ }
-=item VERSION ( [ REQUIRE ] )
+=item C<< $obj->DOES( ROLE ) >>
+
+=item C<< CLASS->DOES( ROLE ) >>
+
+C<DOES> checks if the object or class performs the role C<ROLE>. A role is a
+named group of specific behavior (often methods of particular names and
+signatures), similar to a class, but not necessarily a complete class by
+itself. For example, logging or serialization may be roles.
+
+C<DOES> and C<isa> are similar, in that if either is true, you know that the
+object or class on which you call the method can perform specific behavior.
+However, C<DOES> is different from C<isa> in that it does not care I<how> the
+invocand performs the operations, merely that it does. (C<isa> of course
+mandates an inheritance relationship. Other relationships include aggregation,
+delegation, and mocking.)
+
+By default, classes in Perl only perform the C<UNIVERSAL> role, as well as the
+role of all classes in their inheritance. In other words, by default C<DOES>
+responds identically to C<isa>.
+
+There is a relationship between roles and classes, as each class implies the
+existence of a role of the same name. There is also a relationship between
+inheritance and roles, in that a subclass that inherits from an ancestor class
+implicitly performs any roles its parent performs. Thus you can use C<DOES> in
+place of C<isa> safely, as it will return true in all places where C<isa> will
+return true (provided that any overridden C<DOES> I<and> C<isa> methods behave
+appropriately).
+
+=item C<< $obj->can( METHOD ) >>
+
+=item C<< CLASS->can( METHOD ) >>
+
+=item C<< eval { VAL->can( METHOD ) } >>
+
+C<can> checks if the object or class has a method called C<METHOD>. If it does,
+then it returns a reference to the sub. If it does not, then it returns
+I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
+C<VAL>.
+
+C<can> cannot know whether an object will be able to provide a method through
+AUTOLOAD (unless the object's class has overridden C<can> appropriately), 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.
+
+You may call C<can> as a class (static) method or an object method.
+
+Again, the same rule about having a valid invocand applies -- use an C<eval>
+block or C<blessed> if you need to be extra paranoid.
+
+=item C<VERSION ( [ REQUIRE ] )>
C<VERSION> will return the value of the variable C<$VERSION> in the
package the object is blessed into. If C<REQUIRE> is given then
it will do a comparison and die if the package version is not
-greater than or equal to C<REQUIRE>.
+greater than or equal to C<REQUIRE>, or if either C<$VERSION> or C<REQUIRE>
+is not a "lax" version number (as defined by the L<version> module).
+
+The return from C<VERSION> will actually be the stringified version object
+using the package C<$VERSION> scalar, which is guaranteed to be equivalent
+but may not be precisely the contents of the C<$VERSION> scalar. If you want
+the actual contents of C<$VERSION>, use C<$CLASS::VERSION> instead.
-C<VERSION> can be called as either a static or object method call.
+C<VERSION> can be called as either a class (static) method or an object
+method.
=back
-C<UNIVERSAL> also optionally exports the following subroutines
+=head1 WARNINGS
-=over 4
+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.
-=item isa ( VAL, TYPE )
+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).
-C<isa> returns I<true> if the first argument is a reference and either
-of the following statements is true.
+=head1 EXPORTS
-=over 8
+None by default.
-=item
+You may request the import of three functions (C<isa>, C<can>, and C<VERSION>),
+B<but this feature is deprecated and will be removed>. Please don't do this in
+new code.
-C<VAL> is a blessed reference and is blessed into package C<TYPE>
-or inherits from package C<TYPE>
+For example, previous versions of this documentation suggested using C<isa> as
+a function to determine the type of a reference:
-=item
+ use UNIVERSAL 'isa';
-C<VAL> is a reference to a C<TYPE> of perl variable (er 'HASH')
+ $yes = isa $h, "HASH";
+ $yes = isa "Foo", "Bar";
-=back
+The problem is that this code will I<never> call an overridden C<isa> method in
+any class. Instead, use C<reftype> from L<Scalar::Util> for the first case:
-=item can ( VAL, METHOD )
+ use Scalar::Util 'reftype';
-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.
+ $yes = reftype( $h ) eq "HASH";
-=back
+and the method form of C<isa> for the second:
+
+ $yes = Foo->isa("Bar");
=cut
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