package UNIVERSAL;

our $VERSION = '1.13';

# UNIVERSAL should not contain any extra subs/methods beyond those

# that it exists to define. The existence of import() below is a historical

# accident that can't be fixed without breaking code.

# 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 Carp;

Carp::croak("UNIVERSAL does not export anything");

}

1;

__END__

=head1 NAME

UNIVERSAL - base class for ALL classes (blessed references)

=head1 SYNOPSIS

$is_io = $fd->isa("IO::Handle");

$is_io = Class->isa("IO::Handle");

$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 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 ) >>

=item C<< eval { VAL->isa( TYPE ) } >>

Where

=over 4

=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>.

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>.

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.

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:

use Scalar::Util 'blessed';

if ( blessed( $obj ) && $obj->isa("Some::Class") ) {

...

}

=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>, 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 class (static) method or an object

method.

=back

=head1 WARNINGS

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).

=head1 EXPORTS

None.

Previous versions of this documentation suggested using C<isa> as

a function to determine the type of a reference:

$yes = UNIVERSAL::isa($h, "HASH");

$yes = UNIVERSAL::isa("Foo", "Bar");

The problem is that this code would I<never> call an overridden C<isa> method in

any class. Instead, use C<reftype> from L<Scalar::Util> for the first case:

use Scalar::Util 'reftype';

$yes = reftype( $h ) eq "HASH";

and the method form of C<isa> for the second:

$yes = Foo->isa("Bar");

=cut