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UNIVERSAL - base class for ALL classes (blessed references)


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

$sub = $obj->can("print");
$sub = Class->can("print");

use UNIVERSAL qw( isa can VERSION );
$yes = isa $ref, "HASH" ;
$sub = can $ref, "fandango" ;
$ver = VERSION $obj ;


UNIVERSAL is the base class which all bless references will inherit from, see perlobj.

UNIVERSAL provides the following methods and functions:

$obj->isa( TYPE ), CLASS->isa( TYPE ), isa( VAL, TYPE )
C<TYPE> is a package name
$obj is a blessed reference or a string containing a package name
C<CLASS> is a package name
C<VAL> is any of the above or an unblessed reference

When used as an instance or class method ($obj-isa( TYPE )>), isa returns true if $obj is blessed into package TYPE or inherits from package TYPE.

When used as a class method (CLASS-isa( TYPE )>; sometimes referred to as a static method), isa returns true if CLASS inherits from (or is itself) the name of the package TYPE or inherits from package TYPE.

When used as a function, like

use UNIVERSAL qw( isa ) ;
$yes = isa $h, "HASH";
$yes = isa "Foo", "Bar";


require UNIVERSAL ;
$yes = UNIVERSAL::isa $a, "ARRAY";

, isa returns true in the same cases as above and also if VAL is an unblessed reference to a perl variable of type TYPE, such as "HASH", "ARRAY", or "Regexp".

$obj->can( METHOD ), CLASS->can( METHOD ), can( VAL, METHOD )

can checks if the object or class has a method called METHOD. If it does then a reference to the sub is returned. If it does not then undef is returned. This includes methods inherited or imported by $obj, CLASS, or VAL.

can cannot know whether an object will be able to provide a method through AUTOLOAD, so a return value of 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 perlsub) for methods they will handle via AUTOLOAD. For such 'dummy' subs, 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.

can can be called as a class (static) method, an object method, or a function.

When used as a function, if VAL is a blessed reference or package name which has a method called METHOD, can returns a reference to the subroutine. If VAL is not a blessed reference, or if it does not have a method METHOD, undef is returned.


VERSION will return the value of the variable $VERSION in the package the object is blessed into. If REQUIRE is given then it will do a comparison and die if the package version is not greater than or equal to REQUIRE.

VERSION can be called as either a class (static) method, an object method or or a function.

These subroutines should not be imported via use UNIVERSAL qw(...). If you want simple local access to them you can do

*isa = \&UNIVERSAL::isa;

to import isa into your package.