UNIVERSAL

UNIVERSAL(3)	       Perl Programmers Reference Guide		 UNIVERSAL(3)



NAME
       UNIVERSAL - base class for ALL classes (blessed references)

SYNOPSIS
	   $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 ;

DESCRIPTION
       "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 )"
	   Where

	   "TYPE"
	       is a package name

	   $obj
	       is a blessed reference or a string containing a package name

	   "CLASS"
	       is a package name

	   "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";

	   or

	      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 nec-
	   essarily mean the object will not be able to handle the method
	   call. To get around this some module authors use a forward decla-
	   ration (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 suit-
	   able 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 pack-
	   age name which has a method called "METHOD", "can" returns a ref-
	   erence to the subroutine.  If "VAL" is not a blessed reference, or
	   if it does not have a method "METHOD", undef is returned.

       "VERSION ( [ REQUIRE ] )"
	   "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 a function.

EXPORTS
       None by default.

       You may request the import of all three functions ("isa", "can", and
       "VERSION"), however it isn’t usually necessary to do so.	 Perl magi-
       cally makes these functions act as methods on all objects.  The one
       exception is "isa", which is useful as a function when operating on
       non-blessed references.



perl v5.8.8			  2001-09-21			 UNIVERSAL(3)