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



NAME
       UNIVERSAL - base class for ALL classes (blessed refer-
       ences)

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 pack-
           age "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 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 refer-
           ence 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 ( [ REQUIRE ] )"
           "VERSION" will return the value of the variable $VER-
           SION 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 magically 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 ref-
       erences.



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