Seth Woolley's Man Viewer

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(3,n) 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(3,n) $obj is blessed into package "TYPE" or inher-
           its from package "TYPE".

           When used as a class method ("CLASS->isa( TYPE )": sometimes
           referred to as a static method), "isa" returns true if(3,n) "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(1,8) the same cases as above and also if(3,n) "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(3,n) 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 neces-
           sarily 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(3,n) "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(3,n) it
           does not have a method "METHOD", undef is returned.

       "VERSION ( [ REQUIRE ] )"
           "VERSION" will return the value of the variable $VERSION in(1,8) the
           package the object is blessed into. If "REQUIRE" is given then it
           will do a comparison and die if(3,n) the package version(1,3,5) 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 v5.8.5                       2001-09-21                      UNIVERSAL(3)