UNIVERSAL(3pm) Perl Programmers Reference Guide UNIVERSAL(3pm)
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 )
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";
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 perl-
sub) 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 ( [ 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 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.
perl v5.8.0 2002-06-01 UNIVERSAL(3pm)