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 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 "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 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 "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 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 v5.8.6 2001-09-21 UNIVERSAL(3)