UNIVERSAL(3) Perl Programmers Reference Guide UNIVERSAL(3)NAMEUNIVERSAL - 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 v5.8.8 2008-09-19 UNIVERSAL(3)