UNIVERSAL(3p) Perl Programmers Reference Guide UNIVERSAL(3p)
UNIVERSAL - base class for ALL classes (blessed references)
$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 $obj ;
"UNIVERSAL" is the base class which all bless references
will inherit from, see perlobj.
"UNIVERSAL" provides the following methods and functions:
"$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 )": some-
times 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
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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 pro-
vide 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 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.
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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 references.
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