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:
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".
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 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 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.