Section: Perl Programmers Reference Guide (3pm)
Updated: 2001-09-21
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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 )
is a package name
is a blessed reference or a string containing a package name
is a package name
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";


   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 )
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 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 a function.



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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