UNIVERSAL man page on aLinux

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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");

	   $does_log = $obj->DOES("Logger");
	   $does_log = Class->DOES("Logger");

	   $sub	     = $obj->can("print");
	   $sub	     = Class->can("print");

	   $sub	     = eval { $ref->can("fandango") };
	   $ver	     = $obj->VERSION;

	   # but never do this!
	   $is_io    = UNIVERSAL::isa($fd, "IO::Handle");
	   $sub	     = UNIVERSAL::can($obj, "print");

DESCRIPTION
       "UNIVERSAL" is the base class from which all blessed references
       inherit.	 See perlobj.

       "UNIVERSAL" provides the following methods:

       "$obj->isa( TYPE )"
       "CLASS->isa( TYPE )"
       "eval { VAL->isa( 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 )", 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".

	   If you're not sure what you have (the "VAL" case), wrap the method
	   call in an "eval" block to catch the exception if "VAL" is
	   undefined.

	   If you want to be sure that you're calling "isa" as a method, not a
	   class, check the invocant with "blessed" from Scalar::Util first:

	     use Scalar::Util 'blessed';

	     if ( blessed( $obj ) && $obj->isa("Some::Class") {
		 ...
	     }

       "$obj->DOES( ROLE )"
       "CLASS->DOES( ROLE )"
	   "DOES" checks if the object or class performs the role "ROLE".  A
	   role is a named group of specific behavior (often methods of
	   particular names and signatures), similar to a class, but not
	   necessarily a complete class by itself.  For example, logging or
	   serialization may be roles.

	   "DOES" and "isa" are similar, in that if either is true, you know
	   that the object or class on which you call the method can perform
	   specific behavior.  However, "DOES" is different from "isa" in that
	   it does not care how the invocant performs the operations, merely
	   that it does.  ("isa" of course mandates an inheritance
	   relationship.  Other relationships include aggregation, delegation,
	   and mocking.)

	   By default, classes in Perl only perform the "UNIVERSAL" role.  To
	   mark that your own classes perform other roles, override "DOES"
	   appropriately.

	   There is a relationship between roles and classes, as each class
	   implies the existence of a role of the same name.  There is also a
	   relationship between inheritance and roles, in that a subclass that
	   inherits from an ancestor class implicitly performs any roles its
	   parent performs.  Thus you can use "DOES" in place of "isa" safely,
	   as it will return true in all places where "isa" will return true
	   (provided that any overridden "DOES" and "isa" methods behave
	   appropriately).

       "$obj->can( METHOD )"
       "CLASS->can( METHOD )"
       "eval { VAL->can( METHOD ) }"
	   "can" checks if the object or class has a method called "METHOD".
	   If it does, then it returns a reference to the sub.	If it does
	   not, then it returns undef.	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 (unless the object's class has overriden
	   "can" appropriately), 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.

	   You may call "can" as a class (static) method or an object method.

	   Again, the same rule about having a valid invocant applies -- use
	   an "eval" block or "blessed" if you need to be extra paranoid.

       "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 or an
	   object method.

EXPORTS
       None by default.

       You may request the import of three functions ("isa", "can", and
       "VERSION"), however it is usually harmful to do so.  Please don't do
       this in new code.

       For example, previous versions of this documentation suggested using
       "isa" as a function to determine the type of a reference:

	 use UNIVERSAL 'isa';

	 $yes = isa $h, "HASH";
	 $yes = isa "Foo", "Bar";

       The problem is that this code will never call an overridden "isa"
       method in any class.  Instead, use "reftype" from Scalar::Util for the
       first case:

	 use Scalar::Util 'reftype';

	 $yes = reftype( $h ) eq "HASH";

       and the method form of "isa" for the second:

	 $yes = Foo->isa("Bar");

perl v5.10.0			  2007-12-18			  UNIVERSAL(3)
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