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UNIVERSAL(3pm)         Perl Programmers Reference Guide         UNIVERSAL(3pm)

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 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, as
           well as the role of all classes in their inheritance.  In other
           words, by default "DOES" responds identically to "isa".

           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".  Both $VERSION or "REQUIRE" must be
           "lax" version numbers (as defined by the version module) or
           "VERSION" will die with an error.

           "VERSION" can be called as either a class (static) method or an
           object method.

WARNINGS
       NOTE: "can" directly uses Perl's internal code for method lookup, and
       "isa" uses a very similar method and cache-ing strategy. This may cause
       strange effects if the Perl code dynamically changes @ISA in any
       package.

       You may add other methods to the UNIVERSAL class via Perl or XS code.
       You do not need to "use UNIVERSAL" to make these methods available to
       your program (and you should not do so).

EXPORTS
       None by default.

       You may request the import of three functions ("isa", "can", and
       "VERSION"), but this feature is deprecated and will be removed.  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 v5.12.1                      2010-04-26                    UNIVERSAL(3pm)
 

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