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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 invocand
           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 invocand 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 overridden "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 invocand 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", or if either $VERSION or "REQUIRE" is not a "lax" version
           number (as defined by the version module).

           The return from "VERSION" will actually be the stringified version object using the package
           $VERSION scalar, which is guaranteed to be equivalent but may not be precisely the contents of
           the $VERSION scalar.  If you want the actual contents of $VERSION, use $CLASS::VERSION instead.

           "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 v5.16.2                                     2012-10-11                                   UNIVERSAL(3pm)