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



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 v5.20.2                                2014-12-27                           UNIVERSAL(3perl)