package UNIVERSAL; our $VERSION = '1.03'; # UNIVERSAL should not contain any extra subs/methods beyond those # that it exists to define. The use of Exporter below is a historical # accident that can't be fixed without breaking code. Note that we # *don't* set @ISA here, as we don't want all classes/objects inheriting from # Exporter. It's bad enough that all classes have a import() method # whenever UNIVERSAL.pm is loaded. require Exporter; @EXPORT_OK = qw(isa can VERSION); # Make sure that even though the import method is called, it doesn't do # anything unless called on UNIVERSAL. sub import { return unless $_[0] eq __PACKAGE__; goto &Exporter::import; } 1; __END__ =head1 NAME UNIVERSAL - base class for ALL classes (blessed references) =head1 SYNOPSIS $is_io = $fd->isa("IO::Handle"); $is_io = Class->isa("IO::Handle"); $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"); =head1 DESCRIPTION C is the base class from which all blessed references inherit. See L. C provides the following methods: =over 4 =item C<< $obj->isa( TYPE ) >> =item C<< CLASS->isa( TYPE ) >> =item C<< eval { VAL->isa( TYPE ) } >> Where =over 4 =item C is a package name =item C<$obj> is a blessed reference or a string containing a package name =item C is a package name =item C is any of the above or an unblessed reference =back When used as an instance or class method (C<< $obj->isa( TYPE ) >>), C returns I if $obj is blessed into package C or inherits from package C. When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes referred to as a static method), C returns I if C inherits from (or is itself) the name of the package C or inherits from package C. If you're not sure what you have (the C case), wrap the method call in an C block to catch the exception if C is undefined. If you want to be sure that you're calling C as a method, not a class, check the invocant with C from L first: use Scalar::Util 'blessed'; if ( blessed( $obj ) && $obj->isa("Some::Class") { ... } =item C<< $obj->can( METHOD ) >> =item C<< CLASS->can( METHOD ) >> =item C<< eval { VAL->can( METHOD ) } >> C checks if the object or class has a method called C. If it does, then it returns a reference to the sub. If it does not, then it returns I. This includes methods inherited or imported by C<$obj>, C, or C. C cannot know whether an object will be able to provide a method through AUTOLOAD (unless the object's class has overriden C appropriately), so a return value of I 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 L) for methods they will handle via AUTOLOAD. For such 'dummy' subs, C 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 C as a class (static) method or an object method. Again, the same rule about having a valid invocant applies -- use an C block or C if you need to be extra paranoid. =item C C will return the value of the variable C<$VERSION> in the package the object is blessed into. If C is given then it will do a comparison and die if the package version is not greater than or equal to C. C can be called as either a class (static) method or an object method. =back =head1 EXPORTS None by default. You may request the import of all three functions (C, C, and C), 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 C 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 I call an overridden C method in any class. Instead, use C from L for the first case: use Scalar::Util 'reftype'; $yes = reftype( $h ) eq "HASH"; and the method form of C for the second: $yes = Foo->isa("Bar"); =cut