1package UNIVERSAL; 2 3our $VERSION = '1.00'; 4 5# UNIVERSAL should not contain any extra subs/methods beyond those 6# that it exists to define. The use of Exporter below is a historical 7# accident that can't be fixed without breaking code. Note that we 8# *don't* set @ISA here, don't want all classes/objects inheriting from 9# Exporter. It's bad enough that all classes have a import() method 10# whenever UNIVERSAL.pm is loaded. 11require Exporter; 12*import = \&Exporter::import; 13@EXPORT_OK = qw(isa can VERSION); 14 151; 16__END__ 17 18=head1 NAME 19 20UNIVERSAL - base class for ALL classes (blessed references) 21 22=head1 SYNOPSIS 23 24 $is_io = $fd->isa("IO::Handle"); 25 $is_io = Class->isa("IO::Handle"); 26 27 $sub = $obj->can("print"); 28 $sub = Class->can("print"); 29 30 use UNIVERSAL qw( isa can VERSION ); 31 $yes = isa $ref, "HASH" ; 32 $sub = can $ref, "fandango" ; 33 $ver = VERSION $obj ; 34 35=head1 DESCRIPTION 36 37C<UNIVERSAL> is the base class which all bless references will inherit from, 38see L<perlobj>. 39 40C<UNIVERSAL> provides the following methods and functions: 41 42=over 4 43 44=item $obj->isa( TYPE ), CLASS->isa( TYPE ), isa( VAL, TYPE ) 45 46 C<TYPE> is a package name 47 $obj is a blessed reference or a string containing a package name 48 C<CLASS> is a package name 49 C<VAL> is any of the above or an unblessed reference 50 51When used as an instance or class method (C<$obj->isa( TYPE )>), C<isa> 52returns I<true> if $obj is blessed into package C<TYPE> or inherits from 53package C<TYPE>. 54 55When used as a class method (C<CLASS->isa( TYPE )>; sometimes referred to as a 56static method), C<isa> returns I<true> if C<CLASS> inherits from (or is itself) 57the name of the package C<TYPE> or inherits from package C<TYPE>. 58 59When used as a function, like 60 61 use UNIVERSAL qw( isa ) ; 62 $yes = isa $h, "HASH"; 63 $yes = isa "Foo", "Bar"; 64 65or 66 67 require UNIVERSAL ; 68 $yes = UNIVERSAL::isa $a, "ARRAY"; 69 70, C<isa> returns I<true> in the same cases as above and also if C<VAL> is an 71unblessed reference to a perl variable of type C<TYPE>, such as "HASH", 72"ARRAY", or "Regexp". 73 74=item $obj->can( METHOD ), CLASS->can( METHOD ), can( VAL, METHOD ) 75 76C<can> checks if the object or class has a method called C<METHOD>. If it does 77then a reference to the sub is returned. If it does not then I<undef> is 78returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or 79C<VAL>. 80 81C<can> cannot know whether an object will be able to provide a method 82through AUTOLOAD, so a return value of I<undef> does not necessarily mean 83the object will not be able to handle the method call. To get around 84this some module authors use a forward declaration (see L<perlsub>) 85for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can> 86will still return a code reference, which, when called, will fall through 87to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef 88will cause an error. 89 90C<can> can be called as a class (static) method, an object method, or a 91function. 92 93When used as a function, if C<VAL> is a blessed reference or package name which 94has a method called C<METHOD>, C<can> returns a reference to the subroutine. 95If C<VAL> is not a blessed reference, or if it does not have a method 96C<METHOD>, I<undef> is returned. 97 98=item VERSION ( [ REQUIRE ] ) 99 100C<VERSION> will return the value of the variable C<$VERSION> in the 101package the object is blessed into. If C<REQUIRE> is given then 102it will do a comparison and die if the package version is not 103greater than or equal to C<REQUIRE>. 104 105C<VERSION> can be called as either a class (static) method, an object method or 106or a function. 107 108 109=back 110 111These subroutines should I<not> be imported via S<C<use UNIVERSAL qw(...)>>. 112If you want simple local access to them you can do 113 114 *isa = \&UNIVERSAL::isa; 115 116to import isa into your package. 117 118=cut 119