lib/perl5/5.8.8/UNIVERSAL.pm

   1 package UNIVERSAL;
   2
   3 our $VERSION = '1.01';
   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.
  11 require Exporter;
  12 *import = \&Exporter::import;
  13 @EXPORT_OK = qw(isa can VERSION);
  14
  15 1;
  16 __END__
  17
  18 =head1 NAME
  19
  20 UNIVERSAL - 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
  37 C<UNIVERSAL> is the base class which all bless references will inherit from,
  38 see L<perlobj>.
  39
  40 C<UNIVERSAL> provides the following methods and functions:
  41
  42 =over 4
  43
  44 =item C<< $obj->isa( TYPE ) >>
  45
  46 =item C<< CLASS->isa( TYPE ) >>
  47
  48 =item C<isa( VAL, TYPE )>
  49
  50 Where
  51
  52 =over 4
  53
  54 =item C<TYPE>
  55
  56 is a package name
  57
  58 =item C<$obj>
  59
  60 is a blessed reference or a string containing a package name
  61
  62 =item C<CLASS>
  63
  64 is a package name
  65
  66 =item C<VAL>
  67
  68 is any of the above or an unblessed reference
  69
  70 =back
  71
  72 When used as an instance or class method (C<< $obj->isa( TYPE ) >>),
  73 C<isa> returns I<true> if $obj is blessed into package C<TYPE> or
  74 inherits from package C<TYPE>.
  75
  76 When used as a class method (C<< CLASS->isa( TYPE ) >>: sometimes
  77 referred to as a static method), C<isa> returns I<true> if C<CLASS>
  78 inherits from (or is itself) the name of the package C<TYPE> or
  79 inherits from package C<TYPE>.
  80
  81 When used as a function, like
  82
  83    use UNIVERSAL qw( isa ) ;
  84    $yes = isa $h, "HASH";
  85    $yes = isa "Foo", "Bar";
  86
  87 or
  88
  89    require UNIVERSAL ;
  90    $yes = UNIVERSAL::isa $a, "ARRAY";
  91
  92 C<isa> returns I<true> in the same cases as above and also if C<VAL> is an
  93 unblessed reference to a perl variable of type C<TYPE>, such as "HASH",
  94 "ARRAY", or "Regexp".
  95
  96 =item C<< $obj->can( METHOD ) >>
  97
  98 =item C<< CLASS->can( METHOD ) >>
  99
 100 =item C<can( VAL, METHOD )>
 101
 102 C<can> checks if the object or class has a method called C<METHOD>. If it does
 103 then a reference to the sub is returned. If it does not then I<undef> is
 104 returned.  This includes methods inherited or imported by C<$obj>, C<CLASS>, or
 105 C<VAL>.
 106
 107 C<can> cannot know whether an object will be able to provide a method
 108 through AUTOLOAD, so a return value of I<undef> does not necessarily mean
 109 the object will not be able to handle the method call. To get around
 110 this some module authors use a forward declaration (see L<perlsub>)
 111 for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can>
 112 will still return a code reference, which, when called, will fall through
 113 to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef
 114 will cause an error.
 115
 116 C<can> can be called as a class (static) method, an object method, or a
 117 function.
 118
 119 When used as a function, if C<VAL> is a blessed reference or package name which
 120 has a method called C<METHOD>, C<can> returns a reference to the subroutine.
 121 If C<VAL> is not a blessed reference, or if it does not have a method
 122 C<METHOD>, I<undef> is returned.
 123
 124 =item C<VERSION ( [ REQUIRE ] )>
 125
 126 C<VERSION> will return the value of the variable C<$VERSION> in the
 127 package the object is blessed into. If C<REQUIRE> is given then
 128 it will do a comparison and die if the package version is not
 129 greater than or equal to C<REQUIRE>.
 130
 131 C<VERSION> can be called as either a class (static) method, an object
 132 method or a function.
 133
 134
 135 =back
 136
 137 =head1 EXPORTS
 138
 139 None by default.
 140
 141 You may request the import of all three functions (C<isa>, C<can>, and
 142 C<VERSION>), however it isn't usually necessary to do so.  Perl magically
 143 makes these functions act as methods on all objects.  The one exception is
 144 C<isa>, which is useful as a function when operating on non-blessed
 145 references.
 146
 147 =cut