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   1  package UNIVERSAL;
   2  
   3  our $VERSION = '1.04';
   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, as we 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  @EXPORT_OK = qw(isa can VERSION);
  13  
  14  # Make sure that even though the import method is called, it doesn't do
  15  # anything unless called on UNIVERSAL.
  16  sub import {
  17      return unless $_[0] eq __PACKAGE__;
  18      goto &Exporter::import;
  19  }
  20  
  21  1;
  22  __END__
  23  
  24  =head1 NAME
  25  
  26  UNIVERSAL - base class for ALL classes (blessed references)
  27  
  28  =head1 SYNOPSIS
  29  
  30      $is_io    = $fd->isa("IO::Handle");
  31      $is_io    = Class->isa("IO::Handle");
  32  
  33      $does_log = $obj->DOES("Logger");
  34      $does_log = Class->DOES("Logger");
  35  
  36      $sub      = $obj->can("print");
  37      $sub      = Class->can("print");
  38  
  39      $sub      = eval { $ref->can("fandango") };
  40      $ver      = $obj->VERSION;
  41  
  42      # but never do this!
  43      $is_io    = UNIVERSAL::isa($fd, "IO::Handle");
  44      $sub      = UNIVERSAL::can($obj, "print");
  45  
  46  =head1 DESCRIPTION
  47  
  48  C<UNIVERSAL> is the base class from which all blessed references inherit.
  49  See L<perlobj>.
  50  
  51  C<UNIVERSAL> provides the following methods:
  52  
  53  =over 4
  54  
  55  =item C<< $obj->isa( TYPE ) >>
  56  
  57  =item C<< CLASS->isa( TYPE ) >>
  58  
  59  =item C<< eval { VAL->isa( TYPE ) } >>
  60  
  61  Where
  62  
  63  =over 4
  64  
  65  =item C<TYPE>
  66  
  67  is a package name
  68  
  69  =item C<$obj>
  70  
  71  is a blessed reference or a string containing a package name
  72  
  73  =item C<CLASS>
  74  
  75  is a package name
  76  
  77  =item C<VAL>
  78  
  79  is any of the above or an unblessed reference
  80  
  81  =back
  82  
  83  When used as an instance or class method (C<< $obj->isa( TYPE ) >>),
  84  C<isa> returns I<true> if $obj is blessed into package C<TYPE> or
  85  inherits from package C<TYPE>.
  86  
  87  When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes
  88  referred to as a static method), C<isa> returns I<true> if C<CLASS>
  89  inherits from (or is itself) the name of the package C<TYPE> or
  90  inherits from package C<TYPE>.
  91  
  92  If you're not sure what you have (the C<VAL> case), wrap the method call in an
  93  C<eval> block to catch the exception if C<VAL> is undefined.
  94  
  95  If you want to be sure that you're calling C<isa> as a method, not a class,
  96  check the invocant with C<blessed> from L<Scalar::Util> first:
  97  
  98    use Scalar::Util 'blessed';
  99  
 100    if ( blessed( $obj ) && $obj->isa("Some::Class") {
 101        ...
 102    }
 103  
 104  =item C<< $obj->DOES( ROLE ) >>
 105  
 106  =item C<< CLASS->DOES( ROLE ) >>
 107  
 108  C<DOES> checks if the object or class performs the role C<ROLE>.  A role is a
 109  named group of specific behavior (often methods of particular names and
 110  signatures), similar to a class, but not necessarily a complete class by
 111  itself.  For example, logging or serialization may be roles.
 112  
 113  C<DOES> and C<isa> are similar, in that if either is true, you know that the
 114  object or class on which you call the method can perform specific behavior.
 115  However, C<DOES> is different from C<isa> in that it does not care I<how> the
 116  invocant performs the operations, merely that it does.  (C<isa> of course
 117  mandates an inheritance relationship.  Other relationships include aggregation,
 118  delegation, and mocking.)
 119  
 120  By default, classes in Perl only perform the C<UNIVERSAL> role.  To mark that
 121  your own classes perform other roles, override C<DOES> appropriately.
 122  
 123  There is a relationship between roles and classes, as each class implies the
 124  existence of a role of the same name.  There is also a relationship between
 125  inheritance and roles, in that a subclass that inherits from an ancestor class
 126  implicitly performs any roles its parent performs.  Thus you can use C<DOES> in
 127  place of C<isa> safely, as it will return true in all places where C<isa> will
 128  return true (provided that any overridden C<DOES> I<and> C<isa> methods behave
 129  appropriately).
 130  
 131  =item C<< $obj->can( METHOD ) >>
 132  
 133  =item C<< CLASS->can( METHOD ) >>
 134  
 135  =item C<< eval { VAL->can( METHOD ) } >>
 136  
 137  C<can> checks if the object or class has a method called C<METHOD>. If it does,
 138  then it returns a reference to the sub.  If it does not, then it returns
 139  I<undef>.  This includes methods inherited or imported by C<$obj>, C<CLASS>, or
 140  C<VAL>.
 141  
 142  C<can> cannot know whether an object will be able to provide a method through
 143  AUTOLOAD (unless the object's class has overriden C<can> appropriately), so a
 144  return value of I<undef> does not necessarily mean the object will not be able
 145  to handle the method call. To get around this some module authors use a forward
 146  declaration (see L<perlsub>) for methods they will handle via AUTOLOAD. For
 147  such 'dummy' subs, C<can> will still return a code reference, which, when
 148  called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD is provided,
 149  calling the coderef will cause an error.
 150  
 151  You may call C<can> as a class (static) method or an object method.
 152  
 153  Again, the same rule about having a valid invocant applies -- use an C<eval>
 154  block or C<blessed> if you need to be extra paranoid.
 155  
 156  =item C<VERSION ( [ REQUIRE ] )>
 157  
 158  C<VERSION> will return the value of the variable C<$VERSION> in the
 159  package the object is blessed into. If C<REQUIRE> is given then
 160  it will do a comparison and die if the package version is not
 161  greater than or equal to C<REQUIRE>.
 162  
 163  C<VERSION> can be called as either a class (static) method or an object
 164  method.
 165  
 166  =back
 167  
 168  =head1 EXPORTS
 169  
 170  None by default.
 171  
 172  You may request the import of three functions (C<isa>, C<can>, and C<VERSION>),
 173  however it is usually harmful to do so.  Please don't do this in new code.
 174  
 175  For example, previous versions of this documentation suggested using C<isa> as
 176  a function to determine the type of a reference:
 177  
 178    use UNIVERSAL 'isa';
 179  
 180    $yes = isa $h, "HASH";
 181    $yes = isa "Foo", "Bar";
 182  
 183  The problem is that this code will I<never> call an overridden C<isa> method in
 184  any class.  Instead, use C<reftype> from L<Scalar::Util> for the first case:
 185  
 186    use Scalar::Util 'reftype';
 187  
 188    $yes = reftype( $h ) eq "HASH";
 189  
 190  and the method form of C<isa> for the second:
 191  
 192    $yes = Foo->isa("Bar");
 193  
 194  =cut