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    1 package UNIVERSAL;
    2 
    3 our $VERSION = '1.13';
    4 
    5 # UNIVERSAL should not contain any extra subs/methods beyond those
    6 # that it exists to define. The existence of import() below is a historical
    7 # accident that can't be fixed without breaking code.
    8 
    9 # Make sure that even though the import method is called, it doesn't do
   10 # anything unless called on UNIVERSAL.
   11 sub import {
   12     return unless $_[0] eq __PACKAGE__;
   13     return unless @_ > 1;
   14     require Carp;
   15     Carp::croak("UNIVERSAL does not export anything");
   16 }
   17 
   18 1;
   19 __END__
   20 
   21 =head1 NAME
   22 
   23 UNIVERSAL - base class for ALL classes (blessed references)
   24 
   25 =head1 SYNOPSIS
   26 
   27     $is_io    = $fd->isa("IO::Handle");
   28     $is_io    = Class->isa("IO::Handle");
   29 
   30     $does_log = $obj->DOES("Logger");
   31     $does_log = Class->DOES("Logger");
   32 
   33     $sub      = $obj->can("print");
   34     $sub      = Class->can("print");
   35 
   36     $sub      = eval { $ref->can("fandango") };
   37     $ver      = $obj->VERSION;
   38 
   39     # but never do this!
   40     $is_io    = UNIVERSAL::isa($fd, "IO::Handle");
   41     $sub      = UNIVERSAL::can($obj, "print");
   42 
   43 =head1 DESCRIPTION
   44 
   45 C<UNIVERSAL> is the base class from which all blessed references inherit.
   46 See L<perlobj>.
   47 
   48 C<UNIVERSAL> provides the following methods:
   49 
   50 =over 4
   51 
   52 =item C<< $obj->isa( TYPE ) >>
   53 
   54 =item C<< CLASS->isa( TYPE ) >>
   55 
   56 =item C<< eval { VAL->isa( TYPE ) } >>
   57 
   58 Where
   59 
   60 =over 4
   61 
   62 =item C<TYPE>
   63 
   64 is a package name
   65 
   66 =item C<$obj>
   67 
   68 is a blessed reference or a package name
   69 
   70 =item C<CLASS>
   71 
   72 is a package name
   73 
   74 =item C<VAL>
   75 
   76 is any of the above or an unblessed reference
   77 
   78 =back
   79 
   80 When used as an instance or class method (C<< $obj->isa( TYPE ) >>),
   81 C<isa> returns I<true> if $obj is blessed into package C<TYPE> or
   82 inherits from package C<TYPE>.
   83 
   84 When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes
   85 referred to as a static method), C<isa> returns I<true> if C<CLASS>
   86 inherits from (or is itself) the name of the package C<TYPE> or
   87 inherits from package C<TYPE>.
   88 
   89 If you're not sure what you have (the C<VAL> case), wrap the method call in an
   90 C<eval> block to catch the exception if C<VAL> is undefined.
   91 
   92 If you want to be sure that you're calling C<isa> as a method, not a class,
   93 check the invocand with C<blessed> from L<Scalar::Util> first:
   94 
   95   use Scalar::Util 'blessed';
   96 
   97   if ( blessed( $obj ) && $obj->isa("Some::Class") ) {
   98       ...
   99   }
  100 
  101 =item C<< $obj->DOES( ROLE ) >>
  102 
  103 =item C<< CLASS->DOES( ROLE ) >>
  104 
  105 C<DOES> checks if the object or class performs the role C<ROLE>.  A role is a
  106 named group of specific behavior (often methods of particular names and
  107 signatures), similar to a class, but not necessarily a complete class by
  108 itself.  For example, logging or serialization may be roles.
  109 
  110 C<DOES> and C<isa> are similar, in that if either is true, you know that the
  111 object or class on which you call the method can perform specific behavior.
  112 However, C<DOES> is different from C<isa> in that it does not care I<how> the
  113 invocand performs the operations, merely that it does.  (C<isa> of course
  114 mandates an inheritance relationship.  Other relationships include aggregation,
  115 delegation, and mocking.)
  116 
  117 By default, classes in Perl only perform the C<UNIVERSAL> role, as well as the
  118 role of all classes in their inheritance.  In other words, by default C<DOES>
  119 responds identically to C<isa>.
  120 
  121 There is a relationship between roles and classes, as each class implies the
  122 existence of a role of the same name.  There is also a relationship between
  123 inheritance and roles, in that a subclass that inherits from an ancestor class
  124 implicitly performs any roles its parent performs.  Thus you can use C<DOES> in
  125 place of C<isa> safely, as it will return true in all places where C<isa> will
  126 return true (provided that any overridden C<DOES> I<and> C<isa> methods behave
  127 appropriately).
  128 
  129 =item C<< $obj->can( METHOD ) >>
  130 
  131 =item C<< CLASS->can( METHOD ) >>
  132 
  133 =item C<< eval { VAL->can( METHOD ) } >>
  134 
  135 C<can> checks if the object or class has a method called C<METHOD>. If it does,
  136 then it returns a reference to the sub.  If it does not, then it returns
  137 I<undef>.  This includes methods inherited or imported by C<$obj>, C<CLASS>, or
  138 C<VAL>.
  139 
  140 C<can> cannot know whether an object will be able to provide a method through
  141 AUTOLOAD (unless the object's class has overridden C<can> appropriately), so a
  142 return value of I<undef> does not necessarily mean the object will not be able
  143 to handle the method call. To get around this some module authors use a forward
  144 declaration (see L<perlsub>) for methods they will handle via AUTOLOAD. For
  145 such 'dummy' subs, C<can> will still return a code reference, which, when
  146 called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD is provided,
  147 calling the coderef will cause an error.
  148 
  149 You may call C<can> as a class (static) method or an object method.
  150 
  151 Again, the same rule about having a valid invocand applies -- use an C<eval>
  152 block or C<blessed> if you need to be extra paranoid.
  153 
  154 =item C<VERSION ( [ REQUIRE ] )>
  155 
  156 C<VERSION> will return the value of the variable C<$VERSION> in the
  157 package the object is blessed into. If C<REQUIRE> is given then
  158 it will do a comparison and die if the package version is not
  159 greater than or equal to C<REQUIRE>, or if either C<$VERSION> or C<REQUIRE>
  160 is not a "lax" version number (as defined by the L<version> module).
  161 
  162 The return from C<VERSION> will actually be the stringified version object
  163 using the package C<$VERSION> scalar, which is guaranteed to be equivalent
  164 but may not be precisely the contents of the C<$VERSION> scalar.  If you want
  165 the actual contents of C<$VERSION>, use C<$CLASS::VERSION> instead.
  166 
  167 C<VERSION> can be called as either a class (static) method or an object
  168 method.
  169 
  170 =back
  171 
  172 =head1 WARNINGS
  173 
  174 B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
  175 C<isa> uses a very similar method and cache-ing strategy. This may cause
  176 strange effects if the Perl code dynamically changes @ISA in any package.
  177 
  178 You may add other methods to the UNIVERSAL class via Perl or XS code.
  179 You do not need to C<use UNIVERSAL> to make these methods
  180 available to your program (and you should not do so).
  181 
  182 =head1 EXPORTS
  183 
  184 None.
  185 
  186 Previous versions of this documentation suggested using C<isa> as
  187 a function to determine the type of a reference:
  188 
  189   $yes = UNIVERSAL::isa($h, "HASH");
  190   $yes = UNIVERSAL::isa("Foo", "Bar");
  191 
  192 The problem is that this code would I<never> call an overridden C<isa> method in
  193 any class.  Instead, use C<reftype> from L<Scalar::Util> for the first case:
  194 
  195   use Scalar::Util 'reftype';
  196 
  197   $yes = reftype( $h ) eq "HASH";
  198 
  199 and the method form of C<isa> for the second:
  200 
  201   $yes = Foo->isa("Bar");
  202 
  203 =cut