lib/perl5/5.6.1/UNIVERSAL.pm

   1 package UNIVERSAL;
   2
   3 # UNIVERSAL should not contain any extra subs/methods beyond those
   4 # that it exists to define. The use of Exporter below is a historical
   5 # accident that should be fixed sometime.
   6 require Exporter;
   7 *import = \&Exporter::import;
   8 @EXPORT_OK = qw(isa can);
   9
  10 1;
  11 __END__
  12
  13 =head1 NAME
  14
  15 UNIVERSAL - base class for ALL classes (blessed references)
  16
  17 =head1 SYNOPSIS
  18
  19     $io = $fd->isa("IO::Handle");
  20     $sub = $obj->can('print');
  21
  22     $yes = UNIVERSAL::isa($ref, "HASH");
  23
  24 =head1 DESCRIPTION
  25
  26 C<UNIVERSAL> is the base class which all bless references will inherit from,
  27 see L<perlobj>
  28
  29 C<UNIVERSAL> provides the following methods
  30
  31 =over 4
  32
  33 =item isa ( TYPE )
  34
  35 C<isa> returns I<true> if C<REF> is blessed into package C<TYPE>
  36 or inherits from package C<TYPE>.
  37
  38 C<isa> can be called as either a static or object method call.
  39
  40 =item can ( METHOD )
  41
  42 C<can> checks if the object has a method called C<METHOD>. If it does
  43 then a reference to the sub is returned. If it does not then I<undef>
  44 is returned.
  45
  46 C<can> can be called as either a static or object method call.
  47
  48 =item VERSION ( [ REQUIRE ] )
  49
  50 C<VERSION> will return the value of the variable C<$VERSION> in the
  51 package the object is blessed into. If C<REQUIRE> is given then
  52 it will do a comparison and die if the package version is not
  53 greater than or equal to C<REQUIRE>.
  54
  55 C<VERSION> can be called as either a static or object method call.
  56
  57 =back
  58
  59 The C<isa> and C<can> methods can also be called as subroutines
  60
  61 =over 4
  62
  63 =item UNIVERSAL::isa ( VAL, TYPE )
  64
  65 C<isa> returns I<true> if one of the following statements is true.
  66
  67 =over 8
  68
  69 =item *
  70
  71 C<VAL> is a reference blessed into either package C<TYPE> or a package
  72 which inherits from package C<TYPE>.
  73
  74 =item *
  75
  76 C<VAL> is a reference to a C<TYPE> of Perl variable (e.g. 'HASH').
  77
  78 =item *
  79
  80 C<VAL> is the name of a package that inherits from (or is itself)
  81 package C<TYPE>.
  82
  83 =back
  84
  85 =item UNIVERSAL::can ( VAL, METHOD )
  86
  87 If C<VAL> is a blessed reference which has a method called C<METHOD>,
  88 C<can> returns a reference to the subroutine.   If C<VAL> is not
  89 a blessed reference, or if it does not have a method C<METHOD>,
  90 I<undef> is returned.
  91
  92 =back
  93
  94 These subroutines should I<not> be imported via S<C<use UNIVERSAL qw(...)>>.
  95 If you want simple local access to them you can do
  96
  97   *isa = \&UNIVERSAL::isa;
  98
  99 to import isa into your package.
 100
 101 =cut