1package UNIVERSAL; 2 3our $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. 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 C<< $obj->isa( TYPE ) >> 45 46=item C<< CLASS->isa( TYPE ) >> 47 48=item C<isa( VAL, TYPE )> 49 50Where 51 52=over 4 53 54=item C<TYPE> 55 56is a package name 57 58=item C<$obj> 59 60is a blessed reference or a string containing a package name 61 62=item C<CLASS> 63 64is a package name 65 66=item C<VAL> 67 68is any of the above or an unblessed reference 69 70=back 71 72When used as an instance or class method (C<< $obj->isa( TYPE ) >>), 73C<isa> returns I<true> if $obj is blessed into package C<TYPE> or 74inherits from package C<TYPE>. 75 76When used as a class method (C<< CLASS->isa( TYPE ) >>: sometimes 77referred to as a static method), C<isa> returns I<true> if C<CLASS> 78inherits from (or is itself) the name of the package C<TYPE> or 79inherits from package C<TYPE>. 80 81When used as a function, like 82 83 use UNIVERSAL qw( isa ) ; 84 $yes = isa $h, "HASH"; 85 $yes = isa "Foo", "Bar"; 86 87or 88 89 require UNIVERSAL ; 90 $yes = UNIVERSAL::isa $a, "ARRAY"; 91 92C<isa> returns I<true> in the same cases as above and also if C<VAL> is an 93unblessed reference to a perl variable of type C<TYPE>, such as "HASH", 94"ARRAY", or "Regexp". 95 96=item C<< $obj->can( METHOD ) >> 97 98=item C<< CLASS->can( METHOD ) >> 99 100=item C<can( VAL, METHOD )> 101 102C<can> checks if the object or class has a method called C<METHOD>. If it does 103then a reference to the sub is returned. If it does not then I<undef> is 104returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or 105C<VAL>. 106 107C<can> cannot know whether an object will be able to provide a method 108through AUTOLOAD, so a return value of I<undef> does not necessarily mean 109the object will not be able to handle the method call. To get around 110this some module authors use a forward declaration (see L<perlsub>) 111for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can> 112will still return a code reference, which, when called, will fall through 113to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef 114will cause an error. 115 116C<can> can be called as a class (static) method, an object method, or a 117function. 118 119When used as a function, if C<VAL> is a blessed reference or package name which 120has a method called C<METHOD>, C<can> returns a reference to the subroutine. 121If C<VAL> is not a blessed reference, or if it does not have a method 122C<METHOD>, I<undef> is returned. 123 124=item C<VERSION ( [ REQUIRE ] )> 125 126C<VERSION> will return the value of the variable C<$VERSION> in the 127package the object is blessed into. If C<REQUIRE> is given then 128it will do a comparison and die if the package version is not 129greater than or equal to C<REQUIRE>. 130 131C<VERSION> can be called as either a class (static) method, an object 132method or a function. 133 134 135=back 136 137=head1 EXPORTS 138 139None by default. 140 141You may request the import of all three functions (C<isa>, C<can>, and 142C<VERSION>), however it isn't usually necessary to do so. Perl magically 143makes these functions act as methods on all objects. The one exception is 144C<isa>, which is useful as a function when operating on non-blessed 145references. 146 147=cut 148