1package 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. 6require Exporter; 7*import = \&Exporter::import; 8@EXPORT_OK = qw(isa can); 9 101; 11__END__ 12 13=head1 NAME 14 15UNIVERSAL - 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 26C<UNIVERSAL> is the base class which all bless references will inherit from, 27see L<perlobj> 28 29C<UNIVERSAL> provides the following methods 30 31=over 4 32 33=item isa ( TYPE ) 34 35C<isa> returns I<true> if C<REF> is blessed into package C<TYPE> 36or inherits from package C<TYPE>. 37 38C<isa> can be called as either a static or object method call. 39 40=item can ( METHOD ) 41 42C<can> checks if the object has a method called C<METHOD>. If it does 43then a reference to the sub is returned. If it does not then I<undef> 44is returned. 45 46C<can> can be called as either a static or object method call. 47 48=item VERSION ( [ REQUIRE ] ) 49 50C<VERSION> will return the value of the variable C<$VERSION> in the 51package the object is blessed into. If C<REQUIRE> is given then 52it will do a comparison and die if the package version is not 53greater than or equal to C<REQUIRE>. 54 55C<VERSION> can be called as either a static or object method call. 56 57=back 58 59The C<isa> and C<can> methods can also be called as subroutines 60 61=over 4 62 63=item UNIVERSAL::isa ( VAL, TYPE ) 64 65C<isa> returns I<true> if one of the following statements is true. 66 67=over 8 68 69=item * 70 71C<VAL> is a reference blessed into either package C<TYPE> or a package 72which inherits from package C<TYPE>. 73 74=item * 75 76C<VAL> is a reference to a C<TYPE> of Perl variable (e.g. 'HASH'). 77 78=item * 79 80C<VAL> is the name of a package that inherits from (or is itself) 81package C<TYPE>. 82 83=back 84 85=item UNIVERSAL::can ( VAL, METHOD ) 86 87If C<VAL> is a blessed reference which has a method called C<METHOD>, 88C<can> returns a reference to the subroutine. If C<VAL> is not 89a blessed reference, or if it does not have a method C<METHOD>, 90I<undef> is returned. 91 92=back 93 94These subroutines should I<not> be imported via S<C<use UNIVERSAL qw(...)>>. 95If you want simple local access to them you can do 96 97 *isa = \&UNIVERSAL::isa; 98 99to import isa into your package. 100 101=cut 102