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 should be fixed sometime.
9 *import = \&Exporter::import;
10 @EXPORT_OK = qw(isa can);
17 UNIVERSAL - base class for ALL classes (blessed references)
21 $io = $fd->isa("IO::Handle");
22 $sub = $obj->can('print');
24 $yes = UNIVERSAL::isa($ref, "HASH");
28 C<UNIVERSAL> is the base class which all bless references will inherit from,
31 C<UNIVERSAL> provides the following methods
37 C<isa> returns I<true> if C<REF> is blessed into package C<TYPE>
38 or inherits from package C<TYPE>.
40 C<isa> can be called as either a static or object method call.
44 C<can> checks if the object has a method called C<METHOD>. If it does
45 then a reference to the sub is returned. If it does not then I<undef>
48 C<can> cannot know whether an object will be able to provide a method
49 through AUTOLOAD, so a return value of I<undef> does not necessarily mean
50 the object will not be able to handle the method call. To get around
51 this some module authors use a forward declaration (see L<perlsub>)
52 for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can>
53 will still return a code reference, which, when called, will fall through
54 to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef
57 C<can> can be called as either a static or object method call.
59 =item VERSION ( [ REQUIRE ] )
61 C<VERSION> will return the value of the variable C<$VERSION> in the
62 package the object is blessed into. If C<REQUIRE> is given then
63 it will do a comparison and die if the package version is not
64 greater than or equal to C<REQUIRE>.
66 C<VERSION> can be called as either a static or object method call.
70 The C<isa> and C<can> methods can also be called as subroutines
74 =item UNIVERSAL::isa ( VAL, TYPE )
76 C<isa> returns I<true> if one of the following statements is true.
82 C<VAL> is a reference blessed into either package C<TYPE> or a package
83 which inherits from package C<TYPE>.
87 C<VAL> is a reference to a C<TYPE> of Perl variable (e.g. 'HASH').
91 C<VAL> is the name of a package that inherits from (or is itself)
96 =item UNIVERSAL::can ( VAL, METHOD )
98 If C<VAL> is a blessed reference which has a method called C<METHOD>,
99 C<can> returns a reference to the subroutine. If C<VAL> is not
100 a blessed reference, or if it does not have a method C<METHOD>,
101 I<undef> is returned.
105 These subroutines should I<not> be imported via S<C<use UNIVERSAL qw(...)>>.
106 If you want simple local access to them you can do
108 *isa = \&UNIVERSAL::isa;
110 to import isa into your package.