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1 | package UNIVERSAL; |
2 | ||
3 | our $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. | |
11 | require Exporter; | |
12 | *import = \&Exporter::import; | |
13 | @EXPORT_OK = qw(isa can VERSION); | |
14 | ||
15 | 1; | |
16 | __END__ | |
17 | ||
18 | =head1 NAME | |
19 | ||
20 | UNIVERSAL - 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 | ||
37 | C<UNIVERSAL> is the base class which all bless references will inherit from, | |
38 | see L<perlobj>. | |
39 | ||
40 | C<UNIVERSAL> provides the following methods and functions: | |
41 | ||
42 | =over 4 | |
43 | ||
44 | =item C<< $obj->isa( TYPE ) >> | |
45 | ||
46 | =item C<< CLASS->isa( TYPE ) >> | |
47 | ||
48 | =item C<isa( VAL, TYPE )> | |
49 | ||
50 | Where | |
51 | ||
52 | =over 4 | |
53 | ||
54 | =item C<TYPE> | |
55 | ||
56 | is a package name | |
57 | ||
58 | =item C<$obj> | |
59 | ||
60 | is a blessed reference or a string containing a package name | |
61 | ||
62 | =item C<CLASS> | |
63 | ||
64 | is a package name | |
65 | ||
66 | =item C<VAL> | |
67 | ||
68 | is any of the above or an unblessed reference | |
69 | ||
70 | =back | |
71 | ||
72 | When used as an instance or class method (C<< $obj->isa( TYPE ) >>), | |
73 | C<isa> returns I<true> if $obj is blessed into package C<TYPE> or | |
74 | inherits from package C<TYPE>. | |
75 | ||
76 | When used as a class method (C<< CLASS->isa( TYPE ) >>: sometimes | |
77 | referred to as a static method), C<isa> returns I<true> if C<CLASS> | |
78 | inherits from (or is itself) the name of the package C<TYPE> or | |
79 | inherits from package C<TYPE>. | |
80 | ||
81 | When used as a function, like | |
82 | ||
83 | use UNIVERSAL qw( isa ) ; | |
84 | $yes = isa $h, "HASH"; | |
85 | $yes = isa "Foo", "Bar"; | |
86 | ||
87 | or | |
88 | ||
89 | require UNIVERSAL ; | |
90 | $yes = UNIVERSAL::isa $a, "ARRAY"; | |
91 | ||
92 | C<isa> returns I<true> in the same cases as above and also if C<VAL> is an | |
93 | unblessed reference to a perl variable of type C<TYPE>, such as "HASH", | |
94 | "ARRAY", or "Regexp". | |
95 | ||
96 | =item C<< $obj->can( METHOD ) >> | |
97 | ||
98 | =item C<< CLASS->can( METHOD ) >> | |
99 | ||
100 | =item C<can( VAL, METHOD )> | |
101 | ||
102 | C<can> checks if the object or class has a method called C<METHOD>. If it does | |
103 | then a reference to the sub is returned. If it does not then I<undef> is | |
104 | returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or | |
105 | C<VAL>. | |
106 | ||
107 | C<can> cannot know whether an object will be able to provide a method | |
108 | through AUTOLOAD, so a return value of I<undef> does not necessarily mean | |
109 | the object will not be able to handle the method call. To get around | |
110 | this some module authors use a forward declaration (see L<perlsub>) | |
111 | for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can> | |
112 | will still return a code reference, which, when called, will fall through | |
113 | to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef | |
114 | will cause an error. | |
115 | ||
116 | C<can> can be called as a class (static) method, an object method, or a | |
117 | function. | |
118 | ||
119 | When used as a function, if C<VAL> is a blessed reference or package name which | |
120 | has a method called C<METHOD>, C<can> returns a reference to the subroutine. | |
121 | If C<VAL> is not a blessed reference, or if it does not have a method | |
122 | C<METHOD>, I<undef> is returned. | |
123 | ||
124 | =item C<VERSION ( [ REQUIRE ] )> | |
125 | ||
126 | C<VERSION> will return the value of the variable C<$VERSION> in the | |
127 | package the object is blessed into. If C<REQUIRE> is given then | |
128 | it will do a comparison and die if the package version is not | |
129 | greater than or equal to C<REQUIRE>. | |
130 | ||
131 | C<VERSION> can be called as either a class (static) method, an object | |
132 | method or a function. | |
133 | ||
134 | ||
135 | =back | |
136 | ||
137 | =head1 EXPORTS | |
138 | ||
139 | None by default. | |
140 | ||
141 | You may request the import of all three functions (C<isa>, C<can>, and | |
142 | C<VERSION>), however it isn't usually necessary to do so. Perl magically | |
143 | makes these functions act as methods on all objects. The one exception is | |
144 | C<isa>, which is useful as a function when operating on non-blessed | |
145 | references. | |
146 | ||
147 | =cut |