1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
|
package UNIVERSAL;
our $VERSION = '1.01';
# UNIVERSAL should not contain any extra subs/methods beyond those
# that it exists to define. The use of Exporter below is a historical
# accident that can't be fixed without breaking code. Note that we
# *don't* set @ISA here, don't want all classes/objects inheriting from
# Exporter. It's bad enough that all classes have a import() method
# whenever UNIVERSAL.pm is loaded.
require Exporter;
*import = \&Exporter::import;
@EXPORT_OK = qw(isa can VERSION);
1;
__END__
=head1 NAME
UNIVERSAL - base class for ALL classes (blessed references)
=head1 SYNOPSIS
$is_io = $fd->isa("IO::Handle");
$is_io = Class->isa("IO::Handle");
$sub = $obj->can("print");
$sub = Class->can("print");
use UNIVERSAL qw( isa can VERSION );
$yes = isa $ref, "HASH" ;
$sub = can $ref, "fandango" ;
$ver = VERSION $obj ;
=head1 DESCRIPTION
C<UNIVERSAL> is the base class which all bless references will inherit from,
see L<perlobj>.
C<UNIVERSAL> provides the following methods and functions:
=over 4
=item C<< $obj->isa( TYPE ) >>
=item C<< CLASS->isa( TYPE ) >>
=item C<isa( VAL, TYPE )>
Where
=over 4
=item C<TYPE>
is a package name
=item C<$obj>
is a blessed reference or a string containing a package name
=item C<CLASS>
is a package name
=item C<VAL>
is any of the above or an unblessed reference
=back
When used as an instance or class method (C<< $obj->isa( TYPE ) >>),
C<isa> returns I<true> if $obj is blessed into package C<TYPE> or
inherits from package C<TYPE>.
When used as a class method (C<< CLASS->isa( TYPE ) >>: sometimes
referred to as a static method), C<isa> returns I<true> if C<CLASS>
inherits from (or is itself) the name of the package C<TYPE> or
inherits from package C<TYPE>.
When used as a function, like
use UNIVERSAL qw( isa ) ;
$yes = isa $h, "HASH";
$yes = isa "Foo", "Bar";
or
require UNIVERSAL ;
$yes = UNIVERSAL::isa $a, "ARRAY";
C<isa> returns I<true> in the same cases as above and also if C<VAL> is an
unblessed reference to a perl variable of type C<TYPE>, such as "HASH",
"ARRAY", or "Regexp".
=item C<< $obj->can( METHOD ) >>
=item C<< CLASS->can( METHOD ) >>
=item C<can( VAL, METHOD )>
C<can> checks if the object or class has a method called C<METHOD>. If it does
then a reference to the sub is returned. If it does not then I<undef> is
returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
C<VAL>.
C<can> cannot know whether an object will be able to provide a method
through AUTOLOAD, so a return value of I<undef> does not necessarily mean
the object will not be able to handle the method call. To get around
this some module authors use a forward declaration (see L<perlsub>)
for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can>
will still return a code reference, which, when called, will fall through
to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef
will cause an error.
C<can> can be called as a class (static) method, an object method, or a
function.
When used as a function, if C<VAL> is a blessed reference or package name which
has a method called C<METHOD>, C<can> returns a reference to the subroutine.
If C<VAL> is not a blessed reference, or if it does not have a method
C<METHOD>, I<undef> is returned.
=item C<VERSION ( [ REQUIRE ] )>
C<VERSION> will return the value of the variable C<$VERSION> in the
package the object is blessed into. If C<REQUIRE> is given then
it will do a comparison and die if the package version is not
greater than or equal to C<REQUIRE>.
C<VERSION> can be called as either a class (static) method, an object
method or a function.
=back
=head1 EXPORTS
None by default.
You may request the import of all three functions (C<isa>, C<can>, and
C<VERSION>), however it isn't usually necessary to do so. Perl magically
makes these functions act as methods on all objects. The one exception is
C<isa>, which is useful as a function when operating on non-blessed
references.
=cut
|
