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
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
|
package UNIVERSAL;
our $VERSION = '1.04';
# 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, as we 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;
@EXPORT_OK = qw(isa can VERSION);
# Make sure that even though the import method is called, it doesn't do
# anything unless called on UNIVERSAL.
sub import {
return unless $_[0] eq __PACKAGE__;
goto &Exporter::import;
}
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");
$does_log = $obj->DOES("Logger");
$does_log = Class->DOES("Logger");
$sub = $obj->can("print");
$sub = Class->can("print");
$sub = eval { $ref->can("fandango") };
$ver = $obj->VERSION;
# but never do this!
$is_io = UNIVERSAL::isa($fd, "IO::Handle");
$sub = UNIVERSAL::can($obj, "print");
=head1 DESCRIPTION
C<UNIVERSAL> is the base class from which all blessed references inherit.
See L<perlobj>.
C<UNIVERSAL> provides the following methods:
=over 4
=item C<< $obj->isa( TYPE ) >>
=item C<< CLASS->isa( TYPE ) >>
=item C<< eval { VAL->isa( TYPE ) } >>
Where
=over 4
=item C<TYPE>
is a package name
=item C<$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>.
If you're not sure what you have (the C<VAL> case), wrap the method call in an
C<eval> block to catch the exception if C<VAL> is undefined.
If you want to be sure that you're calling C<isa> as a method, not a class,
check the invocant with C<blessed> from L<Scalar::Util> first:
use Scalar::Util 'blessed';
if ( blessed( $obj ) && $obj->isa("Some::Class") {
...
}
=item C<< $obj->DOES( ROLE ) >>
=item C<< CLASS->DOES( ROLE ) >>
C<DOES> checks if the object or class performs the role C<ROLE>. A role is a
named group of specific behavior (often methods of particular names and
signatures), similar to a class, but not necessarily a complete class by
itself. For example, logging or serialization may be roles.
C<DOES> and C<isa> are similar, in that if either is true, you know that the
object or class on which you call the method can perform specific behavior.
However, C<DOES> is different from C<isa> in that it does not care I<how> the
invocant performs the operations, merely that it does. (C<isa> of course
mandates an inheritance relationship. Other relationships include aggregation,
delegation, and mocking.)
By default, classes in Perl only perform the C<UNIVERSAL> role, as well as the
role of all classes in their inheritance. In other words, by default C<DOES>
responds identically to C<isa>.
There is a relationship between roles and classes, as each class implies the
existence of a role of the same name. There is also a relationship between
inheritance and roles, in that a subclass that inherits from an ancestor class
implicitly performs any roles its parent performs. Thus you can use C<DOES> in
place of C<isa> safely, as it will return true in all places where C<isa> will
return true (provided that any overridden C<DOES> I<and> C<isa> methods behave
appropriately).
=item C<< $obj->can( METHOD ) >>
=item C<< CLASS->can( METHOD ) >>
=item C<< eval { VAL->can( METHOD ) } >>
C<can> checks if the object or class has a method called C<METHOD>. If it does,
then it returns a reference to the sub. If it does not, then it returns
I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
C<VAL>.
C<can> cannot know whether an object will be able to provide a method through
AUTOLOAD (unless the object's class has overriden C<can> appropriately), 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.
You may call C<can> as a class (static) method or an object method.
Again, the same rule about having a valid invocant applies -- use an C<eval>
block or C<blessed> if you need to be extra paranoid.
=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 or an object
method.
=back
=head1 EXPORTS
None by default.
You may request the import of three functions (C<isa>, C<can>, and C<VERSION>),
however it is usually harmful to do so. Please don't do this in new code.
For example, previous versions of this documentation suggested using C<isa> as
a function to determine the type of a reference:
use UNIVERSAL 'isa';
$yes = isa $h, "HASH";
$yes = isa "Foo", "Bar";
The problem is that this code will I<never> call an overridden C<isa> method in
any class. Instead, use C<reftype> from L<Scalar::Util> for the first case:
use Scalar::Util 'reftype';
$yes = reftype( $h ) eq "HASH";
and the method form of C<isa> for the second:
$yes = Foo->isa("Bar");
=cut
|
