Commit | Line | Data |
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def3c102 | 1 | package UNIVERSAL; |
2 | ||
af048c18 | 3 | our $VERSION = '1.05'; |
b75c8c73 | 4 | |
84902520 TB |
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 | |
ea8fae29 | 7 | # accident that can't be fixed without breaking code. Note that we |
71ccbdc2 | 8 | # *don't* set @ISA here, as we don't want all classes/objects inheriting from |
ea8fae29 BS |
9 | # Exporter. It's bad enough that all classes have a import() method |
10 | # whenever UNIVERSAL.pm is loaded. | |
def3c102 | 11 | require Exporter; |
ea8fae29 | 12 | @EXPORT_OK = qw(isa can VERSION); |
def3c102 | 13 | |
2bfd5681 | 14 | # Make sure that even though the import method is called, it doesn't do |
71ccbdc2 | 15 | # anything unless called on UNIVERSAL. |
2bfd5681 MS |
16 | sub import { |
17 | return unless $_[0] eq __PACKAGE__; | |
b3f1e0ca | 18 | require warnings; |
1d9f57de RS |
19 | warnings::warnif( |
20 | 'deprecated', | |
21 | 'UNIVERSAL->import is deprecated and will be removed in a future perl', | |
22 | ); | |
2bfd5681 MS |
23 | goto &Exporter::import; |
24 | } | |
25 | ||
def3c102 | 26 | 1; |
27 | __END__ | |
28 | ||
29 | =head1 NAME | |
30 | ||
31 | UNIVERSAL - base class for ALL classes (blessed references) | |
32 | ||
33 | =head1 SYNOPSIS | |
34 | ||
7d1bbbe8 | 35 | $is_io = $fd->isa("IO::Handle"); |
36 | $is_io = Class->isa("IO::Handle"); | |
def3c102 | 37 | |
7d1bbbe8 | 38 | $does_log = $obj->DOES("Logger"); |
39 | $does_log = Class->DOES("Logger"); | |
ea8fae29 | 40 | |
7d1bbbe8 | 41 | $sub = $obj->can("print"); |
42 | $sub = Class->can("print"); | |
43 | ||
44 | $sub = eval { $ref->can("fandango") }; | |
45 | $ver = $obj->VERSION; | |
71ccbdc2 | 46 | |
47 | # but never do this! | |
7d1bbbe8 | 48 | $is_io = UNIVERSAL::isa($fd, "IO::Handle"); |
49 | $sub = UNIVERSAL::can($obj, "print"); | |
84902520 | 50 | |
def3c102 | 51 | =head1 DESCRIPTION |
52 | ||
71ccbdc2 | 53 | C<UNIVERSAL> is the base class from which all blessed references inherit. |
54 | See L<perlobj>. | |
def3c102 | 55 | |
71ccbdc2 | 56 | C<UNIVERSAL> provides the following methods: |
def3c102 | 57 | |
58 | =over 4 | |
59 | ||
a2b59c1f | 60 | =item C<< $obj->isa( TYPE ) >> |
ea8fae29 | 61 | |
71ccbdc2 | 62 | =item C<< CLASS->isa( TYPE ) >> |
ea8fae29 | 63 | |
71ccbdc2 | 64 | =item C<< eval { VAL->isa( TYPE ) } >> |
ea8fae29 | 65 | |
a2b59c1f CW |
66 | Where |
67 | ||
68 | =over 4 | |
69 | ||
70 | =item C<TYPE> | |
71 | ||
72 | is a package name | |
73 | ||
74 | =item C<$obj> | |
75 | ||
003db2bd | 76 | is a blessed reference or a package name |
a2b59c1f CW |
77 | |
78 | =item C<CLASS> | |
79 | ||
80 | is a package name | |
81 | ||
82 | =item C<VAL> | |
83 | ||
84 | is any of the above or an unblessed reference | |
85 | ||
86 | =back | |
87 | ||
88 | When used as an instance or class method (C<< $obj->isa( TYPE ) >>), | |
89 | C<isa> returns I<true> if $obj is blessed into package C<TYPE> or | |
90 | inherits from package C<TYPE>. | |
91 | ||
71ccbdc2 | 92 | When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes |
a2b59c1f CW |
93 | referred to as a static method), C<isa> returns I<true> if C<CLASS> |
94 | inherits from (or is itself) the name of the package C<TYPE> or | |
95 | inherits from package C<TYPE>. | |
ea8fae29 | 96 | |
71ccbdc2 | 97 | If you're not sure what you have (the C<VAL> case), wrap the method call in an |
98 | C<eval> block to catch the exception if C<VAL> is undefined. | |
def3c102 | 99 | |
71ccbdc2 | 100 | If you want to be sure that you're calling C<isa> as a method, not a class, |
101 | check the invocant with C<blessed> from L<Scalar::Util> first: | |
def3c102 | 102 | |
71ccbdc2 | 103 | use Scalar::Util 'blessed'; |
def3c102 | 104 | |
71ccbdc2 | 105 | if ( blessed( $obj ) && $obj->isa("Some::Class") { |
106 | ... | |
107 | } | |
def3c102 | 108 | |
7d1bbbe8 | 109 | =item C<< $obj->DOES( ROLE ) >> |
110 | ||
111 | =item C<< CLASS->DOES( ROLE ) >> | |
112 | ||
113 | C<DOES> checks if the object or class performs the role C<ROLE>. A role is a | |
114 | named group of specific behavior (often methods of particular names and | |
115 | signatures), similar to a class, but not necessarily a complete class by | |
116 | itself. For example, logging or serialization may be roles. | |
117 | ||
118 | C<DOES> and C<isa> are similar, in that if either is true, you know that the | |
119 | object or class on which you call the method can perform specific behavior. | |
120 | However, C<DOES> is different from C<isa> in that it does not care I<how> the | |
121 | invocant performs the operations, merely that it does. (C<isa> of course | |
122 | mandates an inheritance relationship. Other relationships include aggregation, | |
123 | delegation, and mocking.) | |
124 | ||
bcb8f0e8 RS |
125 | By default, classes in Perl only perform the C<UNIVERSAL> role, as well as the |
126 | role of all classes in their inheritance. In other words, by default C<DOES> | |
127 | responds identically to C<isa>. | |
7d1bbbe8 | 128 | |
129 | There is a relationship between roles and classes, as each class implies the | |
130 | existence of a role of the same name. There is also a relationship between | |
131 | inheritance and roles, in that a subclass that inherits from an ancestor class | |
132 | implicitly performs any roles its parent performs. Thus you can use C<DOES> in | |
133 | place of C<isa> safely, as it will return true in all places where C<isa> will | |
134 | return true (provided that any overridden C<DOES> I<and> C<isa> methods behave | |
135 | appropriately). | |
136 | ||
a2b59c1f CW |
137 | =item C<< $obj->can( METHOD ) >> |
138 | ||
139 | =item C<< CLASS->can( METHOD ) >> | |
140 | ||
71ccbdc2 | 141 | =item C<< eval { VAL->can( METHOD ) } >> |
ea8fae29 | 142 | |
71ccbdc2 | 143 | C<can> checks if the object or class has a method called C<METHOD>. If it does, |
144 | then it returns a reference to the sub. If it does not, then it returns | |
145 | I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or | |
ea8fae29 | 146 | C<VAL>. |
def3c102 | 147 | |
71ccbdc2 | 148 | C<can> cannot know whether an object will be able to provide a method through |
149 | AUTOLOAD (unless the object's class has overriden C<can> appropriately), so a | |
150 | return value of I<undef> does not necessarily mean the object will not be able | |
151 | to handle the method call. To get around this some module authors use a forward | |
152 | declaration (see L<perlsub>) for methods they will handle via AUTOLOAD. For | |
153 | such 'dummy' subs, C<can> will still return a code reference, which, when | |
154 | called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD is provided, | |
155 | calling the coderef will cause an error. | |
04b85669 | 156 | |
71ccbdc2 | 157 | You may call C<can> as a class (static) method or an object method. |
ea8fae29 | 158 | |
71ccbdc2 | 159 | Again, the same rule about having a valid invocant applies -- use an C<eval> |
160 | block or C<blessed> if you need to be extra paranoid. | |
def3c102 | 161 | |
a2b59c1f | 162 | =item C<VERSION ( [ REQUIRE ] )> |
def3c102 | 163 | |
164 | C<VERSION> will return the value of the variable C<$VERSION> in the | |
165 | package the object is blessed into. If C<REQUIRE> is given then | |
166 | it will do a comparison and die if the package version is not | |
167 | greater than or equal to C<REQUIRE>. | |
168 | ||
71ccbdc2 | 169 | C<VERSION> can be called as either a class (static) method or an object |
170 | method. | |
a66bc3b0 | 171 | |
def3c102 | 172 | =back |
173 | ||
ba593fa9 RS |
174 | =head1 WARNINGS |
175 | ||
176 | B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and | |
177 | C<isa> uses a very similar method and cache-ing strategy. This may cause | |
178 | strange effects if the Perl code dynamically changes @ISA in any package. | |
179 | ||
180 | You may add other methods to the UNIVERSAL class via Perl or XS code. | |
181 | You do not need to C<use UNIVERSAL> to make these methods | |
182 | available to your program (and you should not do so). | |
183 | ||
a2b59c1f | 184 | =head1 EXPORTS |
84902520 | 185 | |
a2b59c1f | 186 | None by default. |
84902520 | 187 | |
7d1bbbe8 | 188 | You may request the import of three functions (C<isa>, C<can>, and C<VERSION>), |
1d9f57de RS |
189 | B<but this feature is deprecated and will be removed>. Please don't do this in |
190 | new code. | |
71ccbdc2 | 191 | |
192 | For example, previous versions of this documentation suggested using C<isa> as | |
193 | a function to determine the type of a reference: | |
194 | ||
195 | use UNIVERSAL 'isa'; | |
196 | ||
197 | $yes = isa $h, "HASH"; | |
198 | $yes = isa "Foo", "Bar"; | |
199 | ||
200 | The problem is that this code will I<never> call an overridden C<isa> method in | |
201 | any class. Instead, use C<reftype> from L<Scalar::Util> for the first case: | |
202 | ||
203 | use Scalar::Util 'reftype'; | |
204 | ||
205 | $yes = reftype( $h ) eq "HASH"; | |
206 | ||
207 | and the method form of C<isa> for the second: | |
208 | ||
209 | $yes = Foo->isa("Bar"); | |
84902520 | 210 | |
def3c102 | 211 | =cut |