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, as we 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.
12 @EXPORT_OK = qw(isa can VERSION);
14 # Make sure that even though the import method is called, it doesn't do
15 # anything unless called on UNIVERSAL.
17 return unless $_[0] eq __PACKAGE__;
18 goto &Exporter::import;
26 UNIVERSAL - base class for ALL classes (blessed references)
30 $is_io = $fd->isa("IO::Handle");
31 $is_io = Class->isa("IO::Handle");
33 $does_log = $obj->DOES("Logger");
34 $does_log = Class->DOES("Logger");
36 $sub = $obj->can("print");
37 $sub = Class->can("print");
39 $sub = eval { $ref->can("fandango") };
43 $is_io = UNIVERSAL::isa($fd, "IO::Handle");
44 $sub = UNIVERSAL::can($obj, "print");
48 C<UNIVERSAL> is the base class from which all blessed references inherit.
51 C<UNIVERSAL> provides the following methods:
55 =item C<< $obj->isa( TYPE ) >>
57 =item C<< CLASS->isa( TYPE ) >>
59 =item C<< eval { VAL->isa( TYPE ) } >>
71 is a blessed reference or a string containing a package name
79 is any of the above or an unblessed reference
83 When used as an instance or class method (C<< $obj->isa( TYPE ) >>),
84 C<isa> returns I<true> if $obj is blessed into package C<TYPE> or
85 inherits from package C<TYPE>.
87 When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes
88 referred to as a static method), C<isa> returns I<true> if C<CLASS>
89 inherits from (or is itself) the name of the package C<TYPE> or
90 inherits from package C<TYPE>.
92 If you're not sure what you have (the C<VAL> case), wrap the method call in an
93 C<eval> block to catch the exception if C<VAL> is undefined.
95 If you want to be sure that you're calling C<isa> as a method, not a class,
96 check the invocant with C<blessed> from L<Scalar::Util> first:
98 use Scalar::Util 'blessed';
100 if ( blessed( $obj ) && $obj->isa("Some::Class") {
104 =item C<< $obj->DOES( ROLE ) >>
106 =item C<< CLASS->DOES( ROLE ) >>
108 C<DOES> checks if the object or class performs the role C<ROLE>. A role is a
109 named group of specific behavior (often methods of particular names and
110 signatures), similar to a class, but not necessarily a complete class by
111 itself. For example, logging or serialization may be roles.
113 C<DOES> and C<isa> are similar, in that if either is true, you know that the
114 object or class on which you call the method can perform specific behavior.
115 However, C<DOES> is different from C<isa> in that it does not care I<how> the
116 invocant performs the operations, merely that it does. (C<isa> of course
117 mandates an inheritance relationship. Other relationships include aggregation,
118 delegation, and mocking.)
120 By default, classes in Perl only perform the C<UNIVERSAL> role. To mark that
121 your own classes perform other roles, override C<DOES> appropriately.
123 There is a relationship between roles and classes, as each class implies the
124 existence of a role of the same name. There is also a relationship between
125 inheritance and roles, in that a subclass that inherits from an ancestor class
126 implicitly performs any roles its parent performs. Thus you can use C<DOES> in
127 place of C<isa> safely, as it will return true in all places where C<isa> will
128 return true (provided that any overridden C<DOES> I<and> C<isa> methods behave
131 =item C<< $obj->can( METHOD ) >>
133 =item C<< CLASS->can( METHOD ) >>
135 =item C<< eval { VAL->can( METHOD ) } >>
137 C<can> checks if the object or class has a method called C<METHOD>. If it does,
138 then it returns a reference to the sub. If it does not, then it returns
139 I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
142 C<can> cannot know whether an object will be able to provide a method through
143 AUTOLOAD (unless the object's class has overriden C<can> appropriately), so a
144 return value of I<undef> does not necessarily mean the object will not be able
145 to handle the method call. To get around this some module authors use a forward
146 declaration (see L<perlsub>) for methods they will handle via AUTOLOAD. For
147 such 'dummy' subs, C<can> will still return a code reference, which, when
148 called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD is provided,
149 calling the coderef will cause an error.
151 You may call C<can> as a class (static) method or an object method.
153 Again, the same rule about having a valid invocant applies -- use an C<eval>
154 block or C<blessed> if you need to be extra paranoid.
156 =item C<VERSION ( [ REQUIRE ] )>
158 C<VERSION> will return the value of the variable C<$VERSION> in the
159 package the object is blessed into. If C<REQUIRE> is given then
160 it will do a comparison and die if the package version is not
161 greater than or equal to C<REQUIRE>.
163 C<VERSION> can be called as either a class (static) method or an object
172 You may request the import of three functions (C<isa>, C<can>, and C<VERSION>),
173 however it is usually harmful to do so. Please don't do this in new code.
175 For example, previous versions of this documentation suggested using C<isa> as
176 a function to determine the type of a reference:
180 $yes = isa $h, "HASH";
181 $yes = isa "Foo", "Bar";
183 The problem is that this code will I<never> call an overridden C<isa> method in
184 any class. Instead, use C<reftype> from L<Scalar::Util> for the first case:
186 use Scalar::Util 'reftype';
188 $yes = reftype( $h ) eq "HASH";
190 and the method form of C<isa> for the second:
192 $yes = Foo->isa("Bar");