5 # UNIVERSAL should not contain any extra subs/methods beyond those
6 # that it exists to define. The existence of import() below is a historical
7 # accident that can't be fixed without breaking code.
9 # Make sure that even though the import method is called, it doesn't do
10 # anything unless called on UNIVERSAL.
12 return unless $_[0] eq __PACKAGE__;
15 Carp::croak("UNIVERSAL does not export anything");
23 UNIVERSAL - base class for ALL classes (blessed references)
27 $is_io = $fd->isa("IO::Handle");
28 $is_io = Class->isa("IO::Handle");
30 $does_log = $obj->DOES("Logger");
31 $does_log = Class->DOES("Logger");
33 $sub = $obj->can("print");
34 $sub = Class->can("print");
36 $sub = eval { $ref->can("fandango") };
40 $is_io = UNIVERSAL::isa($fd, "IO::Handle");
41 $sub = UNIVERSAL::can($obj, "print");
45 C<UNIVERSAL> is the base class from which all blessed references inherit.
48 C<UNIVERSAL> provides the following methods:
52 =item C<< $obj->isa( TYPE ) >>
54 =item C<< CLASS->isa( TYPE ) >>
56 =item C<< eval { VAL->isa( TYPE ) } >>
68 is a blessed reference or a package name
76 is any of the above or an unblessed reference
80 When used as an instance or class method (C<< $obj->isa( TYPE ) >>),
81 C<isa> returns I<true> if $obj is blessed into package C<TYPE> or
82 inherits from package C<TYPE>.
84 When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes
85 referred to as a static method), C<isa> returns I<true> if C<CLASS>
86 inherits from (or is itself) the name of the package C<TYPE> or
87 inherits from package C<TYPE>.
89 If you're not sure what you have (the C<VAL> case), wrap the method call in an
90 C<eval> block to catch the exception if C<VAL> is undefined.
92 If you want to be sure that you're calling C<isa> as a method, not a class,
93 check the invocand with C<blessed> from L<Scalar::Util> first:
95 use Scalar::Util 'blessed';
97 if ( blessed( $obj ) && $obj->isa("Some::Class") ) {
101 =item C<< $obj->DOES( ROLE ) >>
103 =item C<< CLASS->DOES( ROLE ) >>
105 C<DOES> checks if the object or class performs the role C<ROLE>. A role is a
106 named group of specific behavior (often methods of particular names and
107 signatures), similar to a class, but not necessarily a complete class by
108 itself. For example, logging or serialization may be roles.
110 C<DOES> and C<isa> are similar, in that if either is true, you know that the
111 object or class on which you call the method can perform specific behavior.
112 However, C<DOES> is different from C<isa> in that it does not care I<how> the
113 invocand performs the operations, merely that it does. (C<isa> of course
114 mandates an inheritance relationship. Other relationships include aggregation,
115 delegation, and mocking.)
117 By default, classes in Perl only perform the C<UNIVERSAL> role, as well as the
118 role of all classes in their inheritance. In other words, by default C<DOES>
119 responds identically to C<isa>.
121 There is a relationship between roles and classes, as each class implies the
122 existence of a role of the same name. There is also a relationship between
123 inheritance and roles, in that a subclass that inherits from an ancestor class
124 implicitly performs any roles its parent performs. Thus you can use C<DOES> in
125 place of C<isa> safely, as it will return true in all places where C<isa> will
126 return true (provided that any overridden C<DOES> I<and> C<isa> methods behave
129 =item C<< $obj->can( METHOD ) >>
131 =item C<< CLASS->can( METHOD ) >>
133 =item C<< eval { VAL->can( METHOD ) } >>
135 C<can> checks if the object or class has a method called C<METHOD>. If it does,
136 then it returns a reference to the sub. If it does not, then it returns
137 I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
140 C<can> cannot know whether an object will be able to provide a method through
141 AUTOLOAD (unless the object's class has overridden C<can> appropriately), so a
142 return value of I<undef> does not necessarily mean the object will not be able
143 to handle the method call. To get around this some module authors use a forward
144 declaration (see L<perlsub>) for methods they will handle via AUTOLOAD. For
145 such 'dummy' subs, C<can> will still return a code reference, which, when
146 called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD is provided,
147 calling the coderef will cause an error.
149 You may call C<can> as a class (static) method or an object method.
151 Again, the same rule about having a valid invocand applies -- use an C<eval>
152 block or C<blessed> if you need to be extra paranoid.
154 =item C<VERSION ( [ REQUIRE ] )>
156 C<VERSION> will return the value of the variable C<$VERSION> in the
157 package the object is blessed into. If C<REQUIRE> is given then
158 it will do a comparison and die if the package version is not
159 greater than or equal to C<REQUIRE>, or if either C<$VERSION> or C<REQUIRE>
160 is not a "lax" version number (as defined by the L<version> module).
162 The return from C<VERSION> will actually be the stringified version object
163 using the package C<$VERSION> scalar, which is guaranteed to be equivalent
164 but may not be precisely the contents of the C<$VERSION> scalar. If you want
165 the actual contents of C<$VERSION>, use C<$CLASS::VERSION> instead.
167 C<VERSION> can be called as either a class (static) method or an object
174 B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
175 C<isa> uses a very similar method and cache-ing strategy. This may cause
176 strange effects if the Perl code dynamically changes @ISA in any package.
178 You may add other methods to the UNIVERSAL class via Perl or XS code.
179 You do not need to C<use UNIVERSAL> to make these methods
180 available to your program (and you should not do so).
186 Previous versions of this documentation suggested using C<isa> as
187 a function to determine the type of a reference:
189 $yes = UNIVERSAL::isa($h, "HASH");
190 $yes = UNIVERSAL::isa("Foo", "Bar");
192 The problem is that this code would I<never> call an overridden C<isa> method in
193 any class. Instead, use C<reftype> from L<Scalar::Util> for the first case:
195 use Scalar::Util 'reftype';
197 $yes = reftype( $h ) eq "HASH";
199 and the method form of C<isa> for the second:
201 $yes = Foo->isa("Bar");