1UNIVERSAL(3pm) Perl Programmers Reference Guide UNIVERSAL(3pm)
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6 UNIVERSAL - base class for ALL classes (blessed references)
7
9 $is_io = $fd->isa("IO::Handle");
10 $is_io = Class->isa("IO::Handle");
11 $does_log = $obj->DOES("Logger");
13 $does_log = Class->DOES("Logger");
14 $sub = $obj->can("print");
16 $sub = Class->can("print");
17 $sub = eval { $ref->can("fandango") };
19 $ver = $obj->VERSION;
20 # but never do this!
22 $is_io = UNIVERSAL::isa($fd, "IO::Handle");
23 $sub = UNIVERSAL::can($obj, "print");
24
26 "UNIVERSAL" is the base class from which all blessed references
27 inherit. See perlobj.
28 "UNIVERSAL" provides the following methods:
30 "$obj->isa( TYPE )"
32 "CLASS->isa( TYPE )"
33 "eval { VAL->isa( TYPE ) }"
34 Where
35 "TYPE"
37 is a package name
38 $obj
40 is a blessed reference or a package name
41 "CLASS"
43 is a package name
44 "VAL"
46 is any of the above or an unblessed reference
47 When used as an instance or class method ("$obj->isa( TYPE )"),
49 "isa" returns true if $obj is blessed into package "TYPE" or
50 inherits from package "TYPE".
51 When used as a class method ("CLASS->isa( TYPE )", sometimes
53 referred to as a static method), "isa" returns true if "CLASS"
54 inherits from (or is itself) the name of the package "TYPE" or
55 inherits from package "TYPE".
56 If you're not sure what you have (the "VAL" case), wrap the method
58 call in an "eval" block to catch the exception if "VAL" is
59 undefined or an unblessed reference. The "isa" operator is an
60 alternative that simply returns false in this case, so the "eval"
61 is not needed.
62 If you want to be sure that you're calling "isa" as a method, not a
64 class, check the invocand with "blessed" from Scalar::Util first:
65 use Scalar::Util 'blessed';
67 if ( blessed( $obj ) && $obj->isa("Some::Class") ) {
69 ...
70 }
71 "$obj->DOES( ROLE )"
73 "CLASS->DOES( ROLE )"
74 "DOES" checks if the object or class performs the role "ROLE". A
75 role is a named group of specific behavior (often methods of
76 particular names and signatures), similar to a class, but not
77 necessarily a complete class by itself. For example, logging or
78 serialization may be roles.
79 "DOES" and "isa" are similar, in that if either is true, you know
81 that the object or class on which you call the method can perform
82 specific behavior. However, "DOES" is different from "isa" in that
83 it does not care how the invocand performs the operations, merely
84 that it does. ("isa" of course mandates an inheritance
85 relationship. Other relationships include aggregation, delegation,
86 and mocking.)
87 By default, classes in Perl only perform the "UNIVERSAL" role, as
89 well as the role of all classes in their inheritance. In other
90 words, by default "DOES" responds identically to "isa".
91 There is a relationship between roles and classes, as each class
93 implies the existence of a role of the same name. There is also a
94 relationship between inheritance and roles, in that a subclass that
95 inherits from an ancestor class implicitly performs any roles its
96 parent performs. Thus you can use "DOES" in place of "isa" safely,
97 as it will return true in all places where "isa" will return true
98 (provided that any overridden "DOES" and "isa" methods behave
99 appropriately).
100 "$obj->can( METHOD )"
102 "CLASS->can( METHOD )"
103 "eval { VAL->can( METHOD ) }"
104 "can" checks if the object or class has a method called "METHOD".
105 If it does, then it returns a reference to the sub. If it does
106 not, then it returns undef. This includes methods inherited or
107 imported by $obj, "CLASS", or "VAL".
108 "can" cannot know whether an object will be able to provide a
110 method through AUTOLOAD (unless the object's class has overridden
111 "can" appropriately), so a return value of undef does not
112 necessarily mean the object will not be able to handle the method
113 call. To get around this some module authors use a forward
114 declaration (see perlsub) for methods they will handle via
115 AUTOLOAD. For such 'dummy' subs, "can" will still return a code
116 reference, which, when called, will fall through to the AUTOLOAD.
117 If no suitable AUTOLOAD is provided, calling the coderef will cause
118 an error.
119 You may call "can" as a class (static) method or an object method.
121 Again, the same rule about having a valid invocand applies -- use
123 an "eval" block or "blessed" if you need to be extra paranoid.
124 "VERSION ( [ REQUIRE ] )"
126 "VERSION" will return the value of the variable $VERSION in the
127 package the object is blessed into. If "REQUIRE" is given then it
128 will do a comparison and die if the package version is not greater
129 than or equal to "REQUIRE", or if either $VERSION or "REQUIRE" is
130 not a "lax" version number (as defined by the version module).
131 The return from "VERSION" will actually be the stringified version
133 object using the package $VERSION scalar, which is guaranteed to be
134 equivalent but may not be precisely the contents of the $VERSION
135 scalar. If you want the actual contents of $VERSION, use
136 $CLASS::VERSION instead.
137 "VERSION" can be called as either a class (static) method or an
139 object method.
140
142 NOTE: "can" directly uses Perl's internal code for method lookup, and
143 "isa" uses a very similar method and cache-ing strategy. This may cause
144 strange effects if the Perl code dynamically changes @ISA in any
145 package.
146 You may add other methods to the UNIVERSAL class via Perl or XS code.
148 You do not need to "use UNIVERSAL" to make these methods available to
149 your program (and you should not do so).
150
152 None.
153 Previous versions of this documentation suggested using "isa" as a
155 function to determine the type of a reference:
156 $yes = UNIVERSAL::isa($h, "HASH");
158 $yes = UNIVERSAL::isa("Foo", "Bar");
159 The problem is that this code would never call an overridden "isa"
161 method in any class. Instead, use "reftype" from Scalar::Util for the
162 first case:
163 use Scalar::Util 'reftype';
165 $yes = reftype( $h ) eq "HASH";
167 and the method form of "isa" for the second:
169 $yes = Foo->isa("Bar");
171perl v5.36.0 2022-08-30 UNIVERSAL(3pm)