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.
60 If you want to be sure that you're calling "isa" as a method, not a
62 class, check the invocand with "blessed" from Scalar::Util first:
63 use Scalar::Util 'blessed';
65 if ( blessed( $obj ) && $obj->isa("Some::Class") ) {
67 ...
68 }
69 "$obj->DOES( ROLE )"
71 "CLASS->DOES( ROLE )"
72 "DOES" checks if the object or class performs the role "ROLE". A
73 role is a named group of specific behavior (often methods of
74 particular names and signatures), similar to a class, but not
75 necessarily a complete class by itself. For example, logging or
76 serialization may be roles.
77 "DOES" and "isa" are similar, in that if either is true, you know
79 that the object or class on which you call the method can perform
80 specific behavior. However, "DOES" is different from "isa" in that
81 it does not care how the invocand performs the operations, merely
82 that it does. ("isa" of course mandates an inheritance
83 relationship. Other relationships include aggregation, delegation,
84 and mocking.)
85 By default, classes in Perl only perform the "UNIVERSAL" role, as
87 well as the role of all classes in their inheritance. In other
88 words, by default "DOES" responds identically to "isa".
89 There is a relationship between roles and classes, as each class
91 implies the existence of a role of the same name. There is also a
92 relationship between inheritance and roles, in that a subclass that
93 inherits from an ancestor class implicitly performs any roles its
94 parent performs. Thus you can use "DOES" in place of "isa" safely,
95 as it will return true in all places where "isa" will return true
96 (provided that any overridden "DOES" and "isa" methods behave
97 appropriately).
98 "$obj->can( METHOD )"
100 "CLASS->can( METHOD )"
101 "eval { VAL->can( METHOD ) }"
102 "can" checks if the object or class has a method called "METHOD".
103 If it does, then it returns a reference to the sub. If it does
104 not, then it returns undef. This includes methods inherited or
105 imported by $obj, "CLASS", or "VAL".
106 "can" cannot know whether an object will be able to provide a
108 method through AUTOLOAD (unless the object's class has overridden
109 "can" appropriately), so a return value of undef does not
110 necessarily mean the object will not be able to handle the method
111 call. To get around this some module authors use a forward
112 declaration (see perlsub) for methods they will handle via
113 AUTOLOAD. For such 'dummy' subs, "can" will still return a code
114 reference, which, when called, will fall through to the AUTOLOAD.
115 If no suitable AUTOLOAD is provided, calling the coderef will cause
116 an error.
117 You may call "can" as a class (static) method or an object method.
119 Again, the same rule about having a valid invocand applies -- use
121 an "eval" block or "blessed" if you need to be extra paranoid.
122 "VERSION ( [ REQUIRE ] )"
124 "VERSION" will return the value of the variable $VERSION in the
125 package the object is blessed into. If "REQUIRE" is given then it
126 will do a comparison and die if the package version is not greater
127 than or equal to "REQUIRE", or if either $VERSION or "REQUIRE" is
128 not a "lax" version number (as defined by the version module).
129 The return from "VERSION" will actually be the stringified version
131 object using the package $VERSION scalar, which is guaranteed to be
132 equivalent but may not be precisely the contents of the $VERSION
133 scalar. If you want the actual contents of $VERSION, use
134 $CLASS::VERSION instead.
135 "VERSION" can be called as either a class (static) method or an
137 object method.
138
140 NOTE: "can" directly uses Perl's internal code for method lookup, and
141 "isa" uses a very similar method and cache-ing strategy. This may cause
142 strange effects if the Perl code dynamically changes @ISA in any
143 package.
144 You may add other methods to the UNIVERSAL class via Perl or XS code.
146 You do not need to "use UNIVERSAL" to make these methods available to
147 your program (and you should not do so).
148
150 None.
151 Previous versions of this documentation suggested using "isa" as a
153 function to determine the type of a reference:
154 $yes = UNIVERSAL::isa($h, "HASH");
156 $yes = UNIVERSAL::isa("Foo", "Bar");
157 The problem is that this code would never call an overridden "isa"
159 method in any class. Instead, use "reftype" from Scalar::Util for the
160 first case:
161 use Scalar::Util 'reftype';
163 $yes = reftype( $h ) eq "HASH";
165 and the method form of "isa" for the second:
167 $yes = Foo->isa("Bar");
169perl v5.30.1 2019-11-29 UNIVERSAL(3pm)