695:, i.e. in a struct or a class in C/C++. In contrast, the unit type can be stored in records in functional programming languages, i.e. it can appear as the type of a field; the above implementation of the unit type in C++ can also be stored. While this may seem a useless feature, it does allow one for instance to elegantly implement a
422:' is used in a manner that simulates some, but not all, of the properties of the unit type, as detailed below. Like most imperative languages, C allows functions that do not return a value; these are specified as having the void return type. Such functions are called procedures in other imperative languages like
434:
The first notable difference between a true unit type and the void type is that the unit type may always be the type of the argument to a function, but the void type cannot be the type of an argument in C, despite the fact that it may appear as the sole argument in the list. This problem is best
541:
must be treated differently from other types. In C++ however, empty classes are allowed, so it is possible to implement a real unit type; the above example becomes compilable as:
124:
that is used in some imperative programming languages serves some of its functions, but because its carrier set is empty, it has some limitations (as detailed below).
414:
is used to designate a function that does not return anything useful, or a function that accepts no arguments. The unit type in C is conceptually similar to an empty
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703:
to the unit type; in the absence of a unit type, one can still implement a set this way by storing some dummy value of another type for each key.
1166:
1161:
1149:
1050:
940:(where a particular class has only one instance, but narrowly-typed non-nullable references to it are usually not held by other classes)
1007:
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that allows only one value (and thus can hold no information). The carrier (underlying set) associated with a unit type can be any
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unit type was added in C++17. Before that, it is possible to define a custom unit type using an empty struct such as
1327:
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418:, but a struct without members is not allowed in the C language specification (this is allowed in C++). Instead, '
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723:(like all other reference types), so it acts as a unit type. In practice, any other non-instantiable type, e.g.
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Statically typed languages give a type to every possible expression. They need to associate a type to the
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unit type and ignore the details of its value. One may also regard the unit type as the type of 0-
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The second notable difference is that the void type is special and can never be stored in a
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type carries no information, it is useless to pass it anyway; but it may arise in
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This issue does not arise in most programming practice in C, because since the
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In Java
Generics, type parameters must be reference types. The wrapper type
251:. In Python's optional static type annotations, this type is represented as
121:
55:
1016:
880:
For example in D, it's possible to declare functions that may only return
435:
illustrated by the following program, which is a compile-time error in C:
1462:
144:, and the argument type of a function that does not require arguments.
393:
368:
176:
71:
715:
is often used when a unit type parameter is needed. Although the
89:
of types and typed functions. It should not be confused with the
1080:
1020:
311:, the unit type is called null, which only value is NULL itself.
1032:
1075:
405:
308:
1028:
671:(For brevity, we're not worried in the above example whether
66:
between any two such sets, so it is customary to talk about
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type can never have any instances, it does have one value,
221:
is a unit type which has one value, namely the symbol
136:
provide a unit type to specify the result type of a
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8:
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19:This article is about the notion used in
873:expression. A type will be defined for
345:is a singleton with only one value: the
274:, reflecting the 0-tuple interpretation.
168:, reflecting the 0-tuple interpretation.
950:
225:. This should not be confused with the
27:. For types of measurement units, see
958:van Rossum, Guido; Levkivskyi, Ivan.
116:The unit type is implemented in most
7:
140:with the sole purpose of causing a
877:and it will only have this value.
14:
247:which allows the single value of
206:and its only value is written as
105:in this category. Similarly, the
430:Difference in calling convention
995:Types and Programming Languages
16:Type that allows only one value
1:
1117:Arbitrary-precision or bignum
966:. Python Software Foundation
964:Python Enhancement Proposals
683:for details on that issue.)
360:is the only instance of the
191:but the value is written as
296:, the unit type is written
270:and its only value is also
164:and its only value is also
1617:
506:// compile-time error here
334:) are built-in unit types.
281:, the unit type is called
262:, the unit type is called
202:, the unit type is called
160:, the unit type is called
18:
1458:Strongly typed identifier
928:, a unit type, can have.
243:, there is a type called
886:
733:
543:
437:
128:In programming languages
1533:Parametric polymorphism
924:is the only value that
175:descendants (including
960:"PEP 483 - Using None"
901:returnThatSpecialThing
384:Void type as unit type
285:and its only value is
187:), the type is called
118:functional programming
31:. For other uses, see
687:Difference in storage
134:programming languages
81:The unit type is the
33:Unit (disambiguation)
1002:. pp. 118–119.
29:Units of measurement
21:computer programming
1538:Primitive data type
1443:Recursive data type
1296:Algebraic data type
1172:Quadruple precision
990:Pierce, Benjamin C.
529:generic programming
330:(its only value is
322:(its only value is
1501:Abstract data type
1182:Extended precision
1141:Reduced precision
101:values and is the
40:mathematical logic
1581:
1580:
1313:Associative array
1177:Octuple precision
938:Singleton pattern
681:singleton pattern
300:and its value is
132:Several computer
109:is the type with
1606:
1553:Type constructor
1438:Opaque data type
1370:Record or Struct
1167:Double precision
1162:Single precision
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232:, which is the
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217:the type named
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120:languages. The
97:, which allows
83:terminal object
38:In the area of
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1511:Data structure
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1150:Half precision
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1137:Floating point
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544:
531:, such as C++
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377:struct empty{}
373:std::monostate
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103:initial object
62:. There is an
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1068:Uninterpreted
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1025:
1023:
1018:
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1009:0-262-16209-1
1005:
1001:
997:
996:
991:
987:
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885:
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732:
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542:
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530:
436:
429:
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127:
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100:
96:
92:
88:
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79:
78:of no types.
77:
73:
69:
65:
61:
60:singleton set
57:
53:
49:
45:
41:
34:
30:
26:
22:
1477:
1338:Intersection
1021:
993:
968:. Retrieved
963:
953:
926:typeof(null)
925:
921:
920:
881:
879:
874:
870:
868:
710:
690:
675:is really a
670:
522:
433:
387:
229:
131:
115:
110:
98:
90:
80:
67:
51:
37:
1599:Type theory
1568:Type theory
1563:Type system
1413:Bottom type
1360:Option type
1301:generalized
1187:Long double
1132:Fixed point
707:In Generics
693:record type
234:bottom type
215:Common Lisp
181:Standard ML
142:side effect
74:, i.e. the
64:isomorphism
48:type theory
25:type theory
1594:Data types
1588:Categories
1473:Empty type
1468:Type class
1418:Collection
1375:Refinement
1353:metaobject
1201:signedness
1060:Data types
983:References
316:JavaScript
302:struct{}{}
95:empty type
1548:Subtyping
1543:Interface
1526:metaclass
1478:Unit type
1448:Semaphore
1428:Exception
1333:Inductive
1323:Dependent
1288:Composite
1266:Character
1248:Reference
1145:Minifloat
1101:Bit array
1017:unit type
1000:MIT Press
865:Null type
677:singleton
606:unit_type
597:unit_type
576:unit_type
567:unit_type
558:unit_type
549:unit_type
533:templates
332:undefined
328:Undefined
122:void type
52:unit type
46:known as
1573:Variable
1463:Top type
1328:Equality
1236:physical
1213:Rational
1208:Interval
1155:bfloat16
992:(2002).
932:See also
673:the_unit
651:the_unit
618:the_unit
588:the_unit
561:the_unit
535:, where
362:NilClass
298:struct{}
245:NoneType
138:function
113:values.
87:category
1516:Generic
1492:Related
1408:Boolean
1365:Product
1241:virtual
1231:Address
1223:Pointer
1196:Integer
1127:Decimal
1122:Complex
1110:Numeric
1019:at the
970:2 March
349:object.
318:, both
150:Haskell
107:Boolean
85:in the
76:product
1506:Boxing
1494:topics
1453:Stream
1390:tagged
1348:Object
1271:String
1006:
907:return
889:typeof
829:String
817:static
814:public
802:return
778:static
775:public
763:return
739:static
736:public
679:; see
657:return
615:return
585:return
509:return
424:Pascal
416:struct
404:, and
371:, the
364:class.
339:Kotlin
326:) and
241:Python
183:, and
156:, and
72:tuples
1401:Other
1385:Union
1318:Class
1308:Array
1091:Tryte
945:Notes
699:as a
555:const
546:class
260:Swift
200:Scala
177:OCaml
54:is a
1521:Kind
1483:Void
1343:List
1258:Text
1096:Word
1086:Trit
1081:Byte
1004:ISBN
972:2024
922:null
910:null
895:null
882:null
875:null
871:null
853:null
832:args
823:main
820:void
805:null
790:Void
781:Void
766:null
751:Void
742:Void
729:null
725:Math
721:null
717:Void
713:Void
630:main
538:void
525:void
503:());
485:void
479:main
467:void
458:void
449:void
440:void
420:void
411:void
354:Ruby
347:Unit
343:Unit
324:null
320:Null
287:null
283:Void
279:Java
264:Void
253:None
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