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nullable type wikipedia jump to content main menu main menu move to sidebar hide navigation main page contents current events random article about wikipedia contact us contribute help learn to edit community portal recent changes upload file special pages search search appearance donate create account log in personal tools donate create account log in contents move to sidebar hide top 1 example 2 compared with null pointers 3 compared with option types 4 language support 5 see also 6 references toggle the table of contents nullable type 3 languages 日本語 한국어 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia feature of some programming languages not to be confused with nullable symbol this article needs more citations please help improve this article by adding citations to reliable sources unsourced material may be challenged and removed find sources nullable type news newspapers books scholar jstor march 2009 learn how and when to remove this message nullable types are a feature of some programming languages which allow a value to be set to the special value null instead of the usual possible values of the data type in statically typed languages a nullable type is an option type citation needed while in dynamically typed languages where values have types but variables do not equivalent behavior is provided by having a single null value null is frequently used to represent a missing value or invalid value such as from a function that failed to return or a missing field in a database as in null in sql in other words null is undefined primitive types such as integers and booleans cannot generally be null but the corresponding nullable types nullable integer and nullable boolean respectively can also assume the null value jargon citation needed this can be represented in ternary logic as false null true as in three valued logic example edit an integer variable may represent integers but 0 zero is a special case because 0 in many programming languages can mean false also this does not provide any notion of saying that the variable is empty a need that arises in many circumstances this need can be achieved with a nullable type in programming languages like c 2 0 a nullable integer for example can be declared by a question mark int x 1 2 46 in programming languages like c 1 0 nullable types can be defined by an external library 3 as new types e g nullableinteger nullableboolean 4 a boolean variable makes the effect more clear its values can be either true or false while a nullable boolean may also contain a representation for undecided however the interpretation or treatment of a logical operation involving such a variable depends on the language the following is an example of t or explicitly system nullable t in c using system public class example static void main string args using shorthand int a 10 double b null using explicit nullable t nullable int c 20 nullable double d 5 5 checking if the nullable variables have values console writeline a hasvalue true console writeline b hasvalue false console writeline c hasvalue true console writeline d hasvalue true accessing the values if a hasvalue console writeline a value 10 if b hasvalue console writeline b value won t be executed because b is null compared with null pointers edit in contrast object pointers can be set to null by default in most common languages meaning that the pointer or reference points to nowhere that no object is assigned the variable does not point to any object nullable references were invented by c a r hoare in 1965 as part of the algol w language hoare later described his invention as a billion dollar mistake 5 this is because object pointers that can be null require the user to check the pointer before using it and require specific code to handle the case when the object pointer is null java has classes that correspond to scalar values such as integer boolean and float combined with autoboxing automatic usage driven conversion between object and value this effectively allows nullable variables for scalar values citation needed compared with option types edit nullable type implementations usually adhere to the null object pattern there is a more general and formal concept that extend the nullable type concept it comes from option types which enforce explicit handling of the exceptional case language support edit the following programming languages support nullable types statically typed languages with native null support include ballerina 6 c 7 dart 8 kotlin 9 ceylon sql sas missing values statically typed languages with library null support include c since version 2 2 5 delphi free pascal vb net 10 java since version 8 scala oxygene f statically typed cli languages dynamically typed languages with null include javascript has a null and undefined values julia has the nothing value which is of type nothing and the union t nothing type idiom 11 perl scalar variables default to undef and can be set to undef php with null type and is_null method native nullable type in version 7 1 12 python has the none value 13 ruby has the nil value and nilclass type see also edit null coalescing operator semipredicate problem union type unit type references edit nullable types c msdn microsoft com retrieved 2013 08 19 1 2 skeet jon 23 march 2019 c in depth manning isbn 978 1617294532 luka developer zone nullabletypes nullabletypes sourceforge net retrieved 2013 08 19 nullabletypes nullabletypes sourceforge net retrieved 2013 08 19 tony hoare 2009 null references the billion dollar mistake qcon london optional type ballerina programming language billwagner nullable value types c reference docs microsoft com retrieved 2021 10 05 sound null safety dart null safety kotlin programming language kathleendollard nullable value types visual basic docs microsoft com retrieved 2021 10 05 types the julia language php rfc nullable_types built in constants python 3 9 5 documentation v t e nulls in computing null character null device null function null modem null object pattern null pointer null in sql null string null coalescing operator see also null coalescing operator nullable type undefined value retrieved from https en wikipedia org w index php title nullable_type oldid 1359418248 category type theory hidden categories articles with short description short description is different from wikidata articles needing additional references from march 2009 all articles needing additional references all articles with unsourced statements articles with unsourced statements from october 2020 all articles that are too technical wikipedia articles that are too technical from july 2019 all articles needing expert attention articles needing expert attention from july 2019 articles with unsourced statements from july 2019 this page was last edited on 15 june 2026 at 04 51 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license additional terms may apply by using this site you agree to the terms of use and privacy policy wikipedia is a registered trademark of the wikimedia foundation inc a non profit organization privacy policy about wikipedia disclaimers contact wikipedia legal safety contacts code of conduct developers statistics cookie statement mobile view search search toggle the table of contents nullable type 3 languages add topic