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.
An integer variable may represent integers, but 0 (zero) is a special case because 0 in many programming languages can mean "false". Also this doesn't give us any notion of saying that the variable is empty, a need for which occurs 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.
Compared with null pointersedit
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]
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 supportedit
The following programming languages support nullable types.
Statically typed languages with native null support include:
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Not to be confused with nullable symbol This article needs additional citations for verification 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 template 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 Contents 1 Example 2 Compared with null pointers 3 Compared with option types 4 Language support 5 See also 6 ReferencesExample editAn integer variable may represent integers but 0 zero is a special case because 0 in many programming languages can mean false Also this doesn t give us any notion of saying that the variable is empty a need for which occurs 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 Compared with null pointers editIn 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 editNullable 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 editThe 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 languagesDynamically typed languages with null include 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 11 Python has the None value 12 Julia has the nothing value which is of type Nothing and the Union T Nothing type idiom 13 Ruby with nil value and NilClass type JavaScript has a null valueSee also editNull coalescing operator Semipredicate problem Union type Unit typeReferences edit Nullable Types C Msdn microsoft com Retrieved 2013 08 19 a b Skeet Jon 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 PHP RFC nullable types Built in Constants Python 3 9 5 documentation Types the Julia Language Retrieved from https en wikipedia org w index php title Nullable type amp oldid 1157650482, wikipedia, wiki, book, books, library,
