Nullable type

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Nullable types are a feature of some programming languages which allow the 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, while in dynamically-typed languages (where values have types, but variables do not), equivalent behavior is provided by having a single null value.

Data type classification of data in computer science

In computer science and computer programming, a data type or simply type is an attribute of data which tells the compiler or interpreter how the programmer intends to use the data. Most programming languages support common data types of real, integer and boolean. A data type constrains the values that an expression, such as a variable or a function, might take. This data type defines the operations that can be done on the data, the meaning of the data, and the way values of that type can be stored. A type of value from which an expression may take its value.

In programming languages and type theory, an option type or maybe type is a polymorphic type that represents encapsulation of an optional value; e.g., it is used as the return type of functions which may or may not return a meaningful value when they are applied. It consists of a constructor which either is empty, or which encapsulates the original data type A.

Contents

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.

Null (SQL) special marker and keyword in SQL

Null is a special marker used in Structured Query Language to indicate that a data value does not exist in the database. Introduced by the creator of the relational database model, E. F. Codd, SQL Null serves to fulfil the requirement that all true relational database management systems (RDBMS) support a representation of "missing information and inapplicable information". Codd also introduced the use of the lowercase Greek omega (ω) symbol to represent Null in database theory. In SQL, NULL is a reserved word used to identify this marker.

SQL is a domain-specific language used in programming and designed for managing data held in a relational database management system (RDBMS), or for stream processing in a relational data stream management system (RDSMS). It is particularly useful in handling structured data where there are relations between different entities/variables of the data. SQL offers two main advantages over older read/write APIs like ISAM or VSAM. First, it introduced the concept of accessing many records with one single command; and second, it eliminates the need to specify how to reach a record, e.g. with or without an index.

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 ]

Integer Number in {..., –2, –1, 0, 1, 2, ...}

An integer is a number that can be written without a fractional component. For example, 21, 4, 0, and −2048 are integers, while 9.75, 5 1/2, and 2 are not.

Example

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] In programming languages like C# 1.0, nullable types can be defined by an external library [2] as new types (e.g. NullableInteger, NullableBoolean). [3]

C Sharp (programming language) Multi-paradigm (object-oriented) programming language

C# is a general-purpose, multi-paradigm programming language encompassing strong typing, lexically scoped, imperative, declarative, functional, generic, object-oriented (class-based), and component-oriented programming disciplines. It was developed around 2000 by Microsoft as part of its .NET initiative, and later approved as an international standard by Ecma (ECMA-334) and ISO. Mono is the name of the free and open-source project to develop a compiler and runtime for the language. C# is one of the programming languages designed for the Common Language Infrastructure (CLI).

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

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". [4] 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.

Pointer (computer programming) programming language data type

In computer science, a pointer is a programming language object that stores the memory address of another value located in computer memory. A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer. As an analogy, a page number in a book's index could be considered a pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number and reading the text found on that page. The actual format and content of a pointer variable is dependent on the underlying computer architecture.

In computing, a null pointer or null reference has a value reserved for indicating that the pointer or reference does not refer to a valid object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown length or the failure to perform some action; this use of null pointers can be compared to nullable types and to the Nothing value in an option type.

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 ]

Java (programming language) Object-oriented programming language

Java is a general-purpose programming language that is class-based, object-oriented, and designed to have as few implementation dependencies as possible. It is intended to let application developers write once, run anywhere (WORA), meaning that compiled Java code can run on all platforms that support Java without the need for recompilation. Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of the underlying computer architecture. The syntax of Java is similar to C and C++, but it has fewer low-level facilities than either of them. As of 2019, Java was one of the most popular programming languages in use according to GitHub, particularly for client-server web applications, with a reported 9 million developers.

Compared with option types

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. Option type implementations usually adhere to the Special Case pattern. [5]

Language support

The following programming languages support nullable types.

Statically typed languages with native null support include:

Statically typed languages with library null support include:

Dynamically-typed languages with null include:

See also

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References

  1. "Nullable Types (C#)". Msdn.microsoft.com. Retrieved 2013-08-19.
  2. "(luKa) Developer Zone - NullableTypes". Nullabletypes.sourceforge.net. Retrieved 2013-08-19.
  3. "NullableTypes". Nullabletypes.sourceforge.net. Retrieved 2013-08-19.
  4. Tony Hoare (2009). "Null References: The Billion Dollar Mistake". QCon London.
  5. "P of EAA: Special Case". Martinfowler.com. Retrieved 2013-08-19.
  6. "Optional Type - Ballerina Programming Language".
  7. "Null Safety - Kotlin Programming Language".
  8. https://wiki.php.net/rfc/nullable_types
  9. https://docs.python.org/3/library/constants.html#None