Strongly typed identifier

A UML class diagram for a strongly typed identifier.

A strongly typed identifier is user-defined data type which serves as an identifier or key that is strongly typed. This is a solution to the "primitive obsession" code smell as mentioned by Martin Fowler. The data type should preferably be immutable if possible. It is common for implementations to handle equality testing, serialization and model binding.

The strongly typed identifier commonly wraps the data type used as the primary key in the database, such as a string, an integer or universally unique identifier (UUID).

Web frameworks can often be configured to model bind properties on view models that are strongly typed identifiers. Object–relational mappers can often be configured with value converters to map data between the properties on a model using strongly typed identifier data types and database columns.

Examples

Passing a strongly typed identifier throughout the layers of an example application.

C#

C# have records which provide immutability and equality testing. ^[1] The record is sealed to prevent inheritance. ^[2] It overrides the built-in ToString() method. ^[3]

This example implementation includes a static method which can be used to initialize a new instance with a randomly generated globally unique identifier (GUID).

/// <summary>/// Represents a user identifier./// </summary>/// <param name="Id">The user identifier.</param>publicsealedrecordUserId(GuidId){/// <summary>/// Initializes a new instance of the <see cref="UserId" /> record./// </summary>/// <returns>A new UserId object.</returns>publicstaticUserIdNew()=>new(Guid.NewGuid());publicoverridestringToString()=>Id.ToString();}

C++

C++ has structs but not immutability so here the id field is marked as private with a method named getId() to get the value.

importstd;usingstd::string;classUserId{private:conststringid;public:explicitUserId(conststring&id):id{id}{}[[nodiscard]]stringgetId()constnoexcept{returnid;}[[nodiscard]]booloperator==(constUserId&rhs)constnoexcept{returnid==rhs.getId();}};

Crystal

Crystal's standard library provides the record macro for creating records which are immutable structs and lets you create override the built-in to_s method. ^[4]

require"uuid"# Represents a user identifier.recordUserId,id:Stringdodefinitialize()@id=UUID.v4.to_senddefto_s(io)io<<idenddefself.emptyself.new(UUID.empty.to_s)endend

D

D have immutable structs. ^[5]

importstd;/** Represents a user identifier. */immutablestructUserId{immutableUUIDid;/** Initializes a new instance of the UserId struct. */this(immutablestringid){this.id=UUID(id);}publicstaticUserIdcreate(){returnUserId(randomUUID.toString());}stringtoString(){returnthis.id.toString();}}

Dart

Dart have classes with operator overloading.

import'package:meta/meta.dart';/// Represents a user identifier.@immutablefinalclassUserId{finalStringid;/// Initializes a new instance of the UserId struct.constUserId(this.id);@overrideoperator==(other)=>otherisUserId&&other.id==id;@overrideintgethashCode=>id.hashCode;@overrideStringtoString()=>id;}

F#

F# lets you create override the Equals, GetHashCode and ToString methods.

openSystem/// <summary>/// Represents a user identifier./// </summary>/// <param name="id">The user identifier.</param>typeUserId(id:Guid)=memberx.id=idstaticmemberNew()=Guid.NewGuid()staticmemberEmpty=Guid.Emptyoverridex.Equals(b)=matchbwith|:?UserIdasp->id=p.id|_->falseoverridex.GetHashCode()=hashidoverridex.ToString()=id.ToString()

Go

Go have structs which provide equality testing. Go however does not provide immutability.

// Represents a user identifier.typeUserIdstruct{idstring}// Creates a new user identifier.funcNewUserId(idstring)UserId{returnUserId{id:id}}func(xUserId)String()string{returnx.id}

Groovy

Groovy have record classes which provide immutability and equality testing. ^[6]

/** * Represents a user identifier. * * @param id The user identifier. */recordUserId(Stringid){StringtoString(){id}}

Haskell

Haskell can create user-defined custom data types using the newtype keyword. ^[7] It provides equality testing using the Eq standard class and printing using the Read and Show standard classes.

-- Represents a user identifier.newtypeUserId=UserIdStringderiving(Eq,Read,Show)

Java

Java have records which provide equality testing. ^[8] The record is declared using the final modifier keyword to prevent inheritance. It overrides the built-in toString() method.

importjava.util.UUID;/** * Represents a user identifier. * @param id The user identifier. */publicfinalrecordUserId(UUIDid){/**     * Initializes a new instance of the UserId record.     * @return A new UserId object.     */publicstaticUserIdnewId(){returnnewUserId(UUID.randomUUID());}publicStringtoString(){returnid.toString();}}

JavaScript

This JavaScript example implementation provides the toJSON method used by the JSON.stringify() ^[9] function to serialize the class into a simple string instead of a composite data type. It calls Object.freeze() to make the instance immutable. ^[10] It overrides the built-in toString() method ^[11] and the valueOf() method. ^[12]

classUserId{#id;constructor(id){if(id==undefined){thrownewTypeError("Argument is null or undefined.");}this.#id=id;Object.freeze(this);}staticempty=newthis.prototype.constructor("00000000-0000-0000-0000-000000000000");staticnew(){returnnewthis.prototype.constructor(crypto.randomUUID());}equals(id){returnidinstanceofthis.constructor&&this.#id===id.valueOf();}toJSON(){returnthis.#id;}toString(){returnthis.#id;}valueOf(){returnthis.#id;}}

Julia

Julia have immutable composite data types. ^[13]

usingUUIDs"Represents a user identifier."structUserIdid::UUIDendBase.string(userId::UserId)=userId.id

Kotlin

Kotlin have "inline classes". ^[14]

/** * Represents a user identifier. * * @property id The user identifier. * @constructor Creates a user identifier. */@JvmInlinepublicvalueclassUserId(publicvalid:String){overridefuntoString()=id}

Nim

Nim have "distinct types". ^{[15] [16]}

## Represents a user identifier.typeUserId*=distinctstring

PHP

This PHP example implementation implements the __toString() magic method. ^[17] Furthermore, it implements the JsonSerializable interface which is used by the built-in json_encode function to serialize the class into a simple string instead of a composite data type. ^[18] The class is declared using the final modifier keyword to prevent inheritance. ^[19] PHP has traits as a way to re-use code. ^[20]

/** * Represents a user identifier. */finalclassUserIdimplementsJsonSerializable{useStronglyTypedIdentifier;}/** * Provides methods for use with strongly typed identifiers. */traitStronglyTypedIdentifier{/**     * Initializes a new instance of the UserId object.     * @param string $id The user identifier.     */publicfunction__construct(publicreadonlystring$id){}/**     * Creates a new user identifier.     */publicstaticfunctionnew():self{returnnewself(bin2hex(random_bytes(16)));}publicfunctionjsonSerialize():string{return$this->id;}publicfunction__toString():string{return$this->id;}}

Python

Python has data classes which provides equality testing and can be made immutable using the frozen parameter. ^[21] It overrides the __str__ dunder method. ^[22]

This example implementation includes a static method which can be used to initialize a new instance with a randomly generated universally unique identifier (UUID).

importuuidfromdataclassesimportdataclassfromuuidimportUUID@dataclass(frozen=True)classUserId:"""Represents a user identifier."""id:UUID@staticmethoddefnew()->Self:"""Create a new user identifier."""return__class__(uuid.uuid4())def__str__(self)->str:returnstr(self.id)

Python also has NewType which can be used to create new data types. ^[23]

fromtypingimportNewTypeUserId:NewType=NewType('UserId',int)

Raku

Raku have classes which provides equality testing and are immutable. It overrides the built-in Str method.

This example overrides the default gist method. ^[24] It uses roles which are mixed in into classes to be re-usable. ^[25]

roleStronglyTypedIdentifier {     hasStr$.idisrequired;      multimethodnew(Str$id) {         self.bless(:$id);     }      multimethodgist(StronglyTypedIdentifier:U:) { self.^name }     multimethodgist(StronglyTypedIdentifier:D:) { self.id }      methodStr() { "$!id" }      methodempty() {         self.new("00000000-0000-0000-0000-000000000000")     } }  classUserIddoesStronglyTypedIdentifier {} 

Ruby

Ruby have data classes which provides equality testing and are immutable. ^[26] It overrides the built-in to_s method.

This example implementation includes a static method which can be used to initialize a new instance with a randomly generated universally unique identifier (UUID).

require'securerandom'# Represents a user identifier.UserId=Data.define(:id)do# Create a new user identifier.defself.createself.new(SecureRandom.uuid)enddefself.emptyself.new('00000000-0000-0000-0000-000000000000')enddefto_sidendend

Rust

In Rust this can be done using a tuple struct containing a single value. ^[27] This example implementation implements the Debug ^[28] and the PartialEq ^[29] traits. The PartialEq trait provides equality testing.

// Represents a user identifier.#[derive(Debug, PartialEq)]pubstructUserId(String);

Scala

Scala have case classes which provide immutability and equality testing. ^[30] The case class is sealed to prevent inheritance.

importjava.util.UUID/** Represents a user identifier.  *  * @constructor  *   Create a new user identifier.  * @param id  *   The user identifier.  */sealedcaseclassUserId(id:UUID)objectUserId:/** Initializes a new instance of the UserId class. */defcreate():UserId=UserId(UUID.randomUUID())

Swift

Swift have the CustomStringConvertible protocol which can be used to provide its own representation to be used when converting an instance to a string, ^[31] and the Equatable protocol which provides equality testing. ^[32]

importFoundation/// Represents a user identifier.structUserId:CustomStringConvertible,Equatable{privateletid:UUIDinit(_id:UUID){self.id=id}vardescription:String{returnid.uuidString.lowercased}/// Creates a new user identifier.staticfuncnew()->Self{returnSelf(UUID())}}

Zig

Zig have structs ^[33] with constants but by design does not have operator overloading ^[34] and method overriding.

/// Represents a user identifier.constUserId=struct{value:i32,/// Initializes a new instance of the UserId struct.pubfninit(value:i32)UserId{returnUserId{.value=value};}};

See also

• Computer programming portal

• Domain-driven design
• Type safety
• Value object

References

1. "Records - C# reference". learn.microsoft.com. Retrieved 23 January 2023.
2. "sealed modifier - C# Reference". learn.microsoft.com. Retrieved 23 January 2023.
3. "Object.ToString Method (System)". learn.microsoft.com. Retrieved 14 June 2023.
4. "Structs - Crystal". crystal-lang.org. Retrieved 21 February 2024.
5. "Structs, Unions - D Programming Language". dlang.org. Retrieved 30 May 2023.
6. "The Apache Groovy programming language - Object orientation". groovy-lang.org. Retrieved 24 December 2023.
7. "Newtype - HaskellWiki". wiki.haskell.org. Retrieved 18 June 2023.
8. "Record Classes". Oracle Help Center. Retrieved 24 January 2023.
9. "JSON.stringify() - JavaScript | MDN". developer.mozilla.org. Retrieved 23 January 2023.
10. "Object.freeze() - JavaScript | MDN". developer.mozilla.org. Retrieved 23 January 2023.
11. "Object.prototype.toString() - JavaScript | MDN". developer.mozilla.org. Retrieved 23 January 2023.
12. "Object.prototype.valueOf() - JavaScript | MDN". developer.mozilla.org. Retrieved 23 January 2023.
13. "Types · The Julia Language". docs.julialang.org. Retrieved 30 May 2023.
14. "Inline classes | Kotlin". Kotlin Help. Retrieved 23 January 2023.
15. "Nim Manual". nim-lang.org. Retrieved 4 August 2023.
16. "Nim by Example - Distinct Types". nim-by-example.github.io. Retrieved 4 August 2023.
17. "PHP: Magic Methods - Manual". www.php.net. Retrieved 23 January 2023.
18. "PHP: JsonSerializable::jsonSerialize - Manual". www.php.net. Retrieved 23 January 2023.
19. "PHP: Final Keyword - Manual". www.php.net. Retrieved 23 January 2023.
20. "PHP: Traits - Manual". www.php.net. Retrieved 2 May 2023.
21. "dataclasses — Data Classes". Python documentation. Python Software Foundation . Retrieved 23 January 2023.
22. "3. Data model". Python documentation. Python Software Foundation . Retrieved 12 June 2023.
23. "typing — Support for type hints". Python documentation. Python Software Foundation . Retrieved 17 June 2023.
24. "Classes and objects | Raku Documentation". docs.raku.org. Retrieved 30 December 2025.
25. "Object orientation | Raku Documentation". docs.raku.org. Retrieved 30 December 2025.
26. "class Data - Documentation for Ruby 3.3". docs.ruby-lang.org. Retrieved 6 February 2023.
27. "New Type Idiom - Rust By Example". doc.rust-lang.org. Retrieved 18 June 2023.
28. "Debug in std::fmt - Rust". doc.rust-lang.org. Retrieved 23 January 2023.
29. "PartialEq in std::cmp - Rust". doc.rust-lang.org. Retrieved 23 January 2023.
30. "Case Classes". Scala Documentation. Retrieved 15 May 2023.
31. "CustomStringConvertible". Apple Developer Documentation. Retrieved 5 May 2023.
32. "Documentation". docs.swift.org. Retrieved 4 May 2023.
33. "Structs | zig.guide". zig.guide. 20 April 2024. Retrieved 15 October 2024.
34. "Documentation - The Zig Programming Language" . Retrieved 15 October 2024.

External links

• https://wiki.c2.com/?PrimitiveObsession