Typeof

typeof, alternately also typeOf, and TypeOf, is an operator provided by several programming languages to determine the data type of a variable. This is useful when constructing programs that must accept multiple types of data without explicitly specifying the type.

In languages that support polymorphism and type casting, the typeof operator may have one of two distinct meanings when applied to an object. In some languages, such as Visual Basic, ^[1] the typeof operator returns the dynamic type of the object. That is, it returns the true, original type of the object, irrespective of any type casting. In these languages, the typeof operator is the method for obtaining run-time type information.

In other languages, such as C# ^[2] or D ^[3] and, to some degree, in C (as part of nonstandard extensions and proposed standard revisions), ^{[4] [5]} the typeof operator returns the static type of the operand. That is, it evaluates to the declared type at that instant in the program, irrespective of its original form. These languages usually have other constructs for obtaining run-time type information, such as typeid.

Examples

C#

In C#:

// Given an object, returns if it is an integer.// The "is" operator can be also used to determine this.publicstaticboolIsInteger(objecto){returno.GetType()==typeof(int);}

C

As of C23 typeof is a part of the C standard. The operator typeof_unqual was also added which is the same as typeof, except it removes cvr-qualification and atomic qualification. ^{[6] [7]}

In a non-standard (GNU) extension of the C programming language, typeof may be used to define a general macro for determining the maximum value of two parameters:

#define max(a,b) ({ typeof (a) _a = (a); typeof (b) _b = (b); _a > _b ? _a : _b; })

Java

In Java the keyword is instanceof. ^[8]

booleanisNumber(Objectobj){returnobjinstanceofNumber;}

Note that obj instanceof Number is not the same as obj.getClass() == Number.class, as instanceof will return true with all subclasses of Number and the equality test will not.

There is a preview feature introduced in Java 23 that allows it to be used with primitives as well. ^[9]

// returns true when num can be converted to an int without losing informationbooleanisInt(floatnum){returnnuminstanceofint;}

JavaScript

In JavaScript:

functionisNumber(n){return(typeofn==='number');}

TypeScript

In TypeScript: ^[10]

function(param:typeofexistingObject){...}

letnewObject:typeofexistingObject;

VB.NET

In VB.NET, the C# variant of "typeof" should be translated into the VB.NET's GetType method. The TypeOf keyword in VB.NET is used to compare an object reference variable to a data type.

The following example uses TypeOf...Is expressions to test the type compatibility of two object reference variables with various data types.

DimrefIntegerAsObject=2MsgBox("TypeOf Object[Integer] Is Integer? "&TypeOfrefIntegerIsInteger)MsgBox("TypeOf Object[Integer] Is Double? "&TypeOfrefIntegerIsDouble)DimrefFormAsObject=NewSystem.Windows.Forms.FormMsgBox("TypeOf Object[Form] Is Form? "&TypeOfrefFormIsSystem.Windows.Forms.Form)MsgBox("TypeOf Object[Form] Is Label? "&TypeOfrefFormIsSystem.Windows.Forms.Label)MsgBox("TypeOf Object[Form] Is Control? "&TypeOfrefFormIsSystem.Windows.Forms.Control)MsgBox("TypeOf Object[Form] Is IComponent? "&TypeOfrefFormIsSystem.ComponentModel.IComponent)

See also

• sizeof
• decltype
• Type introspection

References

1. "TypeOf Operator (Visual Basic)". MSDN . Archived from the original on Nov 28, 2016.
2. "typeof (C#)". MSDN. Archived from the original on Sep 10, 2016.
3. "Declarations - D Programming Language 1.0". Digital Mars. Dec 30, 2012. Archived from the original on Oct 7, 2023.
4. "Typeof" in "Using the GNU Compiler Collection".
5. Meneide, JeanHeyd (2021-03-07). "Not-So-Magic - typeof(…) in C | r2". Open Standards. Retrieved 2021-12-02.
6. "N2927: Not-so-magic - typeof for C". Open Standards. 2022-02-02. Archived from the original on Dec 1, 2023.
7. "Consider renaming remove_quals" (PDF). Open Standards. 2022-02-06. Archived (PDF) from the original on Feb 17, 2024.
8. https://docs.oracle.com/javase/specs/jls/se23/html/jls-15.html#jls-15.20.2
9. https://docs.oracle.com/en/java/javase/23/docs/specs/primitive-types-in-patterns-instanceof-switch-jls.html
10. "Using 'typeof' to infer a type". Learn TypeScript. Retrieved 2022-01-28.