QML Value Types | Qt QML 6.4.1

- Qt 6.4
- Qt QML
- QML Value Types
- Qt 6.4.1 Reference Documentation

# QML Value Types

QML supports built-in and custom value types.

A value type is one that is passed by value rather than by reference, such as an `int` or a `string`. This contrasts with QML Object Types. Object types are passed by reference. If you assign an instance of an object type to two different properties, both properties carry the same value. Modifying the object is reflected in both properties. If you assign an instance of a value type to two different properties, the properties carry separate values. If you modify one of them, the other one stays the same. Unlike an object type, a value type cannot be used to declare QML objects: it is not possible, for example, to declare an `int{}` object or a `size{}` object.

Value types can be used to refer to:

- A single value (e.g. int refers to a single number)
- A value that contains properties and methods (e.g. size refers to a value with `width` and `height` properties)
- The generic type var. It can hold values of any other type but is itself a value type.

When a variable or property holds a value type and it is assigned to another variable or property, then a copy of the value is made.

## Available Value Types

Some value types are supported by the engine by default and do not require an import statement to be used, while others do require the client to import the module which provides them. All of the value types listed below may be used as a `property` type in a QML document, with the following exceptions:

- `list` must be used in conjunction with an object or value type as element
- `enumeration` cannot be used directly as the enumeration must be defined by a registered QML object type

### Built-in Value Types Provided By The QML Language

The built-in value types supported natively in the QML language are listed below:

bool | Binary true/false value
date | Date value
double | Number with a decimal point, stored in double precision
enumeration | Named enumeration value
int | Whole number, e.g. 0, 10, or -20
list | List of QML objects
real | Number with a decimal point
string | Free form text string
url | Resource locator
var | Generic property type
variant | Generic property type
void | Empty value type

### Value Types Provided By QML Modules

QML modules may extend the QML language with more value types. For example, the value types provided by the `QtQuick` module are listed below:

color
font
matrix4x4
point | Value with x and y attributes
quaternion
rect | Value with x, y, width and height attributes
size | Value with width and height attributes
vector2d
vector3d
vector4d

The Qt global object provides useful functions for manipulating values of value types.

You may define your own value types as described in Defining QML Types from C++. In order to use types provided by a particular QML module, clients must import that module in their QML documents.

## Property Change Behavior for Value Types

Some value types have properties: for example, the font type has `pixelSize`, `family` and `bold` properties. Unlike properties of object types, properties of value types do not provide their own property change signals. It is only possible to create a property change signal handler for the value type property itself:

     Text {
         // invalid!
         onFont.pixelSizeChanged: doSomething()

         // also invalid!
         font {
             onPixelSizeChanged: doSomething()
         }

         // but this is ok
         onFontChanged: doSomething()
     }

Be aware, however, that a property change signal for a value type is emitted whenever any of its attributes have changed, as well as when the property itself changes. Take the following code, for example:

     Text {
         onFontChanged: console.log("font changed")

         Text { id: otherText }

         focus: true

         // changing any of the font attributes, or reassigning the property
         // to a different font value, will invoke the onFontChanged handler
         Keys.onDigit1Pressed: font.pixelSize += 1
         Keys.onDigit2Pressed: font.b = !font.b
         Keys.onDigit3Pressed: font = otherText.font
     }

In contrast, properties of an object type emit their own property change signals, and a property change signal handler for an object-type property is only invoked when the property is reassigned to a different object value.

See also The QML Type System.