ContentsVala Reference Manual

10. Classes

A class is definition of a data type. A class can contain fields, constants, methods, properties, and signals. Class types support inheritance, a mechanism whereby a derived class can extend and specialize a base class.

The simplest class declaration looks like this:

class ClassName {

As class types support inheritance, you can specify a base class you want to derive from. A derived class is-a superclass. It gets access to some of its methods etc. It can always be used in place of a and so on....

No classes can have multiple base classes, however GObject subclasses may implement multiple interfaces. By implementing an interface, a classed type has an is-a relationship with an interface. Whenever an instance of that interface is expected, an instance of this class will do.

10.1 Types of class

Vala supports three different types of class:

Any non-compact class can also be defined as abstract. An abstract class cannot be instantiated and is used as a base class for derived classes.

10.2 Types of class members

There are three fundamentally different types of class members, instance, class and static.

The distinction between class and static members is not common to other object models. The essential difference is that a sub-class will receive a copy of all its base classes' class members. This is opposed to static members, of which there is only one copy - sub classes access can their base classes' static members because they are automatically imported into the class' scope.

10.3 Class scope

Class scope is more complicated than other scopes, but conceptually the same. A class has a scope, which consists of its static and class members, as describe above. When an instance of the class is created, it is given its own scope, consisting of the defined instance members, with the class' scope as its parent scope.

Within the code of a class, the instance and class scopes are automatically searched as appropriate after the local scope, so no qualification is normally required. When there is a conflict with a name in the local scope, the this scope can be used, for example:

class ClassName {
        int field_name;
        void function_name(field_name) {
                this.field_name = field_name;

When a name is defined in a class which conflicts with one in a subclass, the base scope can be used, to refer to the scope of the subclass.

10.4 Class member visibility

All class members have a visibility. Visibility is declared using the following mutually exclusive modifiers:

class-member-visibility-modifier: private protected internal public

This defines whether the member is visible to code in different locations:

10.5 Class declaration

class-declaration: [ access-modifier ] class qualified-class-name [ inheritance-list ] { [ class-members ] } qualified-class-name: [ qualified-namespace-name . ] class-name class-name: identifier inheritance-list: : superclasses-and-interfaces superclasses-and-interfaces: ( qualified-class-name | qualified-interface-name ) [ , superclasses-and-interfaces ] class-members: class-member [ class-members ] class-member: class-creation-method-declaration class-constructor-declaration class-destructor-declaration class-constant-declaration class-delegate-declaration class-enum-declaration class-instance-member class-class-member class-static-member inner-class-declaration class-constructor-declaration: class-instance-constructor-declaration class-class-constructor-declaration class-static-constructor-declaration class-instance-member: class-instance-field-declaration class-instance-method-declaration class-instance-property-declaration class-instance-signal-declaration class-class-member: class-class-field-declaration class-class-method-declaration class-class-property-declaration class-static-member: class-static-field-declaration class-static-method-declaration class-static-property-declaration inner-class-declaration: [ access-modifier ] class class-name [ inheritance-list ] { [ class-members ] }

In Vala, a class must have either one or zero superclasses, where have zero superclasses has the result described in section. A class must meet all the prerequisites defined by the interfaces it wishes to implement, by implementing prerequisite interfaces or inheriting from a particular class. This latter requirement means it is potentially possible to have two interfaces that cannot be implemented by a single class.

When declaring which class, if any, a new class subclasses, and which interfaces it implements, the names of those other classes or interfaces can be qualified relative to the class being declared. This means that, for example, if the class is declared as "class foo.Bar" (class "Bar" in namespace "foo") then it may subclass class "Base" in namespace "foo" simply with "class foo.Bar : Base".

If an access modifier for the class is not given, the default "internal" is used.

It is possible to declare a class definition to be "abstract." An abstract class is one they may not be instantiated, instead it first be subclassed by a non-abstract ("concrete") class. An abstract class declaration may include abstract class instance members. These act as templates for methods or properties that must be implemented in all concrete subclasses of the abstract class. It is thus guaranteed that any instance of the abstract class (which must be in fact an instance of a concrete subclass) will have a method or property as described in the abstract class definition.

abstract-class-declaration: [ access-modifier ] abstractclass qualified-class-name [ inheritance-list ] { [ abstract-class-members ] } abstract-class-members: class-members class-instance-abstract-method-declaration class-instance-abstract-property-declaration

10.6 Controlling instantiation

When a class is instantiated, data might be required from the user to set initial properties. To define which properties should be or can be set at this stage, the class declaration should be written as:

class ClassName : GLib.Object {

        public ClassName() {

        public ClassName.with_some_quality (Property1Type property1value) {
                this.property1 = property1value;

This example allows the ClassName class to be instantiated either setting no properties, or setting the property. The convention is to name constructors as "with_" and then a description of what the extra properties will be used for, though following this is optional.

class-creation-method-declaration: [ class-member-visibility-modifier ] class-name [ . creation-method-name ] ( param-list ){ construction-assignments } class-name: identifier creation-method-name: identifier construction-assignments: this . property-name = param-name ;

class-name must be the same as the name of the class. If a creation method is given an extra name, this name is also used with instantiating the class, using the same syntax as for declaring the method, e.g. var a = new Button.with_label ("text").

If the property being set is construct type then assignment is made before construction, else afterwards.

Any number of these are allowed, but only one with each name (including null name.)

10.7 Construction

During instantiation, after construction properties have been set, a series of blocks of code are executed. This is the process that prepares the instance for use. There are three types of construct blocks that a class may define:

class-instance-constructor-declaration: construct{ statement-list }

Code in this block is executed on every instance of the class that is instantiated. It is run after construction properties have been set.

class-class-constructor-declaration: classconstruct{ statement-list }

This block will be executed once at the first use of its class, and once at the first use of each subclass of this class.

class-static-constructor-declaration: staticconstruct{ statement-list }

The first time that a class, or any subclass of it, is instantiated, this code is run. It is guaranteed that this code will run exactly once in a program where such a class is used.

The order of execution for constructors:

class-instance-destructor-declaration: ~ class-name (){ statement-list }

Destruction here. When does it happen? And when for each type of class?

10.8 Class fields

Fields act as variable with a scope of either the class or a particular instance, and therefore have names and types in the same way. Basic declarations are as:

class-instance-field-declaration: [ class-member-visibility-modifier ] qualified-type-name field-name [ = expression ] ; class-class-field-declaration: [ class-member-visibility-modifier ] class qualified-type-name field-name [ = expression ] ; class-static-field-declaration: [ class-member-visibility-modifier ] static qualified-type-name field-name [ = expression ] ;

Initial values are optional. FIXME: how much calculation can be done here? what are the defaults?

10.9 Class constants

Constants defined in a class are basically the same as those defined in a namespace. The only difference is the scope and the choice of visibilities available.

class-constant-declaration: [ class-member-visibility-modifier ] const qualified-type-name constant-name = expression ;

10.10 Class methods

Class methods are methods bound to a particularly class or class instance, i.e. they are executed within the scope of that class or class instance. They are declared the same way as other methods, but within the declaration of a class.

The same visibility modifiers can be used as for fields, although in this case they refer to what code can call the methods, rather than who can see or change values.

The static modifier is applicable to methods also. A static method is independent of any instance of the class. It is therefore only in the class scope, and may only access other static members.

class-instance-method-declaration: [ class-member-visibility-modifier ] [ class-method-type-modifier ] return-type method-name ( [ params-list ] ) method-contracts [ throws exception-list ] { statement-list } class-class-method-declaration: [ class-member-visibility-modifier ] class return-type method-name ( [ params-list ] ) method-contracts [ throws exception-list ] { statement-list } class-static-method-declaration: [ class-member-visibility-modifier ] static return-type method-name ( [ params-list ] ) method-contracts [ throws exception-list ] { statement-list } class-method-type-modifier: virtual override

Methods can be virtual, as described in Concepts/Object oriented programming. Methods in Vala classes are not virtual automatically, instead the "virtual" modifier must be used when it is needed. Virtual methods will only chain up if overridden using the override keyword.

Vala classes may also define abstract methods, by writing the declaration with the "abstract" modifier and replacing the method body with an empty statement ";". Abstract methods are not true methods, as they do not have an associated statement block, and so cannot be invoked. Abstract methods can only exist in abstract classes, and must be overridden in derived classes. For this reason an abstract method is always virtual. The purpose of an abstract method is to define methods that all non-abstract subclasses of the current definition must implement, it is therefore always allowable to invoke the method on an instance of the abstract class, because it is required that that instance must in fact be of a non-abstract subclass.

class-instance-abstract-method-declaration: [ class-member-visibility-modifier ] abstract return-type method-name ( [ params-list ] ) method-contracts [ throws exception-list ] ;

10.11 Properties

Properties are an enhanced version of fields. They allow custom code to be called whenever the property is retrieved or assigned to, but may be treated as fields by external Vala code. Properties also function like methods to some extent, and so can be defined as virtual and overridden in subclasses. Since they are also allowed in interfaces, they allow interfaces to declare data members that implementing classes must expose (see Interfaces.)


class-instance-property-declaration: [ class-member-visibility-modifier ] [ class-method-type-modifier ] qualified-type-name property-name { accessors [ default-value ] }; class-instance-abstract-property-declaration: [ class-member-visibility-modifier ] abstract qualified-type-name property-name { automatic-accessors }; class-class-property-declaration: [ class-member-visibility-modifier ] class qualified-type-name property-name { accessors [ default-value ] }; class-static-property-declaration: [ class-member-visibility-modifier ] static qualified-type-name property-name { accessors [ default-value ] }; property-name: identifier accessors: automatic-accessors [ getter ] [ setter ] [ property-constructor ] automatic-accessors: [ automatic-getter ] [ automatic-setter ] [ automatic-property-constructor ] automatic-getter: [ class-member-visibility-modifier ] get; automatic-setter: [ class-member-visibility-modifier ] set [ construct ] ; automatic-property-constructor: [ class-member-visibility-modifier ] construct; get-accessor: [ class-member-visibility-modifier ] get{ statement-list } set-accessor: [ class-member-visibility-modifier ] set [ construct ] { statement-list } property-constructor: [ class-member-visibility-modifier ] construct{ statement-list } default-value: default= expression ;

Execute Code on Setting/Getting Values 

Properties can either be declared with code that will perform particular actions on get and set, or can simply declare which actions are allowed and allow Vala to implement simple get and set methods. This second pattern (automatic property) will result in fields being added to the class to store values that the property will get and set. If either get or set has custom code, then the other must either be also written in full, or omitted altogether.

When a value is assigned to a property, the set block is invoked, with a parameter called value of the same type as the property. When a value is requested from a property, the get block is invoked, and must return an instance of the same type of the property.

Construct / Set Construct Block 

A property may have zero or one construct blocks. This means either a set construct block or a separate construct block. If this is the case that then the property becomes a construct property, meaning that if it is set in creation method, it will be set (using the construct block, as opposed to any simple set block, where there is a distinction) before class construct blocks are called.

Notify Changes Signals 

Managed properties may be annotated with Notify, See Attributes. This will cause the class instance to emit a notify signal when the property has been assigned to.

Virtual Properties 

Instance properties can be defined virtual with the same semantics as for virtual methods. If in an abstract class, an instance property can be defined as abstract. This is done using the "abstract" keyword on a declaration that is otherwise the same as an automatic property. It is then the responsibility of derived classes to implement the property by providing get or set blocks as appropriate. An abstract property is automatically virtual.

Abstract Properties 

As with methods, it is possible to declare abstract properties. These have much the same semantics as abstract methods, i.e. all non-abstract subclasses will have to implement properties with at least the accessors defined in the abstract property. Any set construct or construct accessor must be defined too in non-abstract classes and use override.

class-instance-abstract-property-declaration: [ class-member-visibility-modifier ] abstract qualified-type-name property-name { automatic-accessors };

10.12 Signals

Signals are a system allowing a classed-type instance to emit events which can be received by arbitrary listeners. Receiving these events is achieved by connecting the signal to a handler, for which Vala has a specific syntax. Signals are integrated with the GLib MainLoop system, which provides a system for queueing events (i.e. signal emissions,) when needed - though this capability is not needed non-threaded applications.

class-instance-signal-declaration: [ class-member-visibility-modifier ] [ class-method-type-modifier ] signal return-type signal-name ( [ params-list ] ); signal-name: identifier

Signals may also provide an extra piece of information called a signal detail. This is a single string, which can be used as an initial hint as to the purpose of the signal emission. In Vala you can register that a signal handler should only be invoked when the signal detail matches a given string. A typical use of signal details is in GObject's own "notify" signal, which says that a property of an object has changed - GObject uses the detail string to say which property has been changed.

To assign a handler to a signal, (or register to receive this type of event from the instance), use the following form of expression:

signal-connection-expression: qualified-signal-name [ signal-detail ] += signal-handler qualified-signal-name: [ qualified-namespace-name . ] variable-identifier . signal-name signal-detail: [ expression ] signal-handler: expression qualified-method-name lambda-expression

This expression will request that the signal handler given be invoked whenever the signal is emitted. In order for such a connection expression to be legal, the handler must have the correct signature. The handler should be defined to accept as parameters the same types as the signal, but with an extra parameter before. This parameter should have the type of the class in which the signal is declared. When a signal is emitted all handlers are called with this parameter being the object by which the signal was emitted.

The time that an arbitrary expression is acceptable in this expression is when that expression evaluates to an instance of a delegate type, i.e. to a method that is a legal handler for the signal. For details on delegates, see Delegates. For details on lambda expressions see Methods/Lambdas.

Note that optional signal detail should be directly appended to the signal name, with no white space, e.g. o.notify["name"] += ...

It is also possible to disconnect a signal handler using the following expression form:

signal-disconnection-expression: qualified-signal-name [ signal-detail ] -= connected-signal-handler connected-signal-handler: expression qualified-method-name

Note that you cannot disconnect a signal handler which was defined inline as a lambda expression and then immediately connected to the signal. If this is the effect you really need to achieve, you must assign the lambda expression to an identifier first, so that the lambda can be referred to again at a later time.

10.13 Class enums

Enums defined in a class are basically the same as those defined in a namespace. The only difference is the scope and the choice of visibilities available. See Enumerated types (Enums).

class-enum-declaration: [ class-member-visibility-modifier ] enum enum-name { [ enum-members ] }

10.14 Class delegates

Delegates defined in a class are basically the same as those defined in a namespace. The only difference is the scope and the choice of visibilities available. See Delegates.

class-delegate-declaration: [ class-member-visibility-modifier ] return-type delegate delegate-name ( method-params-list );

10.15 Examples


// ...

Using Properties 

For more examples see: Samples for Class Properties

Virtual Properties
namespace Properties {
    class Base : Object {
        protected int _number;
        public virtual int number { 
            get { 
                return this._number; 
            set { 
                this._number = value; 
     * This class just use Base class default handle 
     * of number property.
    class Subclass : Base {
        public string name { get; set; }
     * This class override how number is handle internally.
    class ClassOverride : Base {
        public override int number {
            get {
                return this._number;
            set {
                this._number = value * 3;
        public static int main (string[] args) {
            stdout.printf ("Implementing Virtual Properties...\n");
            var bc = new Base ();
            bc.number = 3;
            stdout.printf ("Class number = '" + bc.number.to_string () + "'\n");
            var sc = new Subclass ();
            sc.number = 3;
            stdout.printf ("Class number = '" + sc.number.to_string () + "'\n");
            var co = new ClassOverride ();
            co.number = 3;
            stdout.printf ("Class number = '" + co.number.to_string () + "'\n");
            return 0;
Abstract Properties
namespace Properties {
    abstract class Base : Object {
        public abstract string name { get; set construct; }
        construct {
   = "NO_NAME";
    class Subclass : Base {
        private string _name;
        public override string name {
            get {
                return this._name;
            set construct {
                this._name = value;
        /* This class have a default constructor that initializes
         * name as the construct block on Base, and a .with_name()
         * constructor where the user can set class derived name 
         * property.
        public Subclass.with_name (string name) {
            Object (name:name);
            this._name = name;
        public static int main (string[] args) {
            stdout.printf ("Implementing Abstract Properties...\n");
            var sc = new Subclass.with_name ("TEST_CLASS");
            stdout.printf ("Class name = '" + + "'\n");
            var sc2 = new Subclass ();
            stdout.printf ("Class name = '" + + "'\n");
            return 0;

Compile and run using:

# valac source.vala
# ./source

Using signals 

public class Test : Object {
        public signal void test (int data);

void delegate TestHandler (Test t, int data);

public static void main (string[] args) {

        Test t = new Test();

        TestHandler h = (t, data) => {
                stdout.printf("Data: %d\n", d);

        t.test ();
        t.test += h;
        t.test ();
        t.test -= h;
        t.test ();