3.6. Inheritance

In Vala, a class may derive from one or zero other classes. In practice this is always likely to be one, although there is no implicit inheritance as there is in languages like Java.

When defining a class that inherits from another, you create a relationship between the classes where instances of the subclass are also instances of the superclass. This means that operations on instances of the superclass are also applicable on instances of the subclass. As such, wherever an instance of the superclass is required, an instance of the subclass can be substituted.

When writing the definition of a class it is possible to exercise precise control over who can access what methods and data in the object. The following example demonstrates a range of these options:

class SuperClass : GLib.Object {

    private int data;

    public SuperClass(int data) {
        this.data = data;
    }

    protected void protected_method() {
    }

    public static void public_static_method() {
    }
}

class SubClass : SuperClass {

    public SubClass() {
        base(10);
    }
}

data is an instance data member of SuperClass. There will be a member of this type in every instance of SuperClass, and it is declared private so will only be accessible by code that is a part of SuperClass.

protected_method is an instance method of SuperClass. You will be able to execute this method only an instance of SuperClass or of one of its subclasses, and only from code that belongs to SuperClass or one of its subclasses - this latter rule being the result of the protected modifier.

public_static_method has two modifiers. The static modifier means that this method may be called without owning an instance of SuperClass or of one of its subclasses. As a result, this method will not have access to a this reference when it is executed. The public modifier means that this method can be called from any code, no matter its relationship with SuperClass or its subclasses.

Given these definitions, an instance of SubClass will contain all three members of SuperClass, but will only be able to access the non-private members. External code will only be able to access the public method.

With base a constructor of a subclass can chain up to a constructor of its base class.