3.13. Generics

Vala includes a runtime generics system, by which a particular instance of a class can be restricted with a particular type or set of types chosen at construction time. This restriction is generally used to require that data stored in the object must be of a particular type, for example in order to implement a list of objects of a certain type. In that example, Vala would make sure that only objects of the requested type could be added to the list, and that on retrieval all objects would be cast to that type.

In Vala, generics are handled while the program is running. When you define a class that can be restricted by a type, there still exists only one class, with each instance customised individually. This is in contrast to C++ which creates a new class for each type restriction required - Vala’s is similar to the system used by Java. This has various consequences, most importantly: that static members are shared by the type as a whole, regardless of the restrictions placed on each instance; and that given a class and a subclass, a generic refined by the subclass can be used as a generic refined by the class.

The following code demonstrates how to use the generics system to define a minimal wrapper class:

public class Wrapper<G>: GLib.Object {
    public G data { get; set; }
}

This “Wrapper” class must be restricted with a type in order to instantiate it - in this case the type will be identified as “G”, and so instances of this class will store one object of “G” type.

In order to instantiate this class, a type must be chosen, for example the built in string type (in Vala there is no restriction on what type may be used in a generic). To create an briefly use this class:

void main () {
    var w = new Wrapper<string>();
    w.data = "test";
    stdout.printf(w.data);
}

As you can see, when the data is retrieved from the wrapper, it is assigned to an identifier with no explicit type. This is possible because Vala knows what sort of objects are in each wrapper instance, and therefore can do this work for you.

The fact that Vala does not create multiple classes out of your generic definition means that you can code as follows: (This is called Type Erasure which is same in Java to avoid massive expansion in generated C code.)

class TestClass : GLib.Object {
}

void accept_object_wrapper(Wrapper<Glib.Object> w) {
}

...
var test_wrapper = new Wrapper<TestClass>();
accept_object_wrapper(test_wrapper);
...

Since all TestClass instances are also Objects, the “accept_object_wrapper” method will happily accept the object it is passed, and treat its wrapped object as though it was a GLib.Object instance.