Custom types

GCM allows to define custom types using the keyword type. A custom type is similar to what you can declare in GCM as a class. However, there are a few important differences.

First and most importantly, a custom type can only be referenced from other classes as an attribute, i.e. via att, as shown in the following example:

type GPSPosition {
    att lat : Double
    att lng : Double

class Building {
    att location : GPSPosition

This has a major implication: the custom type is embedded in the encompassing class, meaning that access to the custom type is synchronous instead as for relations and references asynchronous. Custom types can only contain attributes by themselves.

The following code shows what the GCM generator generates for a declared custom type:

public class GPSPosition extends BaseCustomTypeSingle {

    private static final String LAT = "lat";
    private static final int LAT_H = HashHelper.hash(LAT);
    private static final String LNG = "lng";
    private static final int LNG_H = HashHelper.hash(LNG);

    public GPSPosition(EStructArray p_backend) {

    public void init() {

    public final double lat() {
        return (double) getAt(LAT_H);

    public final double lng() {
        return (double) getAt(LNG_H);

    public final void setPosition(final double lat, final double lng) {
        this._backend.estruct(DEF_NODE).setAt(LAT_H, Type.DOUBLE, lat).setAt(LNG_H, Type.DOUBLE, lng);

    public String toString() {
        return "position(" + lat() + "," + lng() + ")";


Next, the following example how to work with custom types:

    SmartCity smartCity = SmartCity.create(0, 0, graph);
    GPSPosition pos = smartCity.getLocation();

    pos = smartCity.getOrCreateLocation();

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