4.5. Methods With Syntax Support

Vala recognizes some methods with certain names and signatures and provides syntax support for them. For example, if a type has a contains() method objects of this type can be used with the in operator. The following table lists these special methods. T and Tn are only type placeholders in this table and meant to be replaced with real types.

Indexers
T2 get(T1 index)	Index access: obj[index]
void set(T1 index, T2 item)	Index assignment: obj[index] = item

Indexers with multiple indices
T3 get(T1 index1, T2 index2)	Index access: obj[index1, index2]
void set(T1 index1, T2 index2, T3 item)	Index assignment: obj[ index1, index2] = item

Indexers
T2 get(T1 index)	Index access: obj[index]
void set(T1 index, T2 item)	Index assignment: obj[index] = item

Others
T slice(long start, long end)	Slicing: obj[start:end]
bool contains(T needle)	in operator: bool b = needle in obj
string to_string()	Support within string templates: @"$obj"
Iterator iterator()	Iterable via foreach
T2 get(T1 index) T1 size { get; }	Iterable via foreach

The Iterator type can have any name and must implement one of these two protocols:

bool next() T get()	Standard iterator protocol iterating until .next() returns false`. The current element retrieved via .get().
T? next_value()	Alternative iterator protocol: If the iterator object has a .next_value() function that returns a nullable type then we iterate by calling this function until it returns null.

This example implements some of these methods:

public class EvenNumbers {
    public int get(int index) {
        return index * 2;
    }

    public bool contains(int i) {
        return i % 2 == 0;
    }

    public string to_string() {
        return "[This object enumerates even numbers]";
    }

    public Iterator iterator() {
        return new Iterator(this);
    }

    public class Iterator {
        private int index;
        private EvenNumbers even;

        public Iterator(EvenNumbers even) {
            this.even = even;
        }

        public bool next() {
            return true;
        }

        public int get() {
            this.index++;
            return this.even[this.index - 1];
        }
    }
}

void main() {
    var even = new EvenNumbers();
    stdout.printf("%d\n", even[5]);   // get()
    if (4 in even) {                  // contains()
        stdout.printf(@"$even\n");    // to_string()
    }
    foreach (int i in even) {         // iterator()
        stdout.printf("%d\n", i);
        if (i == 20) break;
    }
}