27 Interfaces

Intermediate

An interface is used to define an abstract type. An interface contains no data but can define a behavior of a type. If a type declares that it implements an interface it must implement all member functions.

An interface can contain the following members:

Member function
Operator overloading function
Member property

These members are abstract. Implementing types must have the corresponding implementation of the members.

Here any class that implements interface Ia, must declare and define function f.

A class can implement multiple interfaces separated by &, as shown with class C.

interfaces.cj

interface Ia {
    func f(): Unit
}

interface Ib {
    func g(): Int64
}

class C <: Ia & Ib {
    public func f(): Unit {
        println("Function F is implemented")
    }
    public func g(): Int64 {
        println("Function G is implemented")
        return 5
    }
}

main() {
    let classInstance: C = C()
    classInstance.f()
    classInstance.g()
}

// Output:
// Function F is implemented
// Function G is implemented