What is meant by the concept of composition in object-oriented programming?

In object-oriented programming, composition is a principle where one object contains references to other objects to utilize their functionality. It is a way of building complex objects from simpler ones, where one object 'consists' of others.

Composition differs from inheritance in that instead of extending a class through inheritance, an object includes other objects and delegates some of the work to them. This increases flexibility and code reusability.

Example in Swift:

class Engine {
    func start() {
        print("Engine started")
    }
}

class Car {
    private let engine = Engine()  // composition: Car contains Engine

    func startCar() {
        engine.start()  // delegate engine start
    }
}

let car = Car()
car.startCar()  // Outputs: Engine started

Here, the Car class consists of an Engine object and uses its methods without inheriting from it.