Q:

how to make deinit take effect in swift

I have a Car class. Let's say a car goes to the junkyard, this car should no longer be counted in the total population. I have the deinit function, but how do I systematically remove a car from the car population? In other words, how do I get the deinit to take effect?
I have a class variable isJunk but don't know how to use it to make this work.
class Car {
    static var population: Int = 0
    var isJunk: Bool = false
    var color: String
    var capacity: Int
    var driver: Bool?
    var carOn: Bool = false
    init (carColor: String, carCapacity: Int) {
        self.capacity = carCapacity
        self.color = carColor
        Car.population += 1

    }
    deinit {
        Car.population -= 1
    }

    func startCar() {
        self.carOn = true
    }
}

A:

class Car {
    static var population: Int = 0
    init() {
        Car.population += 1

    }
    deinit {
        Car.population -= 1
    }
}

var cars: [Car] = [Car(), Car()]
print("Population:", Car.population) // "Population: 2"

// now the second car is removed from array and we have no other references to it
// it gets removed from memory and deinit is called
cars.removeLast()
print("Population:", Car.population) // "Population: 1"

However, the same can be achieved just by asking the number of items in cars array. And that's usually a better alternative than a private counter of instances.
To keep the items in memory you will always need some kind of register (e.g. an array) for them. And that register can keep them counted.
One possibility:
class CarPopulation {
    var liveCars: [Car] = []
    var junkCars: [Car] = []
}

Or you can keep them in one array and set junk on the car and count non-junk cars when needed:
class CarPopulation {
    var cars: [Car] = []

    func liveCars() -> Int {
        return self.cars.filter { !$0.junk }.count
    }
}

There are many possibilities but extracting the counters to some other class that owns the cars is probably a better solution.

