Double to string sample of vala : Calling a string property that contains "DoubleToString" result from base class

DoubleToStringSample.vala

// The code below can be run and test in https://tio.run/#vala

public class Vehicle
{
    public double Speed {get; set;}
    public string SpeedText 
    {
        owned get{return DoubleToString(Speed, "%.0f");}
    }   

    public Vehicle()
    {

    }

    protected string DoubleToString(double num, string format)
    {
        char[] buf = new char[double.DTOSTR_BUF_SIZE];
        var str = num.format(buf, format);

        return str;
    }
}

public class Car : Vehicle
{
    public Car()
    {

    }
}

public static int main(string[] args) 
{
    var car = new Car();
    car.Speed = 1;	
    stdout.printf(car.SpeedText);
    
    return 0;
}