Re: Getting "this" to work similar to "self" in Python

[John Colvin via Digitalmars-d-learn]

On Wednesday, 22 July 2015 at 22:22:02 UTC, nurfz wrote:
How could I get this D code to work similar to this Python code?
So, here is the D code:

    import std.stdio;

    class Vehicle {
        int speed;
        void printSpeed() {
            writeln(this.speed);
        }
    }

    class Airplane: Vehicle {
        int speed = 100;
    }

    int main() {
        auto v = new Vehicle();
        auto a = new Airplane();

        v.printSpeed();  // 0
        a.printSpeed(); // 0 not 100

        writeln(v.speed); // 0
        writeln(a.speed); // 100
    }

Here is the Python code:

    class Vehicle:
        speed = 0
        def printSpeed(self):
            print(self.speed)

    class Airplane(Vehicle):
        speed = 100

    if __name__ == "__main__":
        v = Vehicle()
        a = Airplane()

        v.printSpeed()  # 0
        a.printSpeed()  # 100

        print(v.speed)  # 0
        print(a.speed)  # 100

I guess I'm confused as to why the D code isn't acting similar 
to the Python code in the sense that you would expect "this" to 
reference the "speed" property of the current instance and not 
statically reference the parent.  Am I having these issues 
because these attributes are being initialized statically?

Would using constructors be the way to go about this? I suppose 
I'm just trying to find a way to implement fairly clean and 
intuitive object oriented inheritance that isn't crippled by 
getters/setters, is resolved at compile time, and doesn't 
impose any kind of runtime cost other than what you would 
assume is associated with fundamental level OOP.

Sorry for the long winded post, but this has just been 
confusing me to no end.  Hopefully you guys can help me out! :)

Fields of classes are not in any way polymorphic in D (this is 
the same as C++ and I think java too). Base class members can be 
accessed like so:

class Vehicle {
    int speed;
    void printSpeed() {
        writeln(this.speed);
    }
}

class Airplane: Vehicle {
    this()
    {
        Vehicle.speed = 100;
    }
}

or if you really want to use the same variable name:

class Airplane: Vehicle {
    alias speed = Vehicle.speed;
    this()
    {
        speed = 100;
    }
}

You can even automatically do that sort of thing by various 
means, the shortest/simplest way I can think of would be:

class Airplane: Vehicle {
    private @property Vehicle base() { return this; }
    alias base this;
    this()
    {
        speed = 100;
    }
}