C++ Inheritance : Base attributes over various derived objects

Question

I'm currently learning about C++ Inheritance, so if this question is a dumb one, I apologize in advance.

Implementing this scenario:

1. A super class has a color attribute, which can be any color (let's assume color is represented by an integer).

2. Let's assume I have an initialization of this super class, with the color red.

3. I am also going to initialize different objects of a sub-class which also share the color red.

My question is, is there any way that I can initialize this attribute color to red (or any color for that matter) and it would be automatically inherited by objects of it's sub-class, instead of setting the attribute to red every-time I initialize one of these objects?

Again apologies if I'm missing a basic concept here, but I can't seem to find anything online on this.

Pseudo-Code per request:

Super-class code:

class Shape {

    int color;

    Shape::Shape(int c) : color(c) { }  //constructor

}

Sub-Class code:

class Square {

    int length, width;

    Square::Square(int l, int w, int c)
    :    length(l),
         width(w),
         color(c)
    { }
}

class Circle {

    int radius;

    Square::Square(int r, int c)
    :    radius(r),
         color(c)
    { }
}

What I'm trying to say is that both square and circle need to have the same color, is there anyway (maybe from the super class? ) to declare this color (ex. red), and both shapes would have this color set the same?

Answer 1

You could accomplish what you want with a static default_color that gets used when no color is explicitly specified, and gets set whenever a color is specified.

struct Shape {
    static int default_color;
    int color;
    Shape(int c) : color(c)
    {
        default_color = c;
    }
    Shape() : color(default_color) {}
};

Shape::default_color = BLACK;

struct Square : public Shape {
    int length, width;
    Square(int l, int w, int c)
    : Shape(c),
      length(l),
      width(w),
    { }

    Square(int l, int w)
    : length(l),
      width(w)
    { }
}

struct Circle : public Shape {
    int radius;
    Circle(int r, int c)
    : Shape(c),
      radius(r)
    { }

    Circle(int r)
    : radius(r)
    { }
}

int main()
{
    Square sq(2, 3, RED);
    Circle cir(10); // Automatically red, since that's
                       the last color explicitly specified
}

I would say this is a poor design though. Global state like this makes it very easy to make mistakes. You now have to think about the state of the entire program whenever you create a shape. It would be better to simply create the Circle as Circle cir(10, sq.color);. That makes it explicit what color your Circle is, and reduces cognitive load on the programmer.