Reference Loop C-CodePudding

I have a class Color, I want to define a constant of such class, and use that constant within the class. So the code may look like this:

class Color;

const Color BLACK = *new Color(0, 0, 0);
const Color WHITE = *new Color(255, 255, 255);

class Color {
    const int r;
    const int g;
    const int b;

    Color CloserBlackOrWhite() {
        int rgb = r   g   b;
        return rgb > (255*3/2) ? BLACK : WHITE;
    }
}

Of course, the issue with this is that BLACK/WHITE can't be defined because they're using an incomplete-type. I tried fixing this by placing the consts below the class decloration.

class Color {
    const int r;
    const int g;
    const int b;

    Color CloserBlackOrWhite() {
        int rgb = r   g   b;
        return rgb > (255*3/2) ? BLACK : WHITE;
    }
}

const Color BLACK = *new Color(0, 0, 0);
const Color WHITE = *new Color(255, 255, 255);

Except now in CloserBlackOrWhite is referencing an unknown value.

So, is there a way to declare the constants before the class decloration?

Or is there some other way to do this that I'm missing?

CodePudding user response：

This works and makes your class interface easier to read. Most of the time we want to move the actual functions to source files anyway.

    class Color {
      protected:
        const int r;
        const int g;
        const int b;

        public:
        Color(int r, int g, int b):r(r), g(g), b(b)
        {
        }

        Color CloserBlackOrWhite();
        };

    const Color BLACK = Color(0, 0, 0);
    const Color WHITE = Color(255, 255, 255);

    Color Color::CloserBlackOrWhite() {
        int rgb = r   g   b;
        return rgb > (255*3/2) ? BLACK : WHITE;
    }

I would add comparison operators also.

CodePudding user response：

You don't need memory allocation here. All you need is to forward-declare your constant colours, and forward-declare your class to be able to do the first step. You also want a public constructor, and at least some public methods.

All this gives you:

class Color;

extern const Color BLACK;
extern const Color WHITE;

class Color {
    const int r;
    const int g;
    const int b;

    public:
    Color(int r, int g, int b):r(r), g(g), b(b)
    {
    }

    Color CloserBlackOrWhite() {
        int rgb = r   g   b;
        return rgb > (255*3/2) ? BLACK : WHITE;
    }
};

const Color BLACK = Color(0, 0, 0);
const Color WHITE = Color(255, 255, 255);