`toString()` with a Delegate Parameter

import std.stdio;
import std.string;

struct Point {
    int x;
    int y;

    string toString() const {
        return format("(%s,%s)", x, y);
    }
}

struct Color {
    ubyte r;
    ubyte g;
    ubyte b;

    string toString() const {
        return format("RGB:%s,%s,%s", r, g, b);
    }
}

struct ColoredPoint {
    Color color;
    Point point;

    string toString() const {
        /* Taking advantage of Color.toString and
         * Point.toString: */
        return format("{%s;%s}", color, point);
    }
}

struct Polygon {
    ColoredPoint[] points;

    string toString() const {
        /* Taking advantage of ColoredPoint.toString: */
        return format("%s", points);
    }
}

void main() {
    auto polygon = Polygon(
        [ ColoredPoint(Color(10, 10, 10), Point(1, 1)),
          ColoredPoint(Color(20, 20, 20), Point(2, 2)),
          ColoredPoint(Color(30, 30, 30), Point(3, 3)) ]);

    writeln(polygon);
}