superstruct

import std.math, std.algorithm;

struct Square {
  float size;
  float area() { return size * size; }
}

struct Circle {
  float r;
  float area() { return r * r * PI; }
}

// Or can they?
alias Shape = SuperStruct!(Square, Circle);

// look! polymorphism!
Shape sqr = Square(2);
Shape cir = Circle(4);
Shape[] shapes = [ sqr, cir ];

// call functions that are shared between the source types!
assert(shapes.map!(x => x.area).sum.approxEqual(2 * 2 + 4 * 4 * PI));

struct Square {
  int top, left, width, height;
}

struct Circle {
  int radius;
  int x, y;

  auto top() { return y - radius; }
  auto top(int val) { return y = val + radius; }
}

alias Shape = SuperStruct!(Square, Circle);

// if a Shape is a Circle, `top` forwards to Circle's top property
Shape cir = Circle(4, 0, 0);
assert(cir.top = 6);
assert(cir.top == 6);

// if a Shape is a Square, `top` forwards to Squares's top field
Shape sqr = Square(0, 0, 4, 4);
assert(sqr.top = 6);
assert(sqr.top == 6);

// Square.left is hidden, as Circle has no such member
static assert(!is(typeof(sqr.left)));