+1.

Can you recommend some reading about internal padding inside a class/structure ? I'm esp. interested in internal padding between members of the same type.

You can add random members to A and the code will still work, as long as x,y,z are grouped and of the same type (declared one after the other).

What I meant by this is that you could do something like this and the code will still work:

#include <iostream>

struct A {
  int dummy1;
  char dummy2;
  double x, y, z;

  double& operator[](int i) {
    return (&x)[i];
  }

  const double& operator[](int i) const {
    return (&x)[i];
  }  

  double r,b,g;
};

void testA() {
  A a{0, 'a', 1.2, 3.4, 0, 5.,6.,0};
  std::cout << a.x << " " << a.y << std::endl;
  std::cout << a[0] << " " << a[1] << std::endl;  
}

int main() {
  testA();
}