#include <iostream>
#include <functional>
 
using namespace std;
 
#define RO_PROP(T, N, G) ro_prop<T> N = ro_prop<T>([this]() { return G; })
 
template <typename T> class ro_prop
{
  private:
    const function<T()> get;
 
  public:
    ro_prop(const function<T()> &&get) : get(get) {}
    operator T() { return get(); }
};
 
struct rect
{
  int width, height;
  RO_PROP(int, area, width * height);
};
 
int main()
{
  rect a { 2, 3 };
  int s = a.area;
 
  cout << s << endl;
 
  return 0;
}

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