c++property.cpp

#include <cstddef>
#include <cstdlib>

template <typename R, auto getter, auto setter, size_t (*offset)()>
struct property {
    inline R *self() {
        return reinterpret_cast<R *>(reinterpret_cast<size_t>(this) - offset());
    }
    inline operator auto() { return (self()->*getter)(); }
    inline void operator=(auto t) { (self()->*setter)(t); }
};

#define def_property(C, N, getter, setter)                                    \
    inline constexpr static size_t _##N##_offset() { return offsetof(C, N); } \
    [[no_unique_address]] property<C, &C::getter, &C::setter,                 \
                                   C::_##N##_offset>                          \
        N

struct X {
   private:
    int get_a();
    void set_a(int);

   public:
    int padding[25];
    def_property(X, a, get_a, set_a);
};

static_assert(sizeof(X) == 100, "!!");

int test(X &x, int val) {
    x.a = val;
    return x.a;
}