get shared_ptr which referes (and has ownership of) specified data member

get_shared_member.cc

#include <memory>

template<class T, class C>
std::shared_ptr<T> get_shared_member(
    std::shared_ptr<C> const& sp, T C::*mp) {
  if (!sp) {
    return {};
  }
  auto& x = *sp;
  return std::shared_ptr<T>(sp, std::addressof(x.*mp));
}
template<class T, class C>
std::shared_ptr<T const> get_shared_member(
    std::shared_ptr<C const> const& sp, T C::*mp) {
  return get_shared_member(std::const_pointer_cast<C>(sp), mp);
}

struct Hoge {
  int x;
  int const y;
};

int main() {
  auto p = std::make_shared<Hoge>(Hoge{1, 2});
  std::shared_ptr<Hoge const> cp = p;

  get_shared_member(p, &Hoge::x);
  get_shared_member(p, &Hoge::y);
  get_shared_member(cp, &Hoge::x);
  get_shared_member(cp, &Hoge::y);
}