Retrieve the type of an unnamed struct for using it in a member function

Question

How to get the type of an unnamed struct for using it in a member function?

struct {
  using P = int;  // What should be instead of int?

  bool operator<(const P& r) {
    return false;
  }
} obj1, obj2;

int main() {
  obj1 < obj2;
  return 0;
}

Answer 1

You can make the operator< a template, and constrain the type with std::enable_if . Eg

template <typename P>
auto operator<(const P& r) -> std::enable_if_t<std::is_same_v<P, std::remove_reference_t<decltype(*this)>>, bool> {
  return false;
}