struct MyErrorCategory final : public std::error_category {
   MyErrorCategory() noexcept = default;
   const char* name() const noexcept { return "MyErrCat"; }
   std::string message(int) const noexcept  {return "message"; }
};

const MyErrorCategory& getMyCategory();

const auto& myErrorCategory = getMyCategory(); // caches reference


enum MyError {/*whatever*/};
// use reference myErrorCategory when creating error_code from MyError

// in .cpp, default Mayers singleton
const MyErrorCategory& getMyCategory() {
   static MyErrorCategory cat;
   return cat;
}

std::error_code make_code_1(int i) {
   return {i, getMyCategory()};
}

std::error_code make_code_2(int i) {
   return {i, myErrorCategory};
}