Custom error category

error_code.cpp

class process_error_category : public std::error_category {
public:
  using error_category::error_category;

  const char* name() const noexcept override {
    return "";
  }

  std::string message(int _Errval) const override {
    return "";
  }
};

static std::error_condition error_of() {
  static process_error_category const cat;
  return std::error_condition(5, cat);
}