<system_error2> does not use std::string to return possibly statically or dynamically allocated strings, and thus avoids dragging in a lot of the standard library which impacts build times.

Instead status code domains have a string_ref, which has a polymorphic implementation which may or may not manage a dynamic memory allocation using an atomic reference counter. Due to this polymorphism, you don’t need to worry which implementation is actually in use under the bonnet when you pass around string_ref instances.

string_ref provides the same member functions as a span<const char>, and so participates ordinarily in STL algorithms and containers. In particular, if you need to string search or slice it, you can construct a string_view on top easily.

  // Return the name of our custom code domain
  virtual _base::string_ref name() const noexcept override final  // NOLINT
  {
    static string_ref v("file i/o error domain");
    return v;  // NOLINT
  }

Now you understand what string_ref does, returning the name of the status code domain is self describing. Note we use the non-managing constructor of string_ref, as the string "file i/o error domain" is statically stored. We cache the returned value locally in static storage.