northwindfoo

  // Structure with a function `str`, as in the paper
  struct StrHolder {

    // Assume `str` returns a `const std::string&` when `this` is const
    // and `std::string&` otherwise.
    template <class Self>
    decltype(auto) str(this Self&& self) { ... }
  };


  // Let's use it here
  void main() {
    const StrHolder s;
    s.str().size();     // OK; calling a const function on `const std::string&`
    s.str() = "abc";    // Ill-formed; compilation error here
  }