Simple constexpression string container

constexpr_string.cpp

  // Introduced by Scott Schurr in "C++ Now! 2012"
  class constexpr_string
  {
  protected: 
    const char* const p;
    const std::size_t s;
  public:
    template<std::size_t N>
    constexpr constexpr_string(const char(&a)[N])
      : p(a), s(N-1)
    {}
    constexpr char operator[](std::size_t n)
    {
      return n < s ? p[n] : throw std::out_of_range("");
    } 
    constexpr std::size_t size()
    { 
      return s;
    }
  };