Design a SmartString class template whose only storage is a char array of STORAGE_SIZE bytes, with constant-time length() and a null-terminated c_str().

Design a class template SmartString that takes a size_t template parameter STORAGE_SIZE, defaulting to 16. The object's only storage is a char array of STORAGE_SIZE bytes: there is no separate member for the length. It must support a default constructor; a constructor from (const char* s, size_t len), where s is guaranteed to contain no null character; c_str(), returning a null-terminated const char*; length(), in constant time; and max_size(), which is STORAGE_SIZE - 1.