>const Cell & Test() {
> return NULL; //I just want to tell something is wrong
There's no such thing as a null reference in C++. If all you want to do is return a single error state, you can follow the same pattern as the iostream library:
This returns a null pointer if the cell is null, or a pointer to the object if it's not. If you aren't comfortable working with overloaded type conversions, you can do the same thing with Test: