How do I write an std::string getNoun(size_t) query?

std::string getNoun(size_t) 

It's a query that will return the noun at the index received in an object type of std::string, and if the index is invalid it will return an empty string. I'm not sure how to get a string from an index. For example; from "The dog jumped over the fence", if we do getNoun(1), it should return "dog" Any answers would help. Thanks!!