Simple string tokenizer wrapper
Your constructor is nigh-identical to this up-voted answer for splitting a string.

The rest of the class doesn't add much value: IMO the vector provides the same API as your class and more besides; so a user might prefer to have the vector than to have your StringTokenizer instance which wraps/encapsulates/hides the vector.

vector.at returns a const reference, instead of returning by value: perhaps your methods should too.