There are multiple answers based on what you are doing with the string.
1) Using the string as an id (will not be modified). Passing it in by const reference is probably the best idea here: (std::string const&)
2) Modifying the string but not wanting the caller to see that change. Passing it in by value is preferable: (std::string)
3) Modifying the string but wanting the caller to see that change. Passing it in by reference is preferable: (std::string &)
4) Sending the string into the function and the caller of the function will never use the string again. Using move semantics might be an option (std::string &&)
