Try
string reversed(temp.rbegin(), temp.rend());
EDIT: Elaborating as requested.
string::rbegin() and string::rend(), which stand for "reverse begin" and "reverse end" respectively, return reverse iterators into the string. These are objects supporting the standard iterator interface (operator* to dereference to an element, i.e. a character of the string, and operator++ to advance to the "next" element), such that rbegin() points to the last character of the string, rend() points to the first one, and advancing the iterator moves it to the previous character (this is what makes it a reverse iterator).
Finally, the constructor we are passing these iterators into is a string constructor of the form:
template <typename Iterator>
string(Iterator first, Iterator last);
which accepts a pair of iterators of any type denoting a range of characters, and initializes the string to that range of characters.