In this case, passing the std::string to std::getline by reference is more than just an "out parameter": it is the buffer in which the result should be stored.

In this case, passing the std::string to std::getline by reference is more than just an "out parameter": it is the buffer in which the result should be stored. Of course this will be more efficient than allocating a new buffer for each line -- and that is a good reason why this particular interface would be worse off with multiple return values. The std::unordered_map::insert example in the article is an excellent example of an interface with multiple return values: it returns a pair (iterator, bool) of an iterator to the given key and a bool indicating if the iterator is to a newly inserted key-value pair or if it was the key-value pair already stored in the map. Neither the iterator nor the bool owns a buffer like in the std::getline example, so the std::getline out parameter design is not needed here.