The strings library also has several other functions and a few operators that have been defined to work differently than they normally do. However, there is only one full class in that library. For further details:
// Bootiful information. I thought object would be referring to classes, glad it's usually said that way.
// My guess is that even though <string> is made up of one class, it's probably still also called a library.. though it makes more sense if it's a group of classes. But hey, it'd probably still be called a library for clarity, or else you'd be mixing the word library and class for the same thing.