Boost serialization storage question

Hello, I wondered how the Boost.Serialization library determines how many bytes to read from an input stream to "fill" a class with data. That is, does it read an entire input stream until the end of the stream and use that as the data to fill a class with, or does it perform a sizeof(class) and read that from the stream ? If it is the latter, then out of curiosity, how does it determine the size of an object with different class versions ( so if an object version 1 has been saved and then tries to load version 2 ) ? Does it write some sort of object size at the beginning of the file ? If it reads the entire stream until the end, what would be a good approach to save multiple objects in a single file, and be able to read just a few objects out of the file ? Thanks in advance for any responses. Regards, Leon Mergen