This a the abstract base class for a persistent document. A document is created either by loading from a string or a file, or by constructing it manually when writing data to it. The document then can be saved to either string or to a file. A document contains one root ContainerNode where all data are stored under.
Document is read and written serially, hence the order of reading must be the same as the order of writing. The PersistentDocument class provides API to read and write to the root node, but for more flexible operations application can use the ContainerNode methods instead. Indeed the read and write API in PersistentDocument is just a shorthand which calls the relevant methods in the ContainerNode. As a tip, normally application only uses the readObject() and writeObject() methods declared here to read/write top level objects, and use the macros that are explained in ContainerNode documentation to read/write more detailed data.