|Important||This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here. ArchiveDisclaimer|
Serialization in MFC
This article explains the serialization mechanism provided in the Microsoft Foundation Class Library (MFC) to allow objects to persist between runs of your program.
Serialization is the process of writing or reading an object to or from a persistent storage medium such as a disk file. Serialization is ideal for situations where it is desired to maintain the state of structured data (such as C++ classes or structures) during or after execution of a program. Using the serialization objects provided by MFC allows this to occur in a standard and consistent manner, relieving the user from the need to perform file operations by hand.
MFC supplies built-in support for serialization in the class CObject. Thus, all classes derived from CObject can take advantage of CObject's serialization protocol.
The basic idea of serialization is that an object should be able to write its current state, usually indicated by the value of its member variables, to persistent storage. Later, the object can be re-created by reading, or deserializing, the object's state from the storage. Serialization handles all the details of object pointers and circular references to objects that are used when you serialize an object. A key point is that the object itself is responsible for reading and writing its own state. Thus, for a class to be serializable, it must implement the basic serialization operations. As shown in the Serialization group of articles, it is easy to add this functionality to a class.
MFC uses an object of the CArchive class as an intermediary between the object to be serialized and the storage medium. This object is always associated with a CFile object, from which it obtains the necessary information for serialization, including the file name and whether the requested operation is a read or write. The object that performs a serialization operation can use the CArchive object without regard to the nature of the storage medium.
A CArchive object uses overloaded insertion (<<) and extraction (>>) operators to perform writing and reading operations. For more information, see Storing and Loading CObjects via an Archive in the article Serialization: Serializing an Object.
Do not confuse the CArchive class with general-purpose iostream classes, which are for formatted text only. The CArchive class is for binary-format serialized objects.
If you want, you can bypass MFC serialization to create your own mechanism for persistent data storage. You will need to override the class member functions that initiate serialization at the user's command. See the discussion in Technical Note 22 of the ID_FILE_OPEN, ID_FILE_SAVE, and ID_FILE_SAVE_AS standard commands.
The following articles cover the two main tasks required for serialization:
The article Serialization: Serialization vs. Database Input/Output describes when serialization is an appropriate input/output technique in database applications.