What is "persistence"? What is a "persistent object"?, C++ FAQ

C++ FAQ / Section 39 / FAQ 39.10

Section 39:

39.1	How do I convert a value (a number, for example) to a `std::string`?
39.2	How do I convert a `std::string` to a number?
39.3	Can I templatize the above functions so they work with other types?
39.4	What should be done with macros that contain `if`?
39.5	What should be done with macros that have multiple lines?
39.6	What should be done with macros that need to paste two tokens together? Updated!
39.7	Why can't the compiler find my header file in `#include "c:\test.h"` ?
39.8	What are the C++ scoping rules for `for` loops?
39.9	Why can't I overload a function by its return type?
39.10	What is "persistence"? What is a "persistent object"?
39.11	How can I create two classes that both know about each other?
39.12	What special considerations are needed when forward declarations are used with member objects?
39.13	What special considerations are needed when forward declarations are used with inline functions?
39.14	Why can't I put a forward-declared class in a `std::vector<>`?
39.15	Why do some people think `x = ++y + y++` is bad?
39.16	What's the deal with "sequence points"?

[39.10] What is "persistence"? What is a "persistent object"?

A persistent object can live after the program which created it has stopped. Persistent objects can even outlive different versions of the creating program, can outlive the disk system, the operating system, or even the hardware on which the OS was running when they were created.

The challenge with persistent objects is to effectively store their member function code out on secondary storage along with their data bits (and the data bits and member function code of all member objects, and of all their member objects and base classes, etc). This is non-trivial when you have to do it yourself. In C++, you have to do it yourself. C++/OO databases can help hide the mechanism for all this.