Making C++ Objects Persistent: the Hidden Pointers

C++ objects of types that have virtual functions or virtual base classes contain volatile (‘‘memory’’)pointers. We call such pointers ‘‘hidden pointers’ because they were not specified by the user. If such C++objects are made persistent, then these pointers become invalid across program invocations. Weencountered this problem in our implementation of O++, which is a database language based on C++. O++extends C++ with the ability to create and access persistent objects. In this paper, we describe the hidden pointers problem in detail and present several solutions to it. Oursolutions are elegant in that they do not require modifying the C++ compiler or the semantics of C++. Wealso discuss another problem that arises because C++ allows base class pointers to point to derived classobjects. C++ has emerged as the de facto standard language for software development, and databasesystems based on C++ have attracted much attention. We hope that the details and techniques presentedwill be useful to database researchers and to implementors of object-oriented database systems based onC++.