We describe the design and implementation of ObjectStream, a system for automatic generation of stream I/O primitives for complex C++ objects. The system consists of a parser-based program generator and a generic object I/O library. The program generator pre-processes C++ class declarations to produce class-specific routines that, when executed with application programs, assists the class-neutral generic library to perform I/O operations for objects of the pre-processed classes.ObjectStream is novel in that it is non-intrusive. It lets programmers keep the original class declarations for application development, without requiring the classes to be modified for I/O or to be derived from a persistent base class. This makes easy the task of managing application source code, and ensures compatibility of objects produced with and without the added I/O capability. The implementation of ObjectStream is compiler- and machine-independent, and its operation requires little guidance from the programmers. We describe in this paper the motivations behind ObjectStream, our development strategies, and several interesting implementation issues. A working example is included to illustrate the ObjectStream system.