Garbage collection in the next C++ standard

• Authors: Hans-J. Boehm;Mike Spertus
• Venue: Proceedings of the 2009 international symposium on Memory management
• Year: 2009

C++ has traditionally relied on manual memory management. Sometimes this has been augmented by limited reference counting, implemented in libraries, and requiring use of separate pointer types. In spite of the fact that conservative garbage collectors have been used with C for decades, and with C++ for almost as long, they have not been well-supported by language standards. This in turn has limited their use. We have led an effort to change this by supporting optional "transparent" garbage collection in the next C++ standard. This is designed to either garbage collect or detect leaks in code using normal unadorned C++ pointers. We initially describe an ambitious effort that would have allowed programmers to explicitly request garbage collection. It faced a number of challenges, primarily in correct interaction with existing libraries relying on explicit destructor invocation. This effort was eventually postponed to the next round of standardization. This initial effort was then temporarily replaced by minimal support in the language that officially allows garbage collected implementations. Such minimal support is included in the current committee draft for the next C++ standard. It imposes an additional language restriction that makes it safe to garbage collect C++ programs. Stating this restriction proved subtle. We also provide narrow interfaces that make it easy to both correct code violating this new restriction, and to supply hints to a conservative garbage collector to improve its performance. These provide interesting implementation challenges. We discuss partial solutions.