Conditional compilation with preprocessors such as cpp is a simple but effective means to implement variability. By annotating code fragments with #ifdef and #endif directives, different program variants with or without these annotated fragments can be created, which can be used (among others) to implement software product lines. Although, such annotation-based approaches are frequently used in practice, researchers often criticize them for their negative effect on code quality and maintainability. In contrast to modularized implementations such as components or aspects, annotation-based implementations typically neglect separation of concerns, can entirely obfuscate the source code, and are prone to introduce subtle errors.
Our goal is to rehabilitate annotation-based approaches by showing how tool support can address these problems. With views, we emulate modularity; with a visual representation of annotations, we reduce source code obfuscation and increase program comprehension; and with disciplined annotations and a product-line-aware type system, we prevent or detect syntax and type errors in the entire software product line. At the same time we emphasize unique benefits of annotations, including simplicity, expressiveness, and being language independent. All in all, we provide tool-based separation of concerns without necessarily dividing source code into physically separated modules; we name this approach virtual separation of concerns.
We argue that with these improvements over contemporary preprocessors, virtual separation of concerns can compete with modularized implementation mechanisms. Despite our focus on annotation-based approaches, we do intend not give a definite answer on how to implement software product lines. Modular implementations and annotation-based implementations both have their advantages; we even present an integration and migration path between them. Our goal is to rehabilitate preprocessors and show that they are not a lost cause as many researchers think. On the contrary, we argue that – with the presented improvements – annotation-based approaches are a serious alternative for product-line implementation.