Bison, the GNU parser generator, is a powerful tool for creating parsers in various programming languages. In addition to its standard features, Bison also provides a set of compiler-specific extensions that can be used in C++ code. These extensions offer additional functionality and flexibility when working with Bison-generated parsers in C++. In this article, we will explore some of the most notable Bison compiler-specific extensions and how they can be utilized in your C++ projects.

Bison Location Tracking Extension

One of the common requirements when working with parsers is the need to track the location of tokens in the input. Bison provides a compiler-specific extension to enable location tracking easily. This extension is based on the use of a special data type called location that represents the position of a token in the input.

To enable location tracking in Bison, you need to include the %locations directive in your Bison grammar file. This directive informs Bison to automatically pass location information to your semantic actions. For instance, if you have a rule expr: NUMBER, Bison will automatically pass the location information of the NUMBER token to the corresponding semantic action.

To access the location information inside your semantic actions, you can use the @n notation, where n represents the position of the token in the rule. For example, @1 represents the location of the first token in the rule.

Using locations in your Bison-based C++ parser can greatly simplify the process of error handling and generating meaningful error messages, as you have access to the position of each token in the input.

Bison Pure Declaration Extension

In some cases, you may want to declare your Bison-generated parser as a pure function, meaning that it does not have any side effects and its return value solely depends on its input. This can be useful in scenarios where you want to optimize your parser by allowing certain compiler optimizations.

To declare your Bison parser as a pure function in C++, you can use the %pure-parser declaration in your Bison grammar file. Once you include this declaration, Bison will generate a C++ parser function that is declared as const, indicating that it does not modify any data members of the parser.

Declaring your parser as a pure function can provide performance benefits and enable certain compiler optimizations. However, it’s important to note that this declaration assumes that your semantic actions do not modify any data outside the parser itself. If your semantic actions have side effects, you should not use the %pure-parser declaration.

Conclusion

Bison’s compiler-specific extensions in C++ offer additional features and flexibility when working with Bison-generated parsers. The location tracking extension simplifies error handling and provides access to token positions in the input. The pure declaration extension allows you to declare your parser as a pure function, enabling compiler optimizations. By leveraging these extensions, you can enhance the functionality and performance of your C++ parsers created with Bison.

#parser #Bison