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3.7.14 %code Summary

The %code directive inserts code verbatim into the output parser source at any of a predefined set of locations. It thus serves as a flexible and user-friendly alternative to the traditional Yacc prologue, %{code%}. This section summarizes the functionality of %code for the various target languages supported by Bison. For a detailed discussion of how to use %code in place of %{code%} for C/C++ and why it is advantageous to do so, see section Prologue Alternatives.

Directive: %code {code}

This is the unqualified form of the %code directive. It inserts code verbatim at a language-dependent default location in the parser implementation.

For C/C++, the default location is the parser implementation file after the usual contents of the parser header file. Thus, the unqualified form replaces %{code%} for most purposes.

For Java, the default location is inside the parser class.

Directive: %code qualifier {code}

This is the qualified form of the %code directive. qualifier identifies the purpose of code and thus the location(s) where Bison should insert it. That is, if you need to specify location-sensitive code that does not belong at the default location selected by the unqualified %code form, use this form instead.

For any particular qualifier or for the unqualified form, if there are multiple occurrences of the %code directive, Bison concatenates the specified code in the order in which it appears in the grammar file.

Not all qualifiers are accepted for all target languages. Unaccepted qualifiers produce an error. Some of the accepted qualifiers are:

  • Language(s): C, C++
  • Purpose: This is the best place to write dependency code required for YYSTYPE and YYLTYPE. In other words, it’s the best place to define types referenced in %union directives. If you use #define to override Bison’s default YYSTYPE and YYLTYPE definitions, then it is also the best place. However you should rather %define api.value.type and api.location.type.
  • Location(s): The parser header file and the parser implementation file before the Bison-generated YYSTYPE and YYLTYPE definitions.
  • Language(s): C, C++
  • Purpose: This is the best place to write additional definitions and declarations that should be provided to other modules.
  • Location(s): The parser header file and the parser implementation file after the Bison-generated YYSTYPE, YYLTYPE, and token definitions.
  • Language(s): C, C++
  • Purpose: The unqualified %code or %code requires should usually be more appropriate than %code top. However, occasionally it is necessary to insert code much nearer the top of the parser implementation file. For example:
    %code top {
      #define _GNU_SOURCE
      #include <stdio.h>
  • Location(s): Near the top of the parser implementation file.
  • Language(s): Java
  • Purpose: This is the best place to write Java import directives.
  • Location(s): The parser Java file after any Java package directive and before any class definitions.

Though we say the insertion locations are language-dependent, they are technically skeleton-dependent. Writers of non-standard skeletons however should choose their locations consistently with the behavior of the standard Bison skeletons.

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