3.4.2 More Than One Value Type

In most programs, you will need different data types for different kinds of tokens and groupings. For example, a numeric constant may need type int or long int, while a string constant needs type char *, and an identifier might need a pointer to an entry in the symbol table.

To use more than one data type for semantic values in one parser, Bison requires you to do two things:

• Specify the entire collection of possible data types. There are several options:
  • let Bison compute the union type from the tags you assign to symbols;
  • use the %union Bison declaration (see section The Union Declaration);
  • define the %define variable api.value.type to be a union type whose members are the type tags (see section Providing a Structured Semantic Value Type);
  • use a typedef or a #define to define YYSTYPE to be a union type whose member names are the type tags.
• Choose one of those types for each symbol (terminal or nonterminal) for which semantic values are used. This is done for tokens with the %token Bison declaration (see section Token Type Names) and for groupings with the %type Bison declaration (see section Nonterminal Symbols).

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