Node:Multiple Types, Next:Actions, Previous:Value Type, Up:Semantics

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, 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, with the %union Bison declaration (see The Collection of Value Types).
• 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 Token Type Names) and for groupings with the %type Bison declaration (see Nonterminal Symbols).