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Bison 1.875

3.7.3 The Collection of Value Types

The %union declaration specifies the entire collection of possible data types for semantic values. The keyword %union is followed by a pair of braces containing the same thing that goes inside a union in C.

For example:

%union { double val; symrec *tptr; }

This says that the two alternative types are double and symrec *. They are given names val and tptr; these names are used in the %token and %type declarations to pick one of the types for a terminal or nonterminal symbol (see section Nonterminal Symbols).

As an extension to POSIX, a tag is allowed after the union. For example:

%union value { double val; symrec *tptr; }

specifies the union tag value, so the corresponding C type is union value. If you do not specify a tag, it defaults to YYSTYPE.

Note that, unlike making a union declaration in C, you need not write a semicolon after the closing brace.