3.4.4 The Union Declaration

The %union declaration specifies the entire collection of possible data types for semantic values. The keyword %union is followed by braced code 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, %nterm and %type declarations to pick one of the types for a terminal or nonterminal symbol (see 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 (see %define Summary).

As another extension to POSIX, you may specify multiple %union declarations; their contents are concatenated. However, only the first %union declaration can specify a tag.

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