10.1.4.1 C++ Unions

The %union directive works as for C, see The Union Declaration. In particular it produces a genuine union, which have a few specific features in C++.

• - The type YYSTYPE is defined but its use is discouraged: rather you should refer to the parser’s encapsulated type yy::parser::semantic_type.
• - Non POD (Plain Old Data) types cannot be used. C++98 forbids any instance of classes with constructors in unions: only pointers to such objects are allowed. C++11 relaxed this constraints, but at the cost of safety.

Because objects have to be stored via pointers, memory is not reclaimed automatically: using the %destructor directive is the only means to avoid leaks. See section Freeing Discarded Symbols.