10.1.2 C++ Semantic Values

The %union directive works as for C, see The Collection of Value Types. In particular it produces a genuine union¹, 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++ forbids any instance of classes with constructors in unions: only pointers to such objects are allowed.

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 Freeing Discarded Symbols.