%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++.
YYSTYPEis defined but its use is discouraged: rather you should refer to the parser's encapsulated type
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.
 In the future techniques to allow complex types within pseudo-unions (similar to Boost variants) might be implemented to alleviate these issues.