On 6 Jul 2005, at 11:40, Evan Lavelle wrote:
I'd like to pass a token class from Flex to Bison (ideally, a
tr1::shared_ptr - a reference counted smart pointer class).
However, in Bison itself I need to deal with a number of symbol
types - tokens, AST nodes, and so on.
This means that I need to use a %union, but this is difficult
because the classes are too complex to put in a union. Using class
pointers in the union is difficult, because this messes up the
reference counting, and I have to do manual resource management.
You do not need to use %union. I use a C++ polymorphic (virtual)
hierarchy together with dynamic_cast; any eventual type errors will
then be revealed at runtime. (In addition, I found it useful to add a
Std::string and a integral type to my YYSTYPE, used only to forward
token names and numbers to the Bison parser.)
You can also use static_cast, as dynamic_cast is slow (just as any
dynamic allocation), if your class hierarchy aren't marked "virtual"
in the derivation. Then, one would probably need Bison's static type
checking.
This can easily be done, as soon Bison gets suitable constructs (like
%typed, not requiring a union for implementation, and %code which
admits proper code placement). But it cannot handle it right now.
Also try out latest Bison alpha <ftp://alpha.gnu.org/gnu/bison/
bison-2.0a.tar.gz>; it should be more reliable than the version you
are using.
Hans Aberg
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