Re: Token types with constructor

[Akim Demaille]

>>> "Hans" == Hans Aberg <address@hidden> writes:

 >> There is just no decent way to store different object types, you have
 >> to store pointers (there are nice proposals for Variant types in
 >> future C++es, but that's not for tomorrow).  Also, introducing an
 >> artificial common root to all the symbols often breaks the AST design,
 >> or worse, weakens types hence type safety.

 > Actually, I use:
 >   class semantic_type {
 >   public:
 >     long number_;
 >     std::string text_;
 >     object object_;

 >     semantic_type() : number_(0) {}
 >   };
 >   #define YYSTYPE semantic_type
 > where the class object spots a reference counted pointer to a common
 > polymorphic hierarchy. Sometimes these types are used in parallel, text_
 > for token name, number_ for lookup table token values (used in the case of
 > definitions), and object_ usually contains the semantic value.

OK.

 > I cannot use the Bison %union then, because of the limitation of C++ to not
 > use non-POD's in unions, but I experimented with a %type feature which does
 > not implement unions. But in this polymorphic approach I use, it turns out
 > not to be every essential, as an erroneous type operation will cause a C++
 > runtime dynamic_cast exception. But others ay want a %type feature for
 > keeping track of a static pointer.

We cannot afford such a solution in Bison, for performance reasons.
First, the copy are expensive because of the size of the stack
members, plus the ctor they involve, and in addition dynamic_cast is
an expensive operation.

The C++ skeleton still has much to do to be really what everybody
needs, but once its interface etc. stable, we will try to find a
better solution, based on the ideas for Variants.  Meanwhile, it won't
change.

 > In order to be able to write a decent C++ parser, I need essentially one
 > feature, a command
 >     %code <name> {...}
 > that would put {...} into a M4 macro derived from <name>. The thing is that
 > code can under C++ put in several more places than in C, and the setup
 > should be properly be synced with Flex. This takes some experimentation to
 > do.

It exists, it's called %define, but I'm not ready to open this publicly.