Re: OOP Parse tree generation?

Nicolas <nojeda@my-deja.com> wrote:
> How would you implement parse tree structures for a language that has
> statements and expressions (a la Pascal), in an object oriented fashion ?
> [Good question. I've never seen a very satisfactory OOP design for a
> parser. -John]


For each non-terminal, you define a base class and for each rule for
expanding that non-terminal you define a derived class. For instance,
if the grammar contains the rule


                    widget -> A b C d


(where upper/lower case indicates nontermina/terminal respectively),
then if you are implementing in C++, you need


    class Widget {
        virtual void xlate() = 0;
    };


in your file of base classes. In your file of derived classes put


    class Widget_AbCd : Widget {
        A* my_A; b_stuff my_b; C* my_C; d_stuff my_d;
        Widget_AbCd( A* w, b_stuff x, C* y, d_stuff z )
            : my_A(w), my_b(x), my_C (y), my_d(z)
        {}
        void xlate(){ ... my_A->xlate() ... my_C->xlate() ... }
    };


The tree gets built bottom-up by passing each node's constructor the
appropriate pointers to its children (who are already in the tree).
Invoking xlate() on the root triggers top-down translation of the
entire tree.


Information used in attribute synthesis/inheritance needs to be
made available to ancestors by placing it in the base class.


Tom Payne

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