Object Oriented Compiler Design Problem

mayurnaik@my-dejanews.com wrote:
> The function symbol* add(symbol*, symbol*) cannot be made a virtual function
> of class symbol, since it is a friend function. But, the moment it is a
> friend function, it does not know whether the symbols to be added are
> Constants or Variables


The problem here is that you need multiple dispatch if you don't want
to use RTTI or union with a type field. Here is a possible solution
with a very low overhead (only one virtual method lookup).


First, make 'add' a pure virtual function that takes only one
parameter, and define two other virtual functions:


class constant;
class variable;


class symbol
{
    // ...
    virtual symbol* add(symbol*) = 0;


    virtual symbol* addToConstant(constant*) = 0;
    virtual symbol* addToVariable(variable*) = 0;
    // ...
};


Now, in class 'constant', implement these virtual functions:


class constant : public symbol
{
    // ...
    virtual symbol* add(symbol* sym)
        { return sym->addToConstant(this); }
    virtual symbol* addToConstant (constant* cst)
        { return new constant(cst->val + this->val); } // constant folding
    virtual symbol* addToVariable (variable* var)
        { /* ... gen intermediate code and return temporary var. */ }


    //...
};


You need to do the same in class 'variable':


class variable : public symbol
{
    // ...
    virtual symbol* add(symbol* sym)
        { return sym->addToVariable(this); }
    virtual symbol* addToConstant (constant* cst)
        { /* ... gen intermediate code and return temporary var. */ }
    virtual symbol* addToVariable (variable* var)
        { /* ... gen intermediate code and return temporary var. */ }


    //...
};


The first production rule now becomes:


#1 term : term '+' primary { $$ = $1->add($2); }




The only problem with this solution is that class 'symbol' on the its
subclass, but this can certainly by solved using an intermediate class
(a la Visitor pattern).


Hope this helps!


--Dominique
+---------------------------------+----------------------------+
| Dominique Boucher, M.Sc. |Email: dboucher@locus.ca |
| Software Designer |Tel : (514) 954-3804 |
| Locus Dialogue Corporation |Fax : (514) 954-3805 |
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