Re: Union Type Object problem

[Hans Aberg]

At 13:37 +0200 2004/08/05, Enrico Oliosi wrote:
>I have to do a parser in Bison and I need to use in %union{} construct
>some type Object.
>I have this example-class Expression:
>
>class Expression {
>public:
>   virtual ~Expression () {}
>   //  It's necessary because we need to clone objects without
>   // knowing the exact type.
>   virtual Expression *clone () = 0;
>   // The value represented by the expression
>   virtual int value () = 0;
>};
>
>this is a very simple class, but when I define the Bison Type in
>union construct in this way:
>
>%union{
>   Expression *exp;
>   int num;
>}

This way, Bison will not clean up the *exp pointers; so will have to do it
explicitly in the actions and using the experimental %destructors option. I
posted in the Bison-Patches list a way to produce a C++ parser. One can the
replace your class Expression with one that maintains a pointer to this
class, and cleanup will take place via the C++ language class features
then. One cannot use the %unions feature this way, though as C/C++ unions
only accept POD.

  Hans Aberg