Re: %union and shared pointers to AST nodes

[Hans Aberg]

On 16 Sep 2005, at 19:14, Frans Englich wrote:

> I have a problem involving bison although a solution may not  
> directly mean
> modifying the bison grammar. Nevertheless, someone might be able to  
> spot a
> solution.
>
> From my actions I call factory functions which returns instances I  
> assign to
> the actions($$), in order to build an AST tree. I write in C++, and  
> recently
> converted all code to return/take shared pointers to objects,  
> instead of raw
> pointers.
>
> When it came to converting the Bison grammar, I of course ran into  
> the problem
> that the union can't house class objects(such as shared_ptr  
> instances), but
> only primitive types.

There are two ways to solve this problem. One is to store a common  
pointer a pointer of type void* in the union. You are then  
essentially doing C-programming, and must do cleanup by hand in the  
actions, and using %destructor.

The other way is to make of the C++ language features. One is then  
making a C++ polymorphic (virtual) class hierarchy, which is stored  
in the semantic type. One cannot then use %union, so wither one does  
not use it, and uses dynamic_cast, or one hopes for Bison to be  
augmented so that one still  use the Bison static type system. I use  
the first one using my own C++ skeleton file and a Bison tweak, and I  
know, more or less, how to do the second one.

There is some C++ support in Bison, but I do not know how it works;  
Akim Demaille is responsible for that.

  Hans Aberg