[Cobolforgcc-devel] Re: %union

[Tim J. <te...@me...>]

Keisuke,

I have experimented with several different options here and none
of them seem ideal to me.

By having a union, it means that you need to say 

ptr->which_one.member 

every time you access something. Alternatively you can do what
gcc does, which is have a union but dfine accessor macros eg

TREE_USER(ptr)=1;

I don't like accessor macros really because you have to keep
looking up what actual fields they are accessing.

As far as I can tell, a union is not type safe. If you use the
wrong member it will still not work.

The approach I took is to define seperate structures and do
casts. At first, I used to cast on every access, which became
tedious very quickly (you still see a fair bit of this code lying
around). So now I cast into a variable which is of the right type
and use that variable. This would not cause any runtime slowdown
as the cast is opimized away. This also gives me a good
opportunity to put in an assert or a type check to improve the
integrity of the program. This is the closest to type safeness I
have been able to get.

eg

void myfun (void* ptr1)
{

struct xxx1 *ptr2;

ptr2=(struct xxx1 *) ptr1;

if (ptr2->type != what_i_expect) 
   abort();


IMHO this gives the best combination of type checking and least
typing, but I woudl like a better solution.

By the way I will be starting work on IF again this weekend which
is a three day weekend here.

Tim Josling

Keisuke Nishida wrote:
> 
> Tim,
> 
> It seems that you don't use %union in your parser and use lots
> of type castings instead.  Is there a good reason to do so?
> I don't think too much use of type casting is a good thing
> because it disables the C compiler from doing type checking.
> 
> Keisuke