OpenEuphoria

Re: gets() and "string" variable type

• Posted by Matt Lewis <matthewwalkerlewis at g?ail.c?m> Sep 15, 2007
• 553 views

Pete Lomax wrote:
> 
> PS [OT] Can someone explain the last line of this snippet from be_execute.e:
> 

> 	top = (object)*(top + ((s1_ptr)obj_ptr)->base);
> 
> My grasp of C is limited and I do not actually understand how it knows to get
> "top" from "base+top*4" rather than just "base+top". Not that it does not seem
> logical, more "how does it know?"  Specifically, which parts of
> 
>    typedef long object;
>    typedef object *object_ptr;
> 
> are applied/applicable when and where? Is it the earlier *(object_ptr) or the
> latter (object)* or both or what?

The type defs just tell the compiler, "When I say 'object' please read it
as 'long', and when I say 'object_ptr' please read it as 'long*'."

The expression you put from be_execute.c is what's often referred to as
pointer arithmetic.  When you add 1 to a pointer, it points to the next
place in memory, *after the current value*.  This applies for structs, too,
BTW.  Another way to have written this would be:

   top = ((s1_ptr)obj_ptr)->base[top]

Which, to me, is usually clearer (and how I tend to code that sort of thing,
so when you see it in the code, it's probably something I wrote).

Matt