Re: C array to Ada pointer to unconstrained array without copying memory

[Duncan Sands <duncan.sands@math.u-psud.fr>]

On Wednesday 08 October 2003 04:14, Robert I. Eachus wrote:
> Duncan Sands wrote:
> > So far so good.  Now suppose I do:
> >
> > Y : Array_Pointer := new Array_Type' (X);  -- (*)
> >
> > The object I want is Y.  What is the problem?  The problem
> > is that line (*) involves allocating memory and copying the
> > data from X to that memory.  I would like to end up with Y
> > without performing the copy (and without reallocating
> > memory for the array data).
>
> What is wrong with:
>
> Y: Array_Type;
> for Y'Address use X'Address;
>
> ..if you want X and Y to be two different views of the same memory.
> Notice that there is no need for an explicit pointer here, but if X is a
>   pointer as in your example, you may want to do:
>
> Y: Array_Type;
> for Y'Address use X.all'Address;
>
> Oh, and if there is initialization for the elements of Array_Type you
> may want to add a pragma Import; to turn off initialization.

Hi Robert, maybe I can explain it like this.  The data I have is on
the heap.  I want the array bounds on the heap as well.  In the
solution you suggest, the bounds of the variable Y will in general
be allocated on the stack (Y is declared in a local block), not on
the heap.  A fat pointer pointing to the array will have one part
pointing to the data (which is on the heap and obtained from C)
and the other part pointing to the dope (= bounds, and on the
stack).  At least this is how GNAT does it (which is all I care
about).  Once you leave the local block, the fat pointer can no
longer be used because the stack has been wound up and the
bounds are gone.  However, if the bounds were on the heap too,
it would be a different story.

Ciao,

Duncan.