Re: Objects and the Stack?

["Warren W. Gay VE3WWG" <ve3wwg@NoSpam.cogeco.ca>]

Nick Roberts wrote:
> Freejack wrote:
>> On Fri, 17 Dec 2004 00:28:23 +0000, Jeffrey Carter wrote:
>>> The fact that pointers are generally not needed in Ada except when 
>>> creating things such as dynamic data structures has nothing to do 
>>> with tagged types. If you create objects of a classwide type, they 
>>> are put on the stack if you declare them there, and in a storage pool 
>>> if you allocate them there.
...
> I suppose so, but I think I should tell you now that what you trying 
> (above) cannot work. The assumption in Ada is that the space reserved 
> within a stack frame for an object cannot be resized (even though the 
> space used /within/ the space reserved for an object can). So the 
> compiler must be able to allocate the maximum size of an object on the 
> stack. Since what you are attempting could cause Stack to grow to any 
> size without limit, the compiler cannot know what maximum to allocate. 
> It won't work, and so you must use the heap (or more correctly, a 
> 'storage pool').
> 
> Ada has no equivalent of alloca() in C. It might be interesting to 
> experiment with a compiler extension to provide this facility. Certain 
> algorithms might benefit from it.

The difference between what alloca() gets you and what you
get in the declare block is a minor difference:

    -- Pseudo Ada here (Ada-ised C code for alloca())
    declare
      My_String : String_Access := Alloca(String(1..8));
    begin
      ...
      Put_Line(My_String.all);

    -- Real Ada here
    declare
       My_String : String(1..8);
    begin
       ...
       Put_Line(My_String);

Apart from getting a pointer from alloca(), I don't see any
advantage. Both forms allocate from the stack frame.

>    declare
>       Name: String := "";
>       pragma Extensible_Object(Name);
>    begin
>       ...
>       Name := Name & ':'; -- changes its size
>       ...
> 
> It's an idea.

This is taking the alloca() idea further than I think it
goes. I could be wrong here, but can you realloc() an
alloca() region?  If so, that is something that Ada does
in fact lack (including a realloc() in general, which I
pine for when growing arrays).

Warren.