Very basic questions about memory allocation

Very basic questions about memory allocation

[JIMHER@yanoesladirecci�n.com]

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I'm starting to work in ADA, but i've found some problems. The principal 
one is that I son't know how to free memory previously allocated with:

type t is ....
...
v:access t;
begin
	v:=new t;
end;


I've tried 'free(v)', 'delete v' and others, but they don't work. I use 
to program in C, and I only want an equivalent to 'free(v)'. I know that 
this is a very trivial question (with a very trivial solution I suppose), 
but is very frustrating.

If it helps, I'm using GNAT for windows.

Thanks in advance
-- 
Jes�s Jim�nez Herranz
vtis0102@CACANOps.uib.es
(Remove CACANO from the address)

Re: Very basic questions about memory allocation

[Frank Ecke]

Upon Fri, 30 Jul 1999 22:46:56 +0200, JIMHER@yanoesladirecci�n.com
<vtis0102@CACANOps.uib.es> spake unto us thuswise:

> I'm starting to work in ADA

I'm pleased to hear that.

> but i've found some problems. The principal 
> one is that I son't know how to free memory previously allocated with:
> 
> type t is ....
> ...
> v:access t;
> begin
> 	v:=new t;
> end;
> 
> 
> I've tried 'free(v)', 'delete v' and others, but they don't work. I use 
> to program in C, and I only want an equivalent to 'free(v)'.

Actually, it is all there, but it's a bit, well, hidden.  Ada is a
strongly typed language.  Therefore, access types are not just
pointers, but each access type has associated with it the type it
points to.  Each time you create a new access type, it is different
from all the previously created access types in your program (this
sentence could be rewritten without using the word ``access'').  As
a consequence, there cannot be a single procedure to deallocate all
objects---each object is of a particular type: you need a
deallocation procedure for each access type in your program.  You
cannot use a Free-procedure intended for strings to deallocate
integer objects.  On the other hand, it would be impossible for the
Ada language to ``have on stock'' all the Free-procedures necessary
in your program.  The solution to this dilemma is given by generics.
Ada provides one generic procedure, called Unchecked_Deallocation,
which you are supposed to instantiate with an actual access type.
See ARM (Ada Reference Manual) 13.11.2.

   Thus, returning to your example code from above, the following
solves the problem:

-- T and U are different
type T is ...;
type U is ...;

type T_Ptr is access T;
type U_Ptr is access U;

-- instantiate two deallocation procedures; one for T and one for U
procedure Free is new Ada.Unchecked_Deallocation(T, T_Ptr);
procedure Delete is new Ada.Unchecked_Deallocation(U, U_Ptr);

Foo : T_Ptr;
Bar : U_Ptr;

begin
  --- create them
  Foo := new T;
  Bar := new U;

  -- use them
  ...

  -- kill them
  Free(Foo);
  Delete(Bar);
end;

   Note that you can create the *illusion* of one general
deallocation procedure: you could replace ``Delete'' by ``Free'' (but
be sure to replace *all* occurrences of ``Delete'' by ``Free'').
Mind you, it is an illusion; there are still two different
Free-procedures.  The secret behind this is the concept of
overloading, which is revealed in ARM 8.6.


I hope this helps,

Frank

--
Frank Ecke <franke@minet.uni-jena.de>


      In a world without walls and fences, who needeth windows and gates?

Re: Very basic questions about memory allocation

[tmoran]

> type t is ....
> ...
> v:access t;
> begin
>         v:=new t;
> end;
  Are you sure you need heap allocation?  Since Ada allows highly
dynamic stack allocation, there is much less need for "new" and "free".
For instance, if "type t is array(integer range <>) of something;"
and X is a floating point number,
  v : t(integer(sqrt(X/2.0)) .. integer(sqrt(X)));
is perfectly legal Ada.  You can just let the compiler handle the
space allocation and deallocation.

Re: Very basic questions about memory allocation

[JIMHER@yanoesladirecci�n.com]

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Thanks a lot for your help. It has been very useful for me.

-- 
Jes�s Jim�nez Herranz
vtis0102@CACANOps.uib.es
(Remove CACANO from the address)