Re: non-local pointer cannot point to local object

["J-P. Rosen" <rosen@adalog.fr>]

Le 23/02/2018 à 09:54, artium@nihamkin.com a écrit :
> Can someone explain why I get "non-local pointer cannot point to
> local object" error, even though it looks like "Arr" and "Arr_Access"
> have the same accessibility level?

> Can I overcome the problem without dynamically allocating memory and without using "Unchecked_Access"?
> 
> with Interfaces.C;
> with Interfaces.C.Strings;
> 
> procedure X is
> 
>    type Integer_Access is access all Integer;
> 
>    Arr_Access : Interfaces.C.Strings.char_array_access;
>    Arr : aliased Interfaces.C.char_array := Interfaces.C.To_C ("From");
> 
>    A : Integer_Access;
>    I : aliased Integer := 6;
> 
> begin
> 
>    Arr_Access := Arr'Access;
>    A := I'Access;
> 
> end X;
> 
What counts for accessibility is the place where the type of the object
is declared, not where the object itself is declared. In your case,
char_array_access is a global type, therefore you cannot assign a
pointer to a local variable.  Otherwise, Arr_Access could later be
assigned to some global variable, and you would end up with a global
variable pointing to a local object. Using Unchecked_Access means that
you swear that you don't do such evil things.

Ada has this nice property that no object can survive to its type (no,
it's not obvious; I think this is not guaranteed by C++ - can anyone
confirm?). Therefore, when you exit the scope of an access type, you are
guaranteed that there is no more value of this type hanging around.



-- 
J-P. Rosen
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