Re: Using Access Types for a simple solution

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2016-07-25 15:35, ldries46 wrote:
> Let me first explain the problem. I do have a matrix of records
>
> type Box is record
>   ...
>   ...
> end record
>
> Grid : array( 1 .. N,  1 .. M) of Box;
>
> Now I must do something with a random set out of elements from the
> matrix. I would like to create an array of pointers to the elements of
> the matrix so that I can use these elements and file the changes
> directly if the array. So I created a type:
>
> type Box_Access is access all Box;
>
> and make an array
>
> Box_Array : array(1 .. Q) of Box_Acess;
>
> When try to use that in a simple statement
>
> Box_Array(n) := Grid(n1, n2)'Access;
>
> I get the error (only for this statement): prefix of 'Access attribute
> must be aliased.
>
> I tried:
>
> Grid : aliased array( 1 .. N,  1 .. M) of Box;
>
> But that also gives the same error.
> I cannot imagine that what I want is not possible in Ada as I can
> realise it in C++ so I do miss something.

Yes, the error message that tells you: "prefix of 'Access attribute must 
be aliased". The prefix of 'Access attribute in Grid(n1, n2)'Access is 
obviously Grid(n1, n2).

Aliased array /= array of aliased elements.

And you should not use pointers anyway. Do sets of array indices instead.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de