Re: Help: Dynamic-size arrays using record discriminant.

[Niklas Holsti <nholsti@icon.fi>]

Alon Matas wrote:
> 
> -- I have this problem:
> -- There is a record with a discriminant (and a default value to the
> -- discriminant). The record contains a constrained array, and the range
> -- of the array depends on the discriminant. For example:
> 
> procedure PROBLEM is
> 
>         type ARRAY_TYPE  is array (INTEGER range <>) of NATURAL;
>         type RECORD_TYPE (SIZE: INTEGER:= 5) is
>                 record
>                         VECTOR: ARRAY_TYPE (1..SIZE);
>                 end record;
> 
> -- Now, I have a variable with the record type:
>         REC: RECORD_TYPE;
>         TEMP: RECORD_TYPE;  -- This variable will be explained later.
> 
> -- The question is how do I change the array size (i.e, the discriminant
> 
> -- value) WITHOUT loosing its current content.

   [text snipped]

> Of course, I can do something like this:
> 
>         REC:= (SIZE => 6, VECTOR => (others => 0));

You can do

    REC := (SIZE => 6, VECTOR => REC.VECTOR & (6 => 0));

However, the compiler is likely to generate a temporary and
copy VECTOR contents back and forth. If this is performance-
critical, you should perhaps consider other data structures
where existing VECTOR data remain in place. For example, the
SIZE could be an ordinary component, instead of a discriminant.
You must then constrain the VECTOR component to some assumed
maximum size.

You should anyway constrain the SIZE discriminant, since some
compilers will allocate RECORD_TYPE memory for the maximum
possible SIZE value (integer'last in your code).

- Niklas