Re: discriminant questions

[Georg Bauhaus <rm.dash-bauhaus@futureapps.de>]

On 23.09.11 09:59, Dmitry A. Kazakov wrote:

> Can I declare an iterator object constant? What is the meaning of its
> "constantness" when the container gets manipulated? See a huge language
> problem here? An iterator can be immutable only if the container is. How do
> I express such stuff in the language?

Well, have one more level of indirection, and one aspect (i.e., only
two more kludges ;-)  Not sure whether the aspect can be checked at
compile time, though, or to what extent.

Compiling: const.adb (source file time stamp: 2011-09-23 10:27:42)

     1. with Ada.Containers.Vectors;
     2.
     3. procedure Const is
     4.
     5.    type E is (Mon, Bin, Ter);
     6.    subtype P is Positive range 1 .. 100;
     7.
     8.    package Vec is new Ada.Containers.Vectors
     9.      (Element_Type => E,
    10.       Index_Type => P);
    11.
    12.    type Const_Vector is access constant Vec.Vector;
    13.    type Const_Cursor is new Vec.Cursor
    14.      with Invariant => Has_Element (Const_Cursor);
    15.
    16.    procedure Change (Element : in out E) is
    17.    begin
    18.       if Element = E'Last then
    19. 	 Element := E'First;
    20.       else
    21. 	 Element := E'Succ (Element);
    22.       end if;
    23.    end Change;
    24.
    25.    function Frozen (V : Vec.Vector) return Const_Vector is
    26.    begin
    27.       return V'Unchecked_Access;
    28.    end Frozen;
    29.
    30.    Container : Vec.Vector;
    31.    Position  : Vec.Cursor;
    32.    Const_Container : Const_Vector;
    33. begin
    34.    Container.Append(Mon);
    35.    Container.Append(Ter);
    36.    Position := Container.First;
    37.    Vec.Update_Element (Container,
    38. 		       Position,
    39. 		       Process => Change'Access);
    40.    Const_Container := Frozen (Container);
    41.    Vec.Update_Element (Const_Container.all,
                                               |
        >>> actual for "Container" must be a variable

    42. 		       Position,
    43. 		       Process => Change'Access);
    44.
    45. end Const;
    46.
    47.
    48.

 48 lines: 1 error
gnatmake: "const.adb" compilation error

Compilation exited abnormally with code 4 at Fri Sep 23 10:27:53