Re: what is happening here? (access types & discriminants...)

[Simon Wright <simon@pogner.demon.co.uk>]

dale@cs.rmit.edu.au (Dale Stanbrough) writes:

> I've got the equivalent of the following code...
> 
> with ada.strings.unbounded; use ada.strings.unbounded;
> 
> package ua is
>    type Unbounded_Array     is array  (Positive range <>) of Unbounded_String;
>    type Unbounded_Array_Ptr is access Unbounded_Array;
> 
> 
>   i_dont_understand : unbounded_Array_ptr (1..100);
> end ua;
> 
> 
> Not quite sure what the definition of i_dont_understand is doing...

Judging by this experiment,

  with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;
  with Ada.Text_Io; use Ada.Text_Io;

  procedure Ua is

    type Unbounded_Array     is array  (Positive range <>) of Unbounded_String;
     type Unbounded_Array_Ptr is access Unbounded_Array;


    I_Dont_Understand : Unbounded_Array_Ptr (13 .. 42)
       := new Unbounded_Array (13 .. 42);

    Me_Neither : Unbounded_Array_Ptr
       := new Unbounded_Array (14 .. 43);

  begin
    Put_Line ("f" & I_Dont_Understand'First'Img &
	      " l" & I_Dont_Understand'Last'Img);
    Put_Line ("f" & Me_Neither'First'Img &
	      " l" & Me_Neither'Last'Img);
  --   I_Dont_Understand := Me_Neither;               ------------------ CE
  --   Put_Line ("f" & I_Dont_Understand'First'Img &
  -- 	    " l" & I_Dont_Understand'Last'Img);
    I_Dont_Understand.all := Me_Neither.all;
    Put_Line ("f" & I_Dont_Understand'First'Img &
	      " l" & I_Dont_Understand'Last'Img);
    Me_Neither := new Unbounded_Array (13 .. 43);
    Put_Line ("f" & Me_Neither'First'Img &
	      " l" & Me_Neither'Last'Img);
  end Ua;

where the commented-out bit raises Constraint_Error, we've declared
i_dont_understand as a pointer capable of pointing only to arrays with
the designated constraint.

So an array assignment (of the right length) is OK, but a pointer
assignment with different constraints isn't.