Re: accessing record element (question to the pros)

[Peter Hermann <ica2ph@csv.ica.uni-stuttgart.de>]

Norbert Caspari <nnc@gmx.li> wrote:
> I'm new to Ada programming and until now I wrote just some small pieces of 
> code to try out new things and to educate myself. In this example I try to 
> build a Union Record, where I can access a single Byte either as a small 
> positive Integer value or as single Bits. I don't understand, why there is 
> a compiler error in line 56 and why it is forbidden in this case to assign 
> just a simple positive Integer value. If I comment this line out, the rest 
> of the code will compile without errors. Please remember, this is just some 
> experimental code with no special sense and no meaningful variable names.
> 
> Could somebody please help me to correct my code, showing me how to access 

how to assign ?

> a numeric value to the variable Yyy.
> 
> Thank you very much for your help!
> 
> Best regards, Norbert
> 
> ----------------------------------------------------------
> GNAT 3.15p  (20020523) Copyright 1992-2002 Free Software Foundation, Inc.
> 
> Compiling: j:/source/uniontest4.adb (source file time stamp: 2006-02-24 
> 14:43:50)
> 
>      1. with Ada.Text_Io;
>      2. use Ada.Text_Io;
>      3. 
>      4. procedure Uniontest4 is
>      5. 
>      6.    type Bit_T is
>      7.          (Off,
>      8.           On);

may be cumbersome to replace 0 by off   etc

why not boolean?

>      9. 
>     10.    for Bit_T use (
>     11.       Off => 0,
>     12.       On  => 1);
>     13. 
>     14.    type Bit8 is array (0 .. 7) of Bit_T;
>     15. 
>     16.    pragma Pack(Bit8);
>     17.    for Bit8'Size use 8;
>     18. 
>     19.    type Uint8 is mod 2**8;
>     20. 
>     21.    for Uint8'Size use 8;
>     22. 
>     23.    type Form is
>     24.          (Bytemode,
>     25.           Bitmode);
>     26. 
>     27.    type Bfeld (Ftyp: Form) is
>     28.    record
>     29.       case Ftyp is
>     30.          when Bitmode =>
>     31.             Bit: Bit8;
>     32.          when Bytemode =>
>     33.             Byte : Uint8;
>     34.       end case;
>     35.    end record;
>     36. 
>     37.    pragma Pack(Bfeld);
>     38. 
>     39.    for Bfeld use record
>     40.       at mod 1;
>     41.       Bit at 0 range 0..7;
>     42.       Byte at 0 range 0..7;
>     43.    end record;
>     44. 
>     45.    type Bfeld_Bitmode is new Bfeld
>     46.          (Bitmode);
>     47.    type Bfeld_Bytemode is new Bfeld
>     48.          (Bytemode);

with these declarations you have lost the advantage of the
discriminated record bfeld

>     49. 
>     50.    Xxx : Bfeld_Bitmode;
>     51.    Yyy : Bfeld_Bytemode;
>     52. 
>     53. begin
>     54. 
>     55.    Xxx.Bit(2) := On;
>     56.    Yyy := 20;

yyy.byte := 20;

>                   |
>         >>> expected type "Bfeld_Bytemode" defined at line 47
>         >>> found type universal integer
> 
>     57. 
>     58.    for K in 0 .. 7
>     59.          loop
>     60.       Put(Bit_T'Image(Xxx.Bit(K)));
>     61.    end loop;
>     62.    New_Line;
>     63. 
>     64. end Uniontest4;
>     65. 
> 
>  65 lines: 2 errors
> 

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