[fpc-pascal] more Variant thoughts (was: Variant record types in Sockates unit)

[Michael Van Canneyt]

On Wed, 4 Oct 2006, Alexander Todorov wrote:

> I made the simple test below:
> 
> program project1;
> 
> {$ifdef fpc}
>   {$mode objfpc}{$H+}
> {$endif}
> 
> {$apptype console}
> 
> type
> 
>  TTagVariant = record
>     AlwaysThere : Integer;
>     case Flag : Boolean of
>       false: (false_1, false_2 : Byte);
>       true : (true_1: Word);
>  end;
> 
> var V : TTagVariant;
> 
> begin
>  V.AlwaysThere := 1;
> 
>  V.Flag := false;
>  V.true_1 := 200; // should compiler raise a run time error here
>  V.false_1 := 100;
>  writeln('V.Flag=', V.Flag, ', V.false_1=', V.false_1, ',
> V.true_1=', V.true_1);
> 
>  V.Flag := true;
>  V.true_1 := 200;
>  V.false_1 := 100;// should compiler raise a run time error here
>  writeln('V.Flag=', V.Flag, ', V.false_1=', V.false_1, ',
> V.true_1=', V.true_1);
> 
>  writeln('Addresses: ', 'V.false_1=', Integer(@V.false_1), ',
> V.true_1=', Integer(@V.true_1));
> end.
> 
> The results are:
> V.Flag=FALSE, V.false_1=100,  V.true_1=100
> V.Flag=TRUE,   V.false_1=100,  V.true_1=100
> Addresses: V.false_1=134632390,  V.true_1=134632390
> 
> What I am thinking about is:
> 1) If the Flag field is set to true and the programmer uses field from
> the variant part which is declared in the false section, should there
> be a run time error or not?

No. No checking is performed on this. 

> Although it will be slowly should the compiler check V.Flag and
> consider the other fields undefined? Something like "Inaccessible
> record field used" error?

No.

The 'Flag' field is purely informational and has no restrictive meaning.
It's a hint to the programmer, but nothing more.
This is how pascal is defined.

Michael.