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

Alexander Todorov alexx.todorov at gmail.com
Wed Oct 4 10:36:03 CEST 2006

I made the simple test below:

program project1;

{$ifdef fpc}
   {$mode objfpc}{$H+}

{$apptype console}


  TTagVariant = record
     AlwaysThere : Integer;
     case Flag : Boolean of
       false: (false_1, false_2 : Byte);
       true : (true_1: Word);

var V : TTagVariant;

  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));

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?
Although it will be slowly should the compiler check V.Flag and
consider the other fields undefined? Something like "Inaccessible
record field used" error?
2) In the test program the addresses of the two fields are the same
due to the default aligning used by the compiler. That's why the
output is the number 100.
In Delphi these addresses differ in 1 byte, so the ouput is :
V.Flag=FALSE, V.false_1=100,  V.true_1=200
V.Flag=TRUE,   V.false_1=100,  V.true_1=200
Is this by design or FPC has an inconsistency with Delphi?

3) Bug in Lazarus code complete. Does not show the Flag field of the
variant. Delphi does.
I will fill a bug report.

Please someone with deeper knowledge on variant records and how the
compiler handles them answer these questions.

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