Any object constructor nulls all data fields — undocumented wrong
behavior
lazarus / 62-General

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Avinash · 2024-02-01 10:52

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i {$MODE
TP}type  PObject = ^TObject;  TObject = object    Value: Integer;   
constructor Init;  end;   constructor TObject.Init;  begin  end; var 
B: TObject;begin  B.Value := 12345;  WriteLn(B.Value);  {Output:
12345}  B.Init;  WriteLn(B.Value);  {Output: 0}  {In Turbo Pascal
output "12345"}end.
In Turbo Pascal, calling a constructor by itself does not destroy the
object's fields. This allows, for example, by calling an empty
constructor to change the VMT-link of an object instance. This method,
unlike direct assignment, is more organic: it is compatible between
different «bitnesses» and the location of the VMT link (which in
general may not be at the beginning of the data). In FPC it turns out
that calling an empty constructor leads to the inevitable destruction
of all data fields. Thus, to replace the VMT link without destroying
the data, only direct assignment remains.

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Thaddy · 2024-02-01 12:18

I am not so sure if TurboPascal's behavior is documented. At least
there is nothing in my official paper books.
So if it is by accident, undefined behavior, implementation detail,
even a bug report won't bring you much further.
I am pretty sure it is implementation detail and not documented. As
such it is not wrong.

Plz show me otherwise referring to official Borland documentation.
(You can't!)

More in general, TP mode behavior in FreePascal is more in line with
what you would expect for init constructors; I am not even sure the
behavior that you describe is consistent over different versions of
TurboPascal.

Or better: I would have known about that behavior... (which I am able
to test when needed, I have 1.0, 3.0, 5.5 and 7.0, some versions with
books and floppies)