[fpc-pascal] Records as properties and Delphi compiler error

[fpclist at silvermono.co.za]

Hi Jonas

Thanks for the reply.

A high level, a class is like a record that has been modified to include 
functions and procedures. I know that I'm over simplifying thing here, please 
bare with me.

I'm trying to understand the logic employed by the creators of Delphi where 
they don't allow to write to the fields of a record type property, but if the 
property points to a class type, then anything goes. In the example bellow, 
where a property is of a class type, both Delphi and FPC compile the code, 
but there is no guarantee that the object referenced to by the property has 
been instantiated before the property is used (The programmer must 
instantiate the TTestProp class within TTestClass prior to any call made to 
the property). IMO, it would be a nice feature if the compiler could be 
modified to issue a warning in such a case.Again, I'm over simplifying, to 
the compiler, it would be similar to checking for a variable declaration 
before the variable is used.

I thing that the "error" in the way that FPC allows record properties to 
access the record fields could be handy if retained. Perhaps this feature 
could be reserved for objfpc mode. What are your thoughts on the matter?

Regards,
Nino

//--------------------------------------------------------------------------
unit Unit1;

(*
   This code sample is written for use with Lazarus.
   The Delphi equivalent also compile using Delphi.
*)

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, FileUtil, LResources, Forms, Controls, Graphics, Dialogs,
  StdCtrls;

type
  TForm1 = class(TForm)
    Button1 : TButton;
    procedure Button1Click(Sender : TObject);
  private
  public
  end;

  TTestProp = class
  private
    fX : Byte;
    fY : Byte;
  public
    property X : Byte read fX write fX;
    property Y : Byte read fY write fY;
  end;

  TTestClass = class
   private
     fTestProp : TTestProp;
   public
     constructor Create();
     destructor Destroy(); override;

     property TestProp : TTestProp read fTestProp write fTestProp;
  end;

var
  Form1 : TForm1;

implementation

constructor TTestClass.Create();
begin
  //* If this line is commented, the compilation will succeed
  //* but a violation will occur at runtime in button click handler
  //* since the object referenced does not exist.
  //*
  //* Note: I'm not suggesting here that the compile should be aware
  //* that this object, whilst being referenced later in the code,
  //* is not instantiated, but it would be a cool feature if the compiler
  //* could be modified to issue at least a warning advising of such an event.
  fTestProp := TTestProp.Create();
end;

destructor TTestClass.Destroy();
begin
  if Assigned(fTestProp) then
    FreeAndNil(fTestProp);

  inherited Destroy();
end;

procedure TForm1.Button1Click(Sender : TObject);
var
  TestClass : TTestClass;

begin
  TestClass := TTestClass.Create();
  try
    ShowMessage('TestProp before' + #13 +
                'X = ' + IntToStr(TestClass.TestProp.X) + #13 +
                'Y = ' + IntToStr(TestClass.TestProp.Y));

    TestClass.TestProp.X := 10;
    TestClass.TestProp.Y := 20 ;

    ShowMessage('TestProp after' + #13 +
                'X = ' + IntToStr(TestClass.TestProp.X) + #13 +
                'Y = ' + IntToStr(TestClass.TestProp.Y));

  finally
    FreeAndNil(TestClass);
  end;
end;

initialization
  {$I unit1.lrs}

end.          

//*************************************
On Saturday 06 June 2009 18:27:03 Jonas Maebe wrote:
> On 06 Jun 2009, at 17:36, fpclist at silvermono.co.za wrote:
> > Is there a reason why the following code fails to compile in Delphi
> > but
> > compile in FPC? Could the reason be that FPC allows the use of global
> > properties?
>
> No, it's an error in FPC which has been fixed in 2.3.1:
> http://wiki.freepascal.org/User_Changes_Trunk#Treating_direct-mapped_proper
>ties_as_regular_fields
>
>
> Jonas
> _______________________________________________
> fpc-pascal maillist  -  fpc-pascal at lists.freepascal.org
> http://lists.freepascal.org/mailman/listinfo/fpc-pascal