Re: [lazarus] class-references

[Albert Zeyer]

Am Montag, den 21.08.2006, 19:17 +0200 schrieb Mattias Gaertner:

On Mon, 21 Aug 2006 14:30:06 +0000
Albert Zeyer <[EMAIL PROTECTED]> wrote:
> > With TPersistent it would work like this
> >    TMyBase = class(TPersistent)
> >    public
> >      function CreateCopy: TMyBase;
> >      procedure Assign(Source: TPersistent); override;
> >    end;
> > 
> > it would be
> >  function TMyBase.CreateCopy: TMyBase;
> >  begin
> >    Result := TMyBaseClass(Self.ClassType).Create;
> >    Result.Assign(Self);
> >  end;
> > 
> > Every class must override the Assign method to copy all added
> > values.
> > 
> 
> But in this case, it would also not call the constructor of my
> inherited classes. 

Why not? Did you test it?

Yes, I tested the following code:

// ------------------------------------
...
type
  TBaseTestObject = class(TPersistent)
    constructor Create();
    procedure DoSomething(); virtual; abstract;
  end;

  TBaseTestClass = class of TBaseTestObject;

  TTestObject = class(TBaseTestObject)
  public
    constructor Create();
    destructor Destroy(); override;
    procedure DoSomething(); override;
  end;

  TSkriptTestForm = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
    mTest: TTestObject;
  end;

var
  SkriptTestForm: TSkriptTestForm;

implementation

procedure TSkriptTestForm.Button1Click(Sender: TObject);
var
  cls: TBaseTestClass;
begin
  cls := TTestObject;
  mTest := cls.Create() as TTestObject;
  mTest.DoSomething();
end;

constructor TBaseTestObject.Create();
begin
  inherited Create();
  WriteLn('TBaseTestObject Create');
end;

constructor TTestObject.Create();
begin
  inherited Create();
  WriteLn('TTestObject Create');
end;

destructor TTestObject.Destroy();
begin
  WriteLn('TTestObject Destroy');
  inherited Destroy();
end;

procedure TTestObject.DoSomething();
begin
  WriteLn('TTestObject DoSomething');
end;

// ------------------------------------

TTestObject.Create won't ever be called (only TBaseTestObject.Create will be, but if I use TClass instead of TBaseTestClass, it also won't be). It also makes no difference if I use TPersistent as base or simply TObject. But the DoSomething call works nevertheless, therefore it seems, that the VMT (and other stuff) is initialized correctly, but my own constructor won't ever be called.

Albert