[solved] Aborting construction of object

[DimProfi]

Is there a possibility to abort object construction and return nil without using exceptions in this theoretical way:

Code: [Select]

type
  TObjectA=class
      constructor Create(filename:string);
  end;

constructor TObjectA.Create(filename:string);
begin
  if not FileExists(filename) then
  begin
     Result:=nil;
     exit;
  end;

  Result:=inherrited Create();
end;


Code: [Select]


  obj:=TObjectA.Create('C:\test.db');

  if not assigned(obj) then
  begin
     showmessage('Database could not be accessed!');
     exit;
  end;

   obj.getData();
   ...
   obj.Free;

May be I'm a bit crazy 

The only alternative would be an object factory, which creates objects and returning it, if object initialization successful.


P.S.: I dont like using exceptions because they can appeare everywhere in a constructor or destructor or in a block of handling other exception.

[BrunoK]

constructor TObjectA.Create(filename:string);
begin
  if not FileExists(filename) then
  begin
     FreeAndNil(Self); // Result:=nil; >>>  FreeAndNil in <uses sysutils >
     exit;
  end;

  Result:=inherited Create();
end;

Might work.

[User137]

This worked for me, form caption became 'Is nil'. Free would do a little unnecessary work here. Checking if it's nil at first, when it never is in constructor.

Code: [Select]

constructor TSomeObject.Create;
begin
  Destroy;
  self:=nil;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  if TSomeObject.Create=nil then caption:='Is nil';
end;

[DimProfi]

  self:=nil;

I thought self is read-only
Thanx!

[DimProfi]

  FreeAndNil(Self); // Result:=nil; >>>  FreeAndNil in <uses sysutils >

Thanx!
Gonna check with Heaptrc if all good with FreeAndNil in constructor.