As far as I know, there is no way to do this. The best you can do is raise an exception and catch this.

Michael.

Hello, Every one

I intend to create my own class such as the following:

>>>

type
  TMyClass = class(TObject)
    public
      constructor Create(fn:String);
      destructor Free;
  end;//TMyClass
implementation
constructor TMyClass.Create(fn:String);
begin
  if not FileExists(fn) then begin
    // ---> Here I want to make the new object nil
  end;
end;//TMyClass.Create
demo:
  myclass:=TMyClass.Create('aaaa.txt');
  if myclass=nil then exit;// ---> Here I can check if the object is valid
<<<<<<<<
  Can you tell me what's the constructor should be like to implement this?
  Thank you,
  mili

program createif;
{$MODE OBJFPC}
uses sysutils;
type TMyClass = class(TObject)
  private
    fFileName:string;
  public
    constructor Create(FN:string; dummy:byte);
    class function Create(FN:string):TMyClass;
end;
constructor TMyClass.Create(FN:string; dummy:byte);
begin
  inherited Create;
  fFileName:=FN;
end;
class function TMyClass.Create(FN:string):TMyClass;
begin
  if FileExists(FN)
  then Result:=TMyClass.Create(FN, 0)
  else Result:=nil;
end;
var
  myclass:TMyClass;
begin
  myclass:=TMyClass.Create('aaaa.txt');
  if myclass=nil then begin
    writeln('object not created - file not found');
    exit;
  end else begin
    writeln('Sucess!!!');
    myclass.Free;
  end;
end.

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On Thu, 22 Aug 2002, mili wrote:

}constructor TMyClass.Create(fn:String);
}begin
}  if not FileExists(fn) then begin
}    // ---> Here I want to make the new object nil
}  end;
}end;//TMyClass.Create
}

You answered your question yourself: "Fail".

constructor TMyClass.Create (fn:String);
 begin
   if FileExists(fn) then Fail;
   //
 end;

AFAIK this should do exactly what needed.

By the way, destructor should not be named Free - Free is another method which calls Destroy. It is usually good idea to override TObject.Destroy.

------

Ingmar

------

"Salamanders can survive up to six hours outside a fire if regularly fed pepper" (Scamander 1927).

And what is this "fail"? Is it a predefined identifier, similar in category to those "exit" and "break"? or would I need to define such method?

Aitor

Quoting Aitor Santamaría Merino :

And what is this "fail"? Is it a predefined identifier, similar in category to those "exit" and "break"? or would I need to define such method?

If you raise an exception at the point 'fail' happent, it will also give you much the same result...

{ define an exception class }
type
  EFileExistsException = class(Exception);

{...}

constructor TMyClass.Create(fn:String);
begin
  { raise an exception in the case that the file exists }
  if FileExists(fn) then
    raise EFileExistsException.Create('Error, File exists');
end;

{...}

...
  try
    { instantiate class }
    X := TMyClass.Create('c:\aaabbbcc.txt'); //this may fail...
    try
      { Use try..finally, be nice, protect our resources }
      //do whatever with object instance
    finally
      { remember to clean up }
      X.Free;
    end;
  except
    { handle exception, names so that we don't just trap everything...
      this code should only happen if the constructor fails, unless we reuse
      the exception class }
    on E: EFileExistsException do begin
      //handle the error in some way...
      writeln(E.Message);
      { X should still be instanciated, so we must free, but as a precaution
        use Assigned() to check that assumption }
      if Assigned(X) then X.free;
    end;
  end;
...

That's how I'd do it. It's less of a kludge than attempting to hide public members behind a fake constructor. This would probably need to be compiled in Delphi mode before anyone cries about it not working ;-)

Hope that helps, anyway,

Matt

On Fri, 23 Aug 2002, Aitor wrote:

}And what is this "fail"? Is it a predefined identifier, similar in }category to those "exit" and "break"?

Exactly. Borland, our honourable ancestor and inventor of these things, calls them "procedures", which is confusing, of course.

Fail can only be used inside a constructor. If I remember correctly, it somehow calls destructor of this object or class. Anyway, it deallocates the instance and causes the constructor to return Nil.

------

Ingmar

------

"Salamanders can survive up to six hours outside a fire if regularly fed pepper" (Scamander 1927).