[fpc-pascal]How to make a constructor failed?

[Full_Name]

Quoting Aitor Santamaría Merino <aitor.sm at wanadoo.es>:

> 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

--

"Computer games don't affect kids; I mean if Pac-Man affected us as kids, 
we'd all be running around in darkened rooms, munching magic pills and 
listening to repetitive electronic music." 
Kristian Wilson, 
Nintendo, Inc, 1989

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