New KB Comments

Thanks Daniel. I added your info on (* *) to the multiple-line comment section and I combined the single line comment info with this post and deleted the single line comment. Originally I wanted to document single line comments from multiple line comments. In hindsight, that was a dumb idea.

Since compiler directives are also embedded in comments, I added a bit of introductory info about that too.

You're welcome and thanks for the kind words. By the way, a few months ago you suggested we improve the look and make it a bit simpler and cleaner. Based on your input (and others) I made quite a few changes. Is the website better looking now? Or, do I have more work to do. I do have some ideas I haven't had time to implement but if you have any suggestions, they are welcome.

Mike,

You are an encyclopedia of knowledge. So if no the session, even if the extracts of the session on UML 1.4 in th eform of knowldege base article would help us learn a lot.

Request you to evaluate the possibility.

And, thank you very much for so much knowledge sharing!

Regards

Arti

Although I still like Paradox, I guess I should add that if you want to explore switching to MS Access, check out my ObjectPAL and Access VBA Cross Reference Guide.

> It's over six months since it was revealed that Corel still did not support Paradox under Vista in WPX4...have they fixed this yet?

to be honest, it's doubtful that Corel will ever touch Paradox again.. all the people that were associated with it have been gone since 2002..

No new info. I agree that they should fix this especially if they want to make good software and be known as a quality software company. I'm hopeful they will find the money to fix these errors. Until they find the money, it's just not going to happen. Paradox 9 works fine on Vista though so you might want to revert to that version. In my opinion, Paradox is the best desktop database available. It's only real competitor is Microsoft Access and even though Microsoft has continued development of Access and it does have some neat features, it's still not as easy to use as Paradox.

It's over six months since it was revealed that Corel still did not support Paradox under Vista in WPX4...have they fixed this yet? I'm particularly concerned about the BDE initialization problem and the 'Paradox has stopped' error on exiting. It seems to me that Corel shouldn't be selling the product as a Vista compatible package with these kinds of errors but I can't find any reviews of X4 that indicate that the problem has been fixed (the reviews only seem concerned with WP itself) and I don't want to pay for an upgraded version if the problem is still there. Any new info out on this?

Hi Arti,

No, we didn't capture that event. I'm hoping to capture some in the future and post them to the website. Until then, I'm mainly focusing on knowldge base articles and responding to message board Q&A.

Mike, will you be sharing the session content on this site?

Regards

Dr. Arti Mehta

I always admire the simplistic text you use to explain any concept.

Regarding introduction to DFD, a small diagram, if possible, would have helped to relate description with the visualization.

Regards

Dr. Arti Mehta

Another form of a multiple line comment

(*

  some comments go here

*)

Speaking of comments:

// I am a Single line comment

Multiple levels of comments. You can nest comments like the example below

(*

  I am a comment

  {

    I am a comment within a comment

  }

*)

The hierarchy of comments:

(* comment *) = highest precedence

{ comment }

// comment

To mark or unmark a series of lines of code with a comment

1) Highlight the lines of code you wish to comment out - hold down the shift key and move cursor up or down.

2) Hit Ctrl + / and the lines will be commented out. If they already were commented out, they will no longer be commented

Before:

if (something) then

begin

  ShowMessage('I did it.');

end;

After:

//if (something) then
//begin
//  ShowMessage('I did it.');
//end;

Whether to use Free, or FreeAndNil, is an interesting topic.  It depends on the circumstances.

My rule of thumb is that if an object is local (that is, it's reference variable will soon go out of scope) just use Free.  But if the object is more "global," use FreeAndNil.

A classic example is a form that will create and show another form.  This is often done within the scope of a button click handler, or an ActionItem's Execute method:

procudure MyForm.Button1Click(Sender : TObject);

var 

  StudentForm : TStudentForm;

begin

  StudentForm := TStudentForm.Create(Self);

  StudentForm.ShowModal;

  StudentForm.Free;

end;

In the above snippet, the local variable, StudentForm, isn't accessible outside of the click handler, and it will go out of scope fairly soon.  In this case, I think Free is sufficient.

However, if I'd made my reference variable more "global," say as a private member, FStudentForm, of TMyForm, any and every method of that class has access to it. In that case, I'll use FreeAndNil:

procedure MyForm.Button1Click(Sender : TObject);

begin

  if (FStudentForm = nil) then

    FStudentForm := TStudentForm.Create(Self);

  FStudentForm.ShowModal;

  FreeAndNil(FStudentForm);

end;

Just to play it safe, you'll probably want to set FStudentForm to nil in TMyForm's OnCreate event.

Of course, it does no harm to use FreeAndNil wherever it's legal (As Mike says, the argument to FreeAndNil must be a TObject, or descendant thereof).

Hmmm...sure, that's a good rule of thumb and thanks for adding the info here (I added it to the flashcard above too). Bottom line.

However, to further the discussion I guess it depends on the situation. It is a routine and not a class method so using Free "feels" more OOP to me so I tend to use it with simple OOP examples when giving demos. ((I know, that's a pretty weak excuse but I think it helps with new coders.)) You do have to be careful to only pass TObjects to this routine too. My own rule of thumb with larger applications is that I use Free when doing simple controlled tasks and FreeAndNil with more complicated things.

Also, when do we stop writing extra checks? You could be even more complete and do something like the following:

If Assigned(MyObject) then
  FreeAndNil(MyObject)
Else
  MyObject := nil;

If by some chance MyObject was not assigned to begin with, then FreeAndNil() will never get called and therefore MyObject will never get set to nil.

Should Be NilAndFree

Finally a minor point but because we're getting into it a bit, I thought it might be important because FreeAndNil actually does the opposite. You can run into problems accessing the object variable while it's being destroyed.

For reference, here are the three VCL code routines in discussion:

In System Unit:

//TObject.Free

procedure TObject.Free;
begin
  if Self <> nil then
    Destroy;
end; 

//TObject.Destroy

destructor TObject.Destroy;
begin
end;

In SysUtils unit:

//FreeAndNil

procedure FreeAndNil(var Obj);

var 

  Temp: TObject;

begin 

  Temp := TObject(Obj); 

  Pointer(Obj) := nil; 

  Temp.Free;

end;

Hi Daniel:

I just wanted to welcome you and say your post is just fine right here. We welcome your input.

Hi Francis,

Yeah, I have one developer notebook in the same state. All my other boxes either worked just fine, or I had a few problems but were able to work around them. On the box of mine that isn't working, I'm having some Windows install issues I just haven't had time to address. It thinks Word 2007 is installed when actually Word 2003 is installed, I'm going to uninstall 2003 and install 2007 and try again.

You can open a support ticket with Microsoft. It's free but typically you deal with someone in India who just wants to get rid of you so he or she can keep their support numbers up and make money. I sure miss the days when I got someone in America to work with. Nothing against foreign countries, but techinical stuff is hard enough as it is and the culture differences sometimes make it even harder! Plus, we Americans built Microsoft with American money. It sure would be nice to keep that money here in America.

Anyway, try exploring your Installation History in Windows Update to see what install failed or open a MS support ticket.

Mike I did exatly as you suggested but it still will not load SP1. I am perplexed by this as my wife has the same system and computer as mine and on hers it loaded fine.

Francis

Newbie posting, so please revise or move if desired:

What Wes was referring to about freeing objects within a list manually is shown below using a combo box, for example, as a list type item holding objects within itself.

class procedure TUtils.FreeAndNilComboBoxObjects(p_cboWork: TComboBox);
{
  Pass in the combo box, and this will clear any objects 

  within the combo box
}
var
  nCount: integer;
begin
  for nCount := 0 to (p_cboWork.Items.Count - 1) do
  begin
    if (p_cboWork.Items.Objects[nCount] <> nil) then
    begin
      p_cboWork.Items.Objects[nCount].free;
      p_cboWork.Items.Objects[nCount] := nil;
    end;
  end;
end;

String Grids can hold objects too, here is sample code

  for nCount := 0 to (p_sgdWork.RowCount - 1) do
  begin
    if (p_sgdWork.objects[0,nCount] <> nil) then
    begin
      p_sgdWork.objects[0,nCount].free;
      p_sgdWork.objects[0,nCount] := nil;
    end;
  end;

WIth Delphi 5 and later, it is my understanding it is better to use

FreeAndNil(objMyObject);

Thoughts and comments?

Today I updated this article as follows:

• Removed references to Linux.
• Updated presentation of OOP concepts.
• Embedded links to related information.

Hey Francis. Got it. Have you tried the full Vista SP1 installation yet? If not, "hide" the SP1 update in Windows Update then download and install the full 453MB Vista SP1.

Vista SP1 is not installed ( under copyright notice say it says upgrade windows vista). Also I have downloaded the manual install and have tried three times. Same problems as before everytime.

Today I updated this article with minor updates to reflect changes in the industry (the popularity of C#) and some further clarifications (class based vs object based languages).

You can know for sure if Vista SP1 is installed by right clicking on My Computer and clicking Properties. If you have it installed, it'll say so right under the Copyright notice.

If you're having trouble with Windows Update installing SP1, try installing SP1 manually. It requires you download the FULL SP1 (434 MB) but that might work for you. Go to Microsoft.com and search for "download Vista SP1".

I have done this proceedure on a number of occasions. It has certainly reboots my system ok but Windows keeps advising me that I have an update (Vista SP1)

How do you actually  install SP1 if this constantly happens?

>>...can we have multiple LEVEL 0 diagrams?

...hmmm....not usually. The level 0 diagram is supposed to show the context of the data-system you are documenting and if you're tempted to create two, then I would think you are either trying to document two systems or are not thinking at a high enough "level" for the level 0 context diagram. In other words, you need to continue leveling.

However, to me, documentation is strictly for the purpose of communication. So, if one of my analysts came to me with two level 0 diagrams, I would simply ask, "how does this communicate better than a single level 0 diagram?"

Does that make sense? Can you think of a reason why you would want to use two level 0 context diagrams?