Solved

Destroying Forms.

Posted on 1997-08-10
8
508 Views
Last Modified: 2010-04-04
(A) and (B) are two forms.

I have been using createnew to create a form (B) just before the call to its (B) Show method.  When the form  (B) is closed I then call form(B).release method upon returning to the calling form (A).

I find, by using the 'UBEAUT' new IDE enhancement in DELPHI 3, that the db components (table, datasource,query) have not been freed (as they report as a pointer - not NIL).  This of course means that the form (B) cannot then be freed.

So in the form (B) I have included calls to the various db component's Destroy methods.  I had hoped that this would allow the form (B) to *DIE*.  No such luck.

Next time I need to open this form (B) I check if it is NIL and  if so recreate it.  But it is not NIL so I attempt to access a component (such as a label for changing the forms title or what ever) and BANG the label has a NIL pointer and therefor causes a ACCESS VIOLATION and messes up the application.

Of course one could just leave the form 'ALIVE' until the next time it is needed but this application has some 30 forms and uses  25%-40% of system resources.  I would like to know if it is normal for responsible PC programmers to want to uses a few resource at any one time as possible by  destorying invisible forms or have I spend too much time over the years programming on machines with < 1meg or memory
0
Comment
Question by:Zonnald
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
8 Comments
 
LVL 3

Expert Comment

by:mirek071497
ID: 1341065
Some source please. I think so you must use :  B.Free;
0
 
LVL 3

Expert Comment

by:mirek071497
ID: 1341066
this is from Delphi help, so i think that you play more with source.

"..The CreateNew method creates a new instance of the current form type without reading any state information from resource files or .DFM files. The form is blank, with no controls and all properties at their default. It can be used in place of Create to bypass loading of the associated form file..."
0
 
LVL 3

Expert Comment

by:mirek071497
ID: 1341067
I think so when You call release the form was freed but valiable not setted to nil. try :
  B.Release;
  B := nil;
0
Technology Partners: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

 
LVL 14

Expert Comment

by:AvonWyss
ID: 1341068
Why are you sure that they aren't freed? Pointers usually aren't set to NIL when memory is freed, so this may be your problem.
0
 

Expert Comment

by:nbtaa2
ID: 1341069
You are calling the Show-methode so the form is modeless, right?
You can't call free, release etc. because you don't know when the form is closed.
You must write this statement in B's FormClose-event:

Action := caFree;

This will free the form from meory and release all of B's controls/components.

-Torfinn

0
 
LVL 1

Author Comment

by:Zonnald
ID: 1341070
Thanks,

The form is modal an I have tried caFree.

With regard to AvonWyss' comment - if it isn't NIL how does one know whether or not it need to be recreated.

Anyway I think that maybe **Win95** may actually be at fault (but don't tell BILL)

I know it is not free as I check using the WATCHES and various other neat debugging tools

Thanks again
0
 
LVL 3

Accepted Solution

by:
sperling earned 50 total points
ID: 1341071
I think you've misunderstood Delphi objects a bit, therefore I try submitting this as an answer.


var
  Form1, Form2 : TForm1;

Form1 := nil;
Form1 := TForm1.Create;  
Form2 := Form1;          
Form1.Free;              

This probably illustrates the problem...

The compiler cannot know how many variables you have pointing to the actual TForm1 instance unless full garbage-collection is implemented. Delphi does not implement garbage-collection for objects, as that would slow everything down quite a bit.

You need to explicitly set Form1 := nil after you've freed it.

E.g.

procedure TForm1.SomeEvent...
begin
  Release;
  Form1 := nil;
end;

This is perfectly legal, as the variable Form1 is simply a pointer to the TForm1 instance. An instance doesn't need to have anything pointing to it, therefore you can e.g. do this

with TForm1.Create(Self) do begin
  ShowModal;
  Release;
end;

The fact that you get an access violation when accessing properties/fields of the form indicates that the form is actually freed, and that Form1 points to unallocated memory.

Regards,

Erik.
0
 
LVL 1

Author Comment

by:Zonnald
ID: 1341072
Thanks I'll try this.

Zonnald
0

Featured Post

Independent Software Vendors: We Want Your Opinion

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

A lot of questions regard threads in Delphi.   One of the more specific questions is how to show progress of the thread.   Updating a progressbar from inside a thread is a mistake. A solution to this would be to send a synchronized message to the…
Introduction Raise your hands if you were as upset with FireMonkey as I was when I discovered that there was no TListview.  I use TListView in almost all of my applications I've written, and I was not going to compromise by resorting to TStringGrid…
There's a multitude of different network monitoring solutions out there, and you're probably wondering what makes NetCrunch so special. It's completely agentless, but does let you create an agent, if you desire. It offers powerful scalability …
Add bar graphs to Access queries using Unicode block characters. Graphs appear on every record in the color you want. Give life to numbers. Hopes this gives you ideas on visualizing your data in new ways ~ Create a calculated field in a query: …

695 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question