asked on

What's the best way of doing this?

We have some old code which is possibly causing us headache. We keep getting access violations all the time, and I can't work out where the problem starts.

The person who originally wrote the code did things a little differently to what I'm used to, and I'm trying to work out if this is OK or not.

The form is dynamically created:

MktForm := MktForm.Create(Self);
MktForm.ShowModal;

in MktForm, this is done:

FormClose
Action := caFree;

FormDestroy
MktForm := nil;

Is this OK to do? Is instructing the Action to free the form on close, plus setting it to Nil in the FormDestroy allowed?

I can't trace the source to where the A/V occurs. It's annoying the hell out of me. It only happens sometimes (when a certain sequence of clicks is made). So I'm trying to find all dynamically created objects and make sure they're all freed. There are a lot of them, but the majority aren't called in most circumstances so it's not causing my general problem.

Any help would be appreciated.

Stu

kretzschmar

the nil in formdestroy is not a good choice

Motaz

When you create any component at run-time and you assign an owner for it, the owner (Form for example) will destroy it when the owner destroyed, so that if the owner find that this component is already destroyed or assigned to nil, an access violation some times occure.

until now, I didn't find a solution for this problem, so that I lave such components without destorying and let thier owner to destroy it.

Motaz

Igor UL7AAjr

my usual way:

MktForm := MktForm.Create(nil);
try
if MktForm.ShowModal = mrOK then
begin
// do something here
end;
finally
MktForm.Release;
// if you have to access MktForm variable from another part of your application
// and be sure that it is initialized or not (checking nil) then
// uncomment line bellow
// MktForm := nil;
end;

------
Igor.

robert_marquardt

with MktForm.Create(Self) do
try
if ShowModal = mrOK then
begin
// do something here
end;
finally
Free;
end;

That is my way of doing it.
I also delete the global variable from MktForm.
I never had a problem with Free for the form.

Igor UL7AAjr

>> MktForm := MktForm.Create(nil);
must be:
MktForm := TMktForm.Create(nil);

>> with MktForm.Create(Self) do
the same error
with TMktForm.Create(Self) do

btw, Release is preferable. I had problems with Free and it was solved regarding Release.

----
Igor

ASKER CERTIFIED SOLUTION

AvonWyss

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Motaz

Robert, free problem is not always occured, I use it for long time in my graduation project while I was developing it, and every thing working fine, in the discussion day it rais access violation when I close my project, and that was the first time, so that you can not be sure if freeing owned component manually safe or not.

Motaz

Coltrui

I've experienced the same problems and this worked for me:

1. Change the 'Self' into 'Application'
2. Make sure that every form is in the 'available form'-section instead of the 'autocreate form'-section.

oraelbis

There is some code from my program:

type
TfrmMinCost = class(TForm)
. . .
private
. . .
public
. . .
end;

procedure ShowMinCostForm;

var
frmMinCost: TfrmMinCost;

implementation

{$R *.DFM}

procedure ShowMinCostForm;
begin
try
if not Assigned(frmMinCost) then
Application.CreateForm(TfrmMinCost,frmMinCost);
if frmMinCost.ShowModal = mrOK then
begin
. . .
end;
finally
frmMinCost.Free;
frmMinCost:=nil;
end;
end;
. . .

end.

Remove this form from Project->Options->Forms Auto-create list.

thegroup

In my experience you could set the OnCreate event handler to:

Action = caFree;

This makes the memory being freed without problems.

oraelbis

Check your modules for not disconnecting connections, such as Socket, HTTP or data access components.
Close all DataSets, before closing forms or application.

Stuart_Johnson

ASKER

Hi Folks,

Sorry for the length of time it's taken for me to post anything. All your comments are great and very helpful. Most of what you have said I already do - remember the code I posted above was someone elses.

What I usually do is never have any forms auto created (except the main form). I then create the forms, but I always use nil as the owner - unless the form is an MDI child. I always free the form using FormName.Free:

MyForm := TMyForm.Create(nil);
try
MyForm.ShowModal;
finally
MyForm.Free;
end;

I never have problems with it.

I just wanted clarification that what was done above wasn't really advisable. The guy who coded our app originally was a dreadful programmer. His code is the worst I've seen (even our 1st year juniors code better than him).

I'll have a good read through what's posted in the morning (as it's fairly late here now and I want to get home). I'll ask for more info if I need it, or award some points.

Thanks again,

Stu

Stuart_Johnson

ASKER

Meikl,

You said "the nil in formdestroy is not a good choice".

Can you tell me why?

Our app is MDI. We check to see what forms are opened using if FormName <> nil then do....

If we don't say FormName := nil; in the FormDestroy, FormName is always assigned "something", so testing for nil doesn't work.

AvonWyss:

I love that memory checking utility! Thanks for the link.

thegroup:

AFAIK FormCreate doesn't have an action property, but FormClose does.

oraelbis:

All datasets are closed before the form is closed. We don't have anything else which could cause problems that we know of (no socket components or stuff expecting data).

Although AvonWyss didn't directly answer the question, he did give me some great tips and a very useful application. The points are yours.

Thanks to everyone for your help and input. I really appreciate it.

Stu

AvonWyss

Stuart, thank you! Have a nice weekend.

Stuart_Johnson

ASKER

No problems at all! You have a great weekend too!

Stu