Re: When to delete, and when to NULL? - C / C++ / MFC Discussion Boards

Masaaki Onishi15-Nov-01 17:07

15-Nov-01 17:07

Hello, the codegurus around the world.;)

Check the RAS notification message called WM_RASDIALEVENT,
and check WPARAM wParam to figure out which RAS event occures.

Good luck! Cool | :cool:

Have a nice day!

-Masaaki Onishi-

Owner drawn button with drop-down menu

Derek Lakin15-Nov-01 3:35

Derek Lakin

15-Nov-01 3:35

As some of you may be aware I have an owner drawn button class (CButtonSSL), which is capable of displaying a drop-down arrow and a menu.

I have recently discovered that if the parent window of the button loses focus then when focus is regained the check state of the menu items is lost, i.e. none of the menu items are checked any more Frown | :(

For those of you without the source code the class stores a CMenu member variable, m_menu. In the call to SetSSLButtonMenu the menu is initialised with a call to m_menu.LoadMenu passing in the resource ID of the menu. The menu item check states are altered in the call to CheckSSLMenuItem which calls m_menu.CheckMenuItem () passing in the menu ID and the check state.

In the OnLButtonDown event handler the popup menu is shown as follows:

CMenu* pMenu = m_menu.GetSubMenu (0);
pMenu->TrackPopupMenu (TPM_RIGHTALIGN | TPM_LEFTBUTTON, x, y, GetParent ());

I have scoured the MSDN and other such resources and get work out why the menu would be losing it's check states. Any suggestions?

Derek Lakin.
Salamander Software Ltd.

When to delete, and when to NULL?

Chambers15-Nov-01 3:33

Chambers

15-Nov-01 3:33

Hi all, I have got a little lost with regards to pointers. I think its quite simple (if you know the answer), basically I have some code like :

void NewPointer()
{
  if (m_pPointer == NULL)
  {
    m_pPointer = new(MyCPointClass)
    if (m_pPointer->NewPoint)
      return;
    else
      delete(m_pPointer);
  }
}

However, the pointer is deleted in this function, and then its deleted elsewhere in my program through a destructor (which is called when i exit the app). If the BOOL function NewPoint returns TRUE every time then everything is OK (my app doesn`t try to delete the same pointer twice). HOWEVER, it does if it returns FALSE. Basically I need to know if a memory leak would occur if I inserted :

m_pPointer = NULL;

instead of actually deleting it in the function? What actually happens to the memory allocated by new in this case? The reason I ask is because, although my program WOULD expel any memory leaks upon exiting the program, I don`t want them building up whilst I am running the damn thing, because in the end I could run out of memory. So, basically, what I need to know is, if I make the m_pPointer = NULL, will a memory leak occur, and why/why not? Thanks again guys for your intuition,

Cheers,
Alan. Smile | :)

"When I left you I was but the learner, now I am the Master" - Darth Vader Mad | :mad:

Re: When to delete, and when to NULL?

Nish Nishant15-Nov-01 3:50

Nish Nishant

15-Nov-01 3:50

What you have new'd you must delete
What you have new'd you must delete
What you have new'd you must delete

If you then make it null that's a good programming practice.

but if you make it null without deleting it then you simply create a memory leak.

Nish

Sonork ID 100.9786 voidmain

www.busterboy.org

Nish is a BIG fan of Goran Ivanisevic

Re: When to delete, and when to NULL?

Jon Sagara15-Nov-01 4:53

Jon Sagara

15-Nov-01 4:53

Nish answered your question, but here's a little debugging tip:

When doing comparisons, put the constant value on the LHS of the comparison operator:

if (NULL == m_pPointer)

If you happen to accidentally leave off one of the equal signs (i.e., NULL = m_pPointer), your code won't compile because you can't assign a value to a literal.

This has saved me lots of debugging headaches. Smile | :)

Jon Sagara

"We assume you already know not to consider something like Windows as a firewall platform. Does the name "Windows" evoke images of security? Silly rabbit, Windows is for desktops." -- Unix System Administration Handbook

Re: When to delete, and when to NULL?

Ray Kinsella15-Nov-01 6:02

Ray Kinsella

15-Nov-01 6:02

Jon Sagara wrote:
When doing comparisons, put the constant value on the LHS of the comparison operator:

Quote/Unquote 'Debugging Windows Application' Microsoft Press Wink | ;)

Regards

Ray

"Je Suis Mort De Rire"

Re: When to delete, and when to NULL?

Jon Sagara15-Nov-01 6:46

Jon Sagara

15-Nov-01 6:46

Yep, it's in there, too. Smile | :)

Re: When to delete, and when to NULL?

Jim A. Johnson15-Nov-01 6:08

Jim A. Johnson

15-Nov-01 6:08

Jon Sagara wrote:
When doing comparisons, put the constant value on the LHS of the comparison operator:

if (NULL == m_pPointer)

If you happen to accidentally leave off one of the equal signs (i.e., NULL = m_pPointer), your code won't compile because you can't assign a value to a literal.

Here's a better tip:

Write it in the readable fashion, as you did, eg:

if (m_pointer == NULL)

and then use the following pragma to prevent assignments in the body of an if statement:

#pragma warning(error : 4706) // Assignment in conditional

This keeps your code readable and safe at the same time. And while the pragma is not portable, the resulting safe, readable code is.

Re: When to delete, and when to NULL?

Jon Sagara15-Nov-01 6:50

Jon Sagara

15-Nov-01 6:50

Jim A. Johnson wrote:
if (NULL == m_pPointer)

I didn't realize that this was non-portable. It has worked for me using VC6 and the GNU compilers.

Also, what is so difficult to read? I actually prefer this method because I know right away what the value of the variable is supposed to be.

Jon Sagara

"We assume you already know not to consider something like Windows as a firewall platform. Does the name "Windows" evoke images of security? Silly rabbit, Windows is for desktops." -- Unix System Administration Handbook

Re: When to delete, and when to NULL?

Michael Dunn15-Nov-01 9:11

Michael Dunn

15-Nov-01 9:11

He meant the #pragma that disallows assignments in if conditions is non-portable.

--Mike--
http://home.inreach.com/mdunn/
#include "witty_sig.h"
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Re: When to delete, and when to NULL?

Jon Sagara15-Nov-01 10:58

Jon Sagara

15-Nov-01 10:58

Context - The Mother of all Flame Wars.

Thanks for the note Michael,

Jon Sagara

"We assume you already know not to consider something like Windows as a firewall platform. Does the name "Windows" evoke images of security? Silly rabbit, Windows is for desktops." -- Unix System Administration Handbook

Re: When to delete, and when to NULL?

Jim A. Johnson15-Nov-01 12:15

Jim A. Johnson

15-Nov-01 12:15

Jon Sagara wrote:
I didn't realize that this was non-portable. It has worked for me using VC6 and the GNU compilers

That's not what I said. Read it again, please.

Re: When to delete, and when to NULL?

Jon Sagara15-Nov-01 12:36

Jon Sagara

15-Nov-01 12:36

This keeps your code readable and safe at the same time. And while the pragma is not portable, the resulting safe, readable code is.

Misunderstanding. My bad. Please forgive me this one transgression. Poke tongue | ;-P

Just out of curiosity, why don't you consider if (NULL == m_pPointer) readable? I admit it looked strange to me at first, but it's such a simple coding trick that you can use to save debugging time without having to remember some obscure pragma directive.

Jon Sagara

"We assume you already know not to consider something like Windows as a firewall platform. Does the name "Windows" evoke images of security? Silly rabbit, Windows is for desktops." -- Unix System Administration Handbook

Re: When to delete, and when to NULL?

Tim Smith15-Nov-01 12:54

Tim Smith

15-Nov-01 12:54

Yeah, I agree. It is all about just getting use to it.

Tim Smith
Descartes Systems Sciences, Inc.

Re: When to delete, and when to NULL?

Colin Urquhart15-Nov-01 15:47

Colin Urquhart

15-Nov-01 15:47

Why not use:

if (!m_pPointer)

instead of

if (NULL == m_pPointer)

Smile | :)

Re: When to delete, and when to NULL?

Michael Dunn15-Nov-01 19:07

Michael Dunn

15-Nov-01 19:07

Because the second is instantly readable, whereas with the first one you have to remember that "!pointer" means "pointer is not zero".

--Mike--
http://home.inreach.com/mdunn/
#include "witty_sig.h"
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Re: When to delete, and when to NULL?

#realJSOP16-Nov-01 0:50

#realJSOP

16-Nov-01 0:50

I agree with Mike, but I would make it

if (m_pPointer == NULL)

instead.

It just looks better to me because it flows more like the way you speak the same phrase. I've never heard anyone say "If NULL is the pointer...", but instead, people say "If the pointer is NULL...".

"...the staggering layers of obscenity in your statement make it a work of art on so many levels." - Jason Jystad, 10/26/2001

Re: When to delete, and when to NULL?

Gert Boddaert15-Nov-01 5:06

Gert Boddaert

15-Nov-01 5:06

void NewPointer()
{
if (m_pPointer == NULL)
{
m_pPointer = new(MyCPointClass);
if (NULL != pPointer && m_pPointer->NewPoint()) // NewPoint is a function, right?
{
return;
}
else
{
delete(m_pPointer);
m_pPointer = NULL; // avoid deleting the memory twice! deleting a NULL pointer is safe, but has a performance penalty since a call to the C-runtime library is done.
}
}
}

void DeletePointer()
{
if (NULL != m_pPointer)
{
delete m_pPointer;
m_pPointer = NULL;
}
}

if you insert
m_pPointer = NULL
somewhere in your code between NewPointer() and DeletePointer(), then yes you have a memory leak.

--------------------------------------------------
If my messages appear curt, I apologize.
I try to be brief to save your time as well as mine.
--------------------------------------------------

Re: Thankyou All.

Chambers15-Nov-01 9:32

Chambers

15-Nov-01 9:32

I would just like to say thanks to everybody for their response, all the answers were superb. I just didn`t think to delete the pointer first and then assign it NULL, but everything fits perfectly in my head now. I wondered how the application could delete those pointers in my program that were unused, without any trouble and it was because I had assigned them all to NULL in the constructor! For some reason I thought delete, deleted the memory AND set it to NULL, thats why I couldn`t grasp how it was actually working. But thanks to everyone who wrote in, I am very grateful,

Cheers,
Alan. Big Grin | :-D

P.S. I liked the tips too. Just when I think I seem to be mastering this language, someone comes along and goes "Here's a better way of going about it...", that always amazes me. Thanks for those too guys.

"When I left you I was but the learner, now I am the Master" - Darth Vader Mad | :mad:

How can I open default mail client from my app in VC?

Karavaev Denis15-Nov-01 2:07

Karavaev Denis

15-Nov-01 2:07

pls

==============
www.design.kg

Re: How can I open default mail client from my app in VC?

Nish Nishant15-Nov-01 2:18

Nish Nishant

15-Nov-01 2:18

use this

ShellExecute(this->m_hWnd,"open","mailto:","","",SW_SHOW);

Nish

Sonork ID 100.9786 voidmain

www.busterboy.org

Nish is a BIG fan of Goran Ivanisevic

It's only opening New message window. I need all programm(etc Outlook, The bat, opera ...)

Karavaev Denis15-Nov-01 3:01

Karavaev Denis

15-Nov-01 3:01

?
==============
www.design.kg

Re: It's only opening New message window. I need all programm(etc Outlook, The bat, opera ...)

Nish Nishant15-Nov-01 3:10

Nish Nishant

15-Nov-01 3:10

you need to open the following registry key

HKEY_CLASSES_ROOT\mailto\shell\open\command

Nish

Sonork ID 100.9786 voidmain

www.busterboy.org

Nish is a BIG fan of Goran Ivanisevic

Re: It's only opening New message window. I need all programm(etc Outlook, The bat, opera ...)

15-Nov-01 3:14

Re: It's only opening New message window. I need all programm(etc Outlook, The bat, opera ...)

Nish Nishant15-Nov-01 3:29

Nish Nishant

15-Nov-01 3:29

(1) open the key HKEY_CLASSES_ROOT\mailto\shell\open\command

(2) read it default value. I got this when I did that here
"%ProgramFiles%\Outlook Express\msimn.exe" /mailurl:%1

(3) you need to replace %ProgramFiles% with the actual path
for that use SHGetSpecialFolderLocation and SHGetSpecialFolderPath [you'll find samples in CP]

(4) forget the /mailurl:%1 part

(5) shellexecute the rest of it

Smile | :)

Nish

Sonork ID 100.9786 voidmain

www.busterboy.org

Nish is a BIG fan of Goran Ivanisevic

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