C / C++ / MFC

Re: public member not working

16-Mar-04 10:45

Can you trim the class, including the member-variable access, down to just a few lines that exhibits the problem?

"The pointy end goes in the other man." - Antonio Banderas (Zorro, 1998)

BlackDice16-Mar-04 10:50

BlackDice

16-Mar-04 10:50

thanks, I got it working. I was trying to cheat and mess with a class that I didn't write and change the access to public, but I found a right way to do it, so I don't have to worry about it. Thank you anyway!

If it's broken, I probably did it

bdiamond

C++ pointer question

patnsnaudy16-Mar-04 10:20

16-Mar-04 10:20

If I have a pointer to an object foo is there anyway to tell if that object actually exists?

Say I only want one window open at a time.

void
foo::openWindow()
{
   static foo* ptr = NULL;
   if (ptr)  
      ptr->closeWindow();
   ptr = this;

   // open this window 
   ...
}

So, the second time through this code I want to know if ptr is still valid, because it may have been deleted without my knowledge.

I know there are other ways to do this, but I want to know if I can figure out if ptr still exists.

I've tried adding a ptr = dynamic_cast<foo*>(ptr);, but this throws an exception if ptr has been deleted, and I'd rather not use a try block.

Thanks in advance

Joe Woodbury16-Mar-04 10:55

Joe Woodbury

16-Mar-04 10:55

I completely fail to see the purpose of your code, but that aside, you can simply check whether the 'this' pointer is NULL.

Anyone who thinks he has a better idea of what's good for people than people do is a swine.
- P.J. O'Rourke

patnsnaudy16-Mar-04 11:11

16-Mar-04 11:11

Yeah but if you create

foo *a = new foo();
foo *b = new foo();

a.openWindow();
b.openWindow(); // This should close a.
delete b;       // ptr in openWindow() still points 
                //  to the space b used to occupy.
a.openWindow(); // Trys to close b, but b no longer exists (ERROR).

I know I could create a member and set it to this in openWindow and NULL in closeWindow, but I do not want to have a member for this.

If you change the code to use a dynamic_cast and put a try-catch(...) around it this will work as is, but the powers that be don't like try-catch, so I'm not allowed to use it.

Christian Graus16-Mar-04 11:29

Christian Graus

16-Mar-04 11:29

patnsnaudy wrote:
but the powers that be don't like try-catch, so I'm not allowed to use it.

Not being allowed to use try..catch is wrong, but so is a design that throws exceptions based on bad coding, so on aggregate, I'm with the 'powers that be'.

Christian

I have drunk the cool-aid and found it wan and bitter. - Chris Maunder

Ravi Bhavnani16-Mar-04 17:35

Ravi Bhavnani

16-Mar-04 17:35

It's good coding hygiene to set a deleted pointer to NULL. This will help prevent run-time errors and could make your code simpler.

/ravi

My new year's resolution: 2048 x 1536
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TFrancis16-Mar-04 10:58

TFrancis

16-Mar-04 10:58

If there's some function call out there that tells you if a spot in memory is valid that'd be a real easy way of doing this. But I don't know of one.

My suggestion. Make ptr a static member of the foo class. Then, in your foo destructor, if ptr == this, set ptr = NULL.

Tim

patnsnaudy16-Mar-04 11:02

16-Mar-04 11:02

I know I can do this, but I did not want to add a member for this.

Thanks though!

Jerry

David Crow17-Mar-04 3:03

17-Mar-04 3:03

TFrancis wrote:
If there's some function call out there that tells you if a spot in memory is valid that'd be a real easy way of doing this. But I don't know of one.

How about:

IsBadReadPtr()
IsBadCodePtr()
IsBadStringPtr()
IsBadWritePtr()

"The pointy end goes in the other man." - Antonio Banderas (Zorro, 1998)

Christian Graus16-Mar-04 11:31

Christian Graus

16-Mar-04 11:31

Add an operator HWND to the class that returns the window handle and call ::IsWindow.

Christian

I have drunk the cool-aid and found it wan and bitter. - Chris Maunder

patnsnaudy16-Mar-04 12:55

16-Mar-04 12:55

There is already a function to return HWND I think, if this is derived from the window class (I'll have to check tomorrow at work). ::IsWindow is a great suggestion! I was looking for a magical function that would look at a piece of memory and tell me if it had been deleted, but IsWindow should work.

Thanks

Prakash Nadar16-Mar-04 14:55

Prakash Nadar

16-Mar-04 14:55

it will not work if the pointer to the dialog is deleteed.

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prakashnadar@msn.com

patnsnaudy17-Mar-04 3:56

17-Mar-04 3:56

By it will not work does that mean that it will crash if the pointer has been deleted?

Prakash Nadar17-Mar-04 16:45

Prakash Nadar

17-Mar-04 16:45

yes,it will very unpredictable, it may crash in some cases and it will work in some cases. but then working with deleted pointer is very bad idea.

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prakashnadar@msn.com

Ryan Binns16-Mar-04 17:10

Ryan Binns

16-Mar-04 17:10

To determine if a pointer points to readable memory, use the IsBadReadPtr() function. If it returns FALSE, then the pointer cannot be read.

Hope this helps,

Ryan

"Punctuality is only a virtue for those who aren't smart enough to think of good excuses for being late" John Nichol "Point Of Impact"

I gave in (sln)

patnsnaudy18-Mar-04 2:12

18-Mar-04 2:12

I just created a static pointer to the window and set it to this in the openWindow function and NULL in the dtor.

Thanks

How to TURN UNICODE ON.

HAHAHA_NEXT16-Mar-04 10:19

HAHAHA_NEXT

16-Mar-04 10:19

Hi all.

Using TCHAR is fine, but i wanted to know if there would be a way of turning UNICODE on for my application (#define _UNICODE does nothing). Needed for testing (I fear LOTS of errors Dead | X|

).

Please help.

David Crow16-Mar-04 10:34

16-Mar-04 10:34

Have you also defined UNICODE (in addition to _UNICODE)? Make sure they are both defined as close to the top of stdafx.h as possible.

With both defined, TCHAR will resolve to unsigned short. Otherwise it will resolve to char.

"The pointy end goes in the other man." - Antonio Banderas (Zorro, 1998)

HAHAHA_NEXT16-Mar-04 10:39

HAHAHA_NEXT

16-Mar-04 10:39

Txs. This was fast.

Now how do i get i get sprintf working.
(i do not want to use swprintf because it is unicode only)

For printf there is at least _tprintf , but i can find anything for
sprintf. Is there an euivelent and if not how can i redirect the ourput
to a TCHAR variable.

Thx in advance.

David Crow16-Mar-04 10:46

16-Mar-04 10:46

#ifdef _UNICODE
swprintf(...);
#else
sprintf(...);
#end

"The pointy end goes in the other man." - Antonio Banderas (Zorro, 1998)