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The problem I had is that If I used this code:
CString MyString;
mFile->Read((LPTSTR)(LPCTSTR)MyString, iMyLength);
MyString.Replace("something", AnotherStringWhichIsAMemberVarOfTheClass);
When reading the file, the var AnotherStringWhichIsAMemberVarOfTheClass got screwed it's value with what I wanted to read (from the file).
Then, by using the Try-All-The-Methods-Of-The-Class solution I noticed that If I wrote:
CString MyString;
mFile->Read(MyString.GetBuffer(iMyLength), iMyLength);
MyString.ReleaseBuffer();
MyString.Replace("something", AnotherStringWhichIsAMemberVarOfTheClass);
Everything went right, but I still don't understand why. I'm modifying MyString's content and you've told me that then it'd be better to use GetBuffer(), but contents of AnotherString(...) get screwed when reading the file and before modifying it.
Could you tell me why ????
Thanks for everything.
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OK, it's very simple: CString internally holds a buffer of characters. From our standpoint (programmers using the CString class who don't know exactly how it works) CString's buffer grows/shrinks based on its contents.
In the first scenario (the one that breaks) you instanciate a CString object (MyString) using its default constructor. This means that it's empty -- it's got an empty buffer. Then you read the file and try to copy its characters into the empty CString object. Since it's got an empty buffer, you end up inadvertently copying the characters to whatever memory is adjacent to it. So you basically overwrite memory which you hadn't allocated for that purpose. This is very bad, as you noticed.
In the second scenario, you call GetBuffer to not only return a writable buffer (LPTSTR), but you also tell the object to allocate iMyLength bytes for you to copy into. After reading from the file, you tell the CString object that you're done using the buffer (ReleaseBuffer) and that it should clean it up (find the terminating null character) and make the object ready for use.
I hope this is clear to you, but let me just say that usually if you have to do nasty casting like (LPTSTR)(LPCTSTR)MyString, it's a sign that something's not quite right. You should never do that with a CString object unless you're certain that its buffer won't be changed (ie., you're passing a CString object to poorly written code that should have used LPCTSTR instead of LPTSTR).
The general rule is: if the function takes an LPCTSTR (or const char*), use the object directly (no casting needed). If the function takes an LPTSTR (or char*), use GetBuffer/ReleaseBuffer.
Regards,
Alvaro
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Hello, the codegurus around the world.;)
One rule is that you had better use MFC as much as possilbe in your code
if you want to use CString.
If you want to read the content of the file, you can use CStdio calss.
Since you use CString value, and want to pass const char* (LPCSTR) of the function,
it sometimes waste your code line (GetBuffer...) or the memory. (but, maybe not big deal).
However, if we use COM or Unicode, we face this kind of problem so often.
The string conversion gives us so much headache. 
Have a nice day!
-Masaaki Onishi-
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(edited)
Alvaro's explanation points to the reason. But note that even the following won't work:
CString a = "Ophelia";
CString b = a; // *1*
strcpy((LPTSTR)(LPCTSTR) b, "Hamlet"); // *2*
will change a and b, because:
CString uses a "copy on write" mechanism. At line *1*, b will reference the same string buffer as a. Only when you change one of the strings, an actual copy is made.
*2* now modifies the shared string buffer without CString having any chance of knowing it. Hence no copy, hence a shared buffer gets modified.
-----------------------
CString doesn't have a cast to LPTSTR for a reason. And blindly casting to LPTSTR is definitely not a good idea.
Peter
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thank you all!, nice explanation and I've understood it!
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Read this :
http://www.codeproject.com/string/cstringmgmt.asp
----
Xian
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Does anyone know if I can talk to the parallel port the same way I'd do so with the serial port ( eg CreateFile(...,"COM1",...) -> CreateFile(...,"LPT1",...) ).
It'd be easy to test it out, but I don't have the hardware that I want to talk to - and I won't buy it unless I know I can talk to it over the parallel port under Win2k.
If anyone's interested, I'm trying to figure out whether or not I could build a Win2k driver for this Seiko 320x240 monochrome LCD screen (for an oboard computer in my car).
--
Russell Morris
Georgia Institute of Technology
"Hello, I'm doctor stupid. I'm going to take out your liver bones!" - Ralph Wiggum
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Check this out:
http://www.naughter.com/parallelport.html
I've used his serial port wrapper class with great success.
Mike Mullikin
"Real programmers don't document their code. It was hard to write - it should be hard to read!"
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Friggin' perfect!
I'm somewhat disappointed, though, as now I'll have to spend $200 on this LCD screen so I can start playing with it! 
Thanks.
--
Russell Morris
Georgia Institute of Technology
"Hello, I'm doctor stupid. I'm going to take out your liver bones!" - Ralph Wiggum
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look at this
http://www.techsoft.no/bendik/
or this
http://www.codeguru.com/treeview/CMultiTree.shtml
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Hi,
I would like to have an OnKeyDown event like, for the virtual keys ( -> <- ALT CTRL), is there anyway to do that, or I have to go to the PretranslateMessage... ?
Thanks, Bye!
Braulio
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Hi,
I need to find a tool or learn some techniques that can help me trace GDI leakage. I've found some tools on MSDN which work for Windows Me and 2000, but i also need to check on NT4. Besides, the tools i found on MSDN weren't very good anyway. They reported massively spiralling leakages for palletes and regions and bitmaps when i'm not creating any! Surely to leak, i would have to create and forget to destroy. So those tools may justfifably have been telling me about leaks - but definately they were incorrectly identifying the resource. And they didn't help you find the code.
Any ideas please ?
Jase
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
View your digital photos and images with ease using the ultimate desktop image manager for Microsoft Windows
Download your free copy of SlideShow Desktop today from http://www.slideshowdesktop.com
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Hi Jase !
There is a tool called GDIObj.exe ( Try to find this on the web), if you don´t have it tell me, and I send it to you.
HTH
Braulio
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Hi Braulio,
Thanks for you reply. I searched using google.com and found a gdiobj.exe on ftp.microsoft. It turned out to be a self extracting zip file from 1995 containing source code which i couldn't build (it doesn't have a workspace, so i would have to work out the libraries and dependencies myself in order to link it). It also came with 2 exe's which were dependant upon mfc30.dll so i coldn' run them.
I would really appreciate it if you could send me your copy.
jpj@totalise.co.uk
Thanks
Jase
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
View your digital photos and images with ease using the ultimate desktop image manager for Microsoft Windows
Download your free copy of SlideShow Desktop today from http://www.slideshowdesktop.com
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Bounds checker is probably the best tool for tracking GDI leaks, but it ain't cheap. For memory leaks I would recommend PurifyNT, but, again, it's expensive.
In my experience of Windows programming (10 years now) the most common leaks are:
* Deleting pens, brushes or fonts that are still selected in the DC.
* Calling GetDC without a corresponding ReleaseDC.
Regards.
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Thanks for your reply.
I have tried Purify but when i have Purify running, my dialog will not display. As for bounds checker, i would love to take look, as I have heard much about it. However, they don't provide an evaluation copy which in my book is criminal. I'm not paying all that money without testing it first ...
I shall double check my code for the instances you mention.
Jase
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
View your digital photos and images with ease using the ultimate desktop image manager for Microsoft Windows
Download your free copy of SlideShow Desktop today from http://www.slideshowdesktop.com
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I agree - not having an evaluation copy to play with is pants!
Here are some classic leak examples:
void CMyView::OnDraw(CDC* pDC)
{
CFont font;
// Create a font
font.CreatePointFont(120, _T("Arial"), pDC);
// Select the font into the DC
pDC->SelectObject(&font);
// Draw
pDC->TextOut(0, 0, _T("Leak!"));
// The original font isn't selected back into the DC
// and the font we created is about to be deleted.
// This will soon deprive you of resources (Win9x especially)
}
void CMyView::DrawSomething()
{
// Get a DC
CDC* pDC = GetDC();
// Draw
pDC->TextOut(0, 0, _T("Leak!"));
// You must call ReleaseDC, else this will leak!
}
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Hi,
I can see why these examples would cause leakage, and in looking through my source files - I'm not guilty of either of them (mores he pity, cos then at least i would have found some leaks ...
Jase
Jase
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
View your digital photos and images with ease using the ultimate desktop image manager for Microsoft Windows
Download your free copy of SlideShow Desktop today from http://www.slideshowdesktop.com
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I am more than happy to look at your code if you like - I quite enjoy looking for leaks! Plus I have a pucker copy of BC I can run it through ...
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Thanks for your kind offer. However, my source is for a commercial product so i can't show you the code ...
Jase
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
View your digital photos and images with ease using the ultimate desktop image manager for Microsoft Windows
Download your free copy of SlideShow Desktop today from http://www.slideshowdesktop.com
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Out of interest - how did you get your hands on a copy. I've looked on their website and it doesn't even give a price. It sems you have to go through a salesman / vendor in order to get a copy. And did you then enter a registration code to it ? (that is, it was 'unlockable') ?
Jase
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
View your digital photos and images with ease using the ultimate desktop image manager for Microsoft Windows
Download your free copy of SlideShow Desktop today from http://www.slideshowdesktop.com
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Also, calling DeleteObject instead of DeleteDC.
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We used to use boundschecker, but it din't seem to work after I upgraded to VC 6.0. (i.e. it was reporting memory leaks that weren't there)
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MSDN mag ran an article in the last 12-18 months and not in the last 6, which was a program that showed you the bitmaps/pens/etc that had not been deleted. It rocked, if you cannot find it LMK and I'll dig it up.
Also, W2000 makes you work pretty hard to leak GDI memory, because it cleans up after you. This I know from bitter experience.
Christian
As I learn the innermost secrets of the around me, they reward me in many ways to keep quiet.
Men with pierced ears are better prepared for marriage. They've experienced pain and bought Jewellery.
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I think the tool you mentioned is the tool i mentioned earlier. It is only 95/98 , and you're right - it shows the pensd , palletes and fonts that you are leaking. And it does indeed display lots of them! Only i don't use any, so its not my app thats leaking them 
From the outset, it looks pretty good. But in truth, it doesn't reflect the state of my application one iota.
It's called GDIUsage - is this the one ? You have to build it from source - available from MSJ online.
Jase
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
View your digital photos and images with ease using the ultimate desktop image manager for Microsoft Windows
Download your free copy of SlideShow Desktop today from http://www.slideshowdesktop.com
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