structure)). I will look into the CByteArray (and CArray) and see if I can
get one of them to work for me.
byte packets).
expands the memory allocation as it is used. Hopefully CByteArray or CArray
will do the same thing for me.
"Joseph M. Newcomer" <newcomer@flounder.com> wrote in message
Well, it isn't always that simple. You have to be concerned about exactly
what you are
replacing them with. Note that CString can be either Unicode WCHARs or
ANSI CHARs, and if
you willy-nilly replace your char/BYTE values with CString, you open
yourself for a series
of disasters if the result is incompatible with converting your app to
Unicode.
A CString, in particular, is an absolutely AWFUL way to represent
something like a binary
byte stream. A CByteArray or CArray<BYTE, BYTE> is much better. Do not
ever think that a
CString is a repository of 8-bit bytes; it is not. It is a repository of
CHARACTERS, and
characters are NOT necessarily equal to bytes.
On Tue, 11 Jul 2006 15:54:38 -0400, "Arlis Rose"
<arlisATendevouraerospace.com> wrote:
Okay, for some reason this one has me stumped. I am trying to replace all
of my char * (and unsigned char *) with CString's so to (hopefully) clean
up
the code and make it much easier to track down, fix, and avoid errors
(such
as not releasing memory, or not allocating enough memory, etc.).
My problem has arising when I try to read a lot of (essentially) random
characters form a text file using the CFile::Read operator. I am going to
show both version of my code below (totally stripped down), hopefully
someone can tell me how to use the CString properly in this case (or if I
cannot use a CString what it is I should be using):
Example 1 (which works):
CFile imageFile;
unsigned char * tempImage = new unsigned char [15000];
imageFile.Open("USE.jpg", CFile::typeBinary | CFile::modeRead, NULL);
radarMapSize = imageFile.GetLength();
imageFile.Read(tempImage , radarMapSize);
imageFile.Close();
//Read data into the radarMapDataArray structure
for (int i=0; i<int(radarMapSize/63)+1; i++) {
for (int p=i*63; p<i*63+63;p++) {
int gh = (p%63);
radarMapDataArray[i][p%63]=tempImage[p];
}
}
(please ignore the fact I took out my error checking (around opening the
file opening for example)...it is the 3rd last line that concerns me
(radarMapDataArray[i]p%63]=tempImage[p];
When I rewrite this to use the CString it looks like so:
****
DON'T DO THIS!!!! IT IS WRONG!!!!! In a Unicode app you are screwed. If
the data can
contain embedded NUL bytes, such as an image might contain, you are
screwed! THIS IS
WRONG!
****
CFile imageFile;
CString imageString;
*****
ABSOLUTELY WRONG!!!!!!! NEVER USE A CSTRING FOR ANYTHING OTHER THAT
CHARACTER VALUES!!!!!
CArray<BYTE, BYTE> image;
*****
imageFile.Open("USE.jpg", CFile::typeBinary | CFile::modeRead, NULL);
radarMapSize = imageFile.GetLength();
imageFile.Read(tempImage.GetBuffer(radarMapSize), radarMapSize);
******
You naively assume that GetBuffer is going to work...
image.SetSize(radarMapSize);
would be the correct declaration. Note that it will throw a
CMemoryException if it can't
get enough space...
*****
tempImage.ReleaseBuffer();
*****
At this point, the first NUL character (not byte) in the CString truncates
your "string"
to some nonsensical value. DO NOT USE CSTRING FOR BINARY DATA!
*****
imageFile.Close();
//Read data into the radarMapDataArray structure
for (int i=0; i<int(radarMapSize/63)+1; i++) {
for (int p=i*63; p<i*63+63;p++) {
int gh = (p%63);
radarMapDataArray[i][p%63]=tempImage.GetAt(p);
}
*****
I can't even figure out what this is trying to do. But it won't work,
because your
CString is hosed, and almost certainly the GetAt is going to give a bounds
error, because
CString is so completely incorrect for this type of application.
******
}
and that (what I though was) equivalent line
radarMapDataArray[i][p%63]=tempImage.GetAt(p); will now crash.
*****
That was a no-brainer to predict. I saw this coming before I even read
this section!
Don't use CString for this! Ever!
*****
This is
happening (I believe) because the file I am reading contains extended
ASCII
characters. So of course that line crashes at different points depending
on
how many characters the CString actually contains (for example right now
when I put in a breakpoint it only looks like the CString tempImage has 8
characters).
****
No surprise here. I'm amazed it didn't crash trying to access the [0]th
element. Wrong
representation for the wrong reasons.
****
So I think I understand what is happening (the CString is only 8
characters
long so my call to GetAt() is trying to access memory that isn't allocated
yet) but what I don't understand is how to make the two programs above
equal. I have tried putting a cast operation before my call to
tempImage.GetBuffer(radarMapSize) (I put (unsigned char *)) and this
compiles and runs, but (naturally) doesn't do anything to fix the problem.
*****
Simple: don't use CString. It is completely and utterly inappropriate for
this task.
*****
(Note - I also put a checkpoint in the first section of the code and my
unsigned char * LOOKS to contain the same data as the CString would (only
the 8 characters) however it's call using pointers [] doesn't crash
whereas
my GetAt() does :S
Can someone please point out to me what in the world I am doing wrong...I
must be missing something??
*****
What you are missing is that a CString represents characters, and ONLY
characters. It is
completely unsuited for binary bytes. I'm not sure what led you to
believe that a CString
was equivalent to an unsigned char * for binary data (that is, an LPBYTE),
but it isn't.
CStrings hold characters, not bytes, and you must not confuse the two.
The are not always
interchangeable.
*****
Thanks,
Arlis T. Rose
Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm