Managed C++/CLI

15-Oct-07 9:18

Hi, I'm using Visual Studio 2005 C++/CLI. I need to be able to control the position of where I need to read/write a file. I tried fileStream->Position = 0, streamReader->BaseStream->Position = 0, fileStream->Seek(0, SeekOrigin::Begin), streamReader->BaseStream->Seek(0, SeekOrigin::Begin). None of these appear to work. How do I go about doing this?

Thanks

Buck

Mark Salsbery15-Oct-07 9:44

15-Oct-07 9:44

streamReader->BaseStream->CanSeek is true?

If not, I guess Seek and Position should throw exceptions.

How are they not appearing to work?

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

BuckBrown15-Oct-07 10:17

Re: StreamReader, StreamWriter and File Position? [modified]

15-Oct-07 10:17

I have a file that I am using unique "Header" lines. In order to determine if all the headers are present I use String^ fileContents = streamReader->ReadToEnd(), then fileContents->Contains(uniqe_search_string). Then I try to reposition the file to the begining in order to read from the begining of the file until the end of the file is reached with...

while(streamReader->Peek() >= 0) { streamReader->ReadLine(); }

It's in the Peek() function. If I don't ReadToEnd(), then ReadLine() three times and then Position = 0, then I can WriteLine at the begining of the file. It would seem that once the Peek() function has reached the end of file that the stream needs to be closed and then reopened for the Peek() function to be cleared. I'll play with this a while to see if I can make it work the way I want.

Buck

Mark Salsbery15-Oct-07 11:02

15-Oct-07 11:02

I see two options to fix this....

BuckBrown wrote:
while(streamReader-> Peek() >= 0) { streamReader->ReadLine(); }

1) You can replace the above line with

   while(streamReader->ReadLine())

2) You can change the above line to these two lines

   streamReader->DiscardBufferedData();
   while(streamReader-> Peek() >= 0) { streamReader->ReadLine(); }

Mark

*edit* freakin smilies Roll eyes | :rolleyes:

Last modified: 15mins after originally posted --

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

BuckBrown16-Oct-07 9:05

16-Oct-07 9:05

Hi Mark,

This has been exhibiting some real strange behavoir. I have seen my output begin writing at position 0 as it should after a fileStream->Position = 0, and then I have seen it begin writing at approximately position 100 (when I designated position 0). As I try to debug this problem something seems to "magically" change where position 0 actually is and will stay that way for half an hour and then change to some other position when I designate position 0. If I start with no file existing then the file is created and the following written into it...

[Line Test]
[OVP Test]
[SCP Test]
[UVLO Test]

After that I...

fileStream->Position = 0;
for(int index = 0; index < 3; index++)
{
string = streamReader->ReadLine();
streamWriter->WriteLine(string + "X"); // Notice the appended X
}

And the output looks like...

[Line Test]
[OVP Test]
[SCP Test]
[UVLO Test]
[Line Test]X
[OVP Test]X
[SCP Test]X
[UVLO Test]X

Which is not what I was expecting. Change the above code by one line to set the position to the begining of the file before each streamWriter->WriteLine() to get the code below...

fileStream->Position = 0;
for(int index = 0; index < 3; index++)
{
string = streamReader->ReadLine();
fileStream->Position = 0; // fileStream->Seek(0, SeekOrigin::Begin); behaves the same.
streamWriter->WriteLine(string + "X"); // Notice the appended X
}

And I get the ouput I want, but not what was expected (I expected one line in the file "[UVLO Test]X"). The output file now looks like this...

[Line Test]X
[OVP Test]X
[SCP Test]X
[UVLO Test]X

I don't understand why the file position is not being set the way I would expect it.

Thanks

Buck

Mark Salsbery16-Oct-07 9:26

16-Oct-07 9:26

Are both the streamreader and streamwriter using the same stream? Or a separate stream to/from the same file?

If so...

Results are going to be really unpredictable.

You may be getting lucky because the reader has buffered the entire file before you do any writing.

You can't expect to read and write to the same file at the same time, at least not at overlapping locations.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

BuckBrown16-Oct-07 10:11

16-Oct-07 10:11

Is your 'if so...' pertaining to the first case or the second case? I am setting up my file as follows...

fileStream = gcnew FileStream(stringPath, FileMode::Open, FileAccess::ReadWrite);
streamWriter = gcnew StreamWriter(fileStream);
streamReader = gcnew StreamReader(fileStream);

Buck

Mark Salsbery16-Oct-07 10:29

16-Oct-07 10:29

BuckBrown wrote:
Is your 'if so...' pertaining to the first case or the second case?

Both, actually Smile | :)

After posting I thought about how you were getting those results
and figured you HAD to be using a common stream.

That's not going to work, at least (especially) not in a read and write loop
like you're doing.

There's two pointers each for both the reader and the writer involved here.
One is the stream position, which you have control of.
The other is the reader and writer's internal buffer pointer, which you don't
have control of.

In your first example, first iteration of the loop...
You request to read a line. For efficiency, the reader reads an
entire buffer full (or the entire file, whichever is smaller).
In this case, it looks like the entire file is buffered so the stream pointer
now points to the end of file. All reads come from the readers buffer, all
writes now go to the end of the file, since that's where the stream pointer
was at the point of the first write. I hope that makes sense.

Now let's say you manage to control all of this (using unbuffered I/O and random
access (seeking) to the stream). If you read a string, add a 'X', write it back
to the file, and then try to read the next string, your 'X' will have overwritten the
next string in the file, unless you shift the remaining bytes in the file "to the right"
to make room for the 'X'.

This is why reading and writing each iteration of the loop is not going to work.
You need to do all the reading in one loop, make appropriate changes, then write
the results back to the stream. Sharing the stream like you're doing is fine - just set the
stream position back to the appropriate place before starting the write loop.

Of course, if there's data in the file beyond what you've read, that would all have to be
shifted to make room for anything you add.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

BuckBrown16-Oct-07 11:12

16-Oct-07 11:12

Thanks,

How would you go about shifting the data. This would require inserting and deleting characters at a given fileStream->Position and I haven't found any docs on this.

Buck

Mark Salsbery16-Oct-07 11:27

16-Oct-07 11:27

To insert bytes in a file, typically something like:

   open source file
   create/open temp file
   read from source file, writing to temp file, until insertion point reached.
   write insert data to temp file
   read from source file, writing to temp file, until end of source file reached.
   close files
   delete source file
   rename temp file to src file name

That's good for files of unlimited size. For "small" files you may be able to simplify
by doing something like:

   read entire stream from just after insertion point to end (into a buffer)
   use Stream::SetLength to expand the stream the number of bytes being inserted
   write the inserted bytes at the insertion point
   write the buffered bytes after the inserted bytes

Maybe there's new stream/file classes that have insertion built in....I've never seen
any though.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

BuckBrown16-Oct-07 11:30

16-Oct-07 11:30

Thanks Mark,

I thought I would be able to do this without a temporary file, but that's okay, I will just write it as you have psuedo coded it.

Mark Salsbery16-Oct-07 11:51

How to get HWND from System::Windows::Forms::Panel^ panel [modified]

16-Oct-07 11:51

The second method I described doesn't require a second file (or a separate stream)
but it wouldn't necessarily be good for files GBs in size.

That's why I put "small" in quotes. What's small is relative to your situation, but sometimes
even megabytes could be small enough to just allocate a buffer, read a huge block into
it from a stream, and write it back to the stream at a different location Smile | :)

Cheers,
Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Newbie0014-Oct-07 0:35

Newbie00

14-Oct-07 0:35

Hi, i'm not sure if i've chosen correct place for my post, but

I'm writing an app in VC++ 2005 EE which is going to display some 3D figures on System::Windows::Forms::Panel^ panel

The problem:

I've written my own class which handles all OpenGL stuff. So all I have to do is:
Create new 3D figure object, and draw it with panel's Paint method.
Constructor needs panel's HWND, so i did something like this:

TFigure *Figure; //pointer to my base class objects<br />
Figure = new TCuboid( (HWND)panel->Handle.ToPointer() , panel->Width, panel->Height, 3, 3, 7 );

But this solution doesn't work properly. When i'm trying to initialize OpenGl machine
with this code (inside TCuboid construcor):

m_gHDC = GetDC( hwnd ); //identifier (window HWND) <br />
SetupPixelFormat();<br />
m_hRC = wglCreateContext( m_gHDC ); //identifier (OpenGL graphic context)

the variable: m_hRC is NULL and it shouldn't be.
I've tried my class code with other IDE and all was fine. So i think the problem is with getting panel's HWND.

Can somebody help me please?

-- modified at 9:53 Sunday 14th October, 2007

Re: How to get HWND from System::Windows::Forms::Panel^ panel

Mark Salsbery14-Oct-07 5:53