question about a code...

[MrPolite]

BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

            int stride = bmData.Stride;
            System.IntPtr Scan0 = bmData.Scan0;

            unsafe
            {
                byte * p = (byte *)(void *)Scan0;
.
.
. 

what does that red line exactly do? I know it declares a pointer p, but what does the "(void *)" on the right side of the equal sign do?

also, it accesses p like an array (ie, p[0] ) why is that?

[PT Exorcist]

i see no red line :S

[PT Exorcist]

(*void) if i am not in error that will cast it to a pointer type var and (byte*) will finally cast it to a byte pointer

[MrPolite]

aha ok. (yeah it wouldnt let me colorize it for some reason)
weird though, so you have to convert it to pointer first, then to a byte-pointer? why cant you just convert it directly to a byte-pointer

[MrPolite]

ok forget about that
how about my last question?
why is it accessing it with p[0] like an array?

[CornedBee]

Coloring doesn't work in PHP tags, use CODE tags instead.

[MrPolite]

Originally posted by CornedBee
Coloring doesn't work in PHP tags, use CODE tags instead.

aaha ok

as for the other question I had, I think p[0] just means the pointer at its current location, p[1] would be the next position in the memory... just a guess, because that's how it seems to be working

[CornedBee]

Yes, it works the same as in C++.

[MrPolite]

umm another question
how come I can say if (p==null) ...
but not if (p[2]==null) ...

it gives this error for the second one
Operator '==' cannot be applied to operands of type 'byte' and '<null>'

[CornedBee]

Because a byte value can never be null, and p[2] is a byte value.
A pointer however can be null, and p is a pointer.

[MrPolite]

Originally posted by CornedBee
Because a byte value can never be null, and p[2] is a byte value.
A pointer however can be null, and p is a pointer.

hmm so this way the value of p will be (byte)p ?

also, how would I check to see if p[2] exists or not?

[CornedBee]

hmm so this way the value of p will be (byte)p ?

Huh? (byte)p might compile, but it certainly will not yield anything of any sense.

also, how would I check to see if p[2] exists or not?

You can't. The thing of pointers is that they are simply that: pointers. They store a memory address and NOTHING else. Why do you think you can only use them in unsafe areas? To know if accessing p[2] is valid, you need to store the length of the array that is pointed to by p separatly.

The maximum value you may use inside the square brackets is (Stride * Height) - 1

[MrPolite]

hmm MrPolite has to study pointers
last time I used them was probaby 6th grade... and you can probably guess that I didnt really learn anything about them back then


thanks for the answers

[CornedBee]

Learn plain old-school C and you'll know all about them