What does an "Access Violation &q...

[Joe Creaney]

I am trying to write a win32 bitmap object. The algorithm works fine in a
function but when I create an object out of the same code I get bugs. When I
debug the program I get an access violation (segmentation Fault).

[Joe Creaney]

What does access violation mean?

[cpns]

It means that the process has attempted to access a memory location that is
invalid. It may be causes by actions such as dereferencing an invalid pointer,
or overflowing the process stack, or overrunning an array bound.

[Joe Creaney]

Is there a way to post code or to find out why this is happening. I am trying
to create a color bitmap object. The algorithm works as a function but not as
an object. Where would I look to try to correct this? I have to use a pointer
or a vector to hold the data for the bitmap.

[cpns]

s there a way to post code

Paste it into the composer and mark with code tags (use the buttons or
manually type BBCode) - that's if SourceForge does not spontaneously change
its mark-up syntax again!

or to find out why this is happening

The best way is to use a debugger; but good-luck with Dev-C++'s debugger - it
sucks. There are better free tools.

The algorithm works as a function but not as an object

You mean "as a class"? An object is an instance of a class, you cannot have an
object without a class to instantiate.

I have to use a pointer or a vector to hold the data for the bitmap.

Why the imperative? Remember that a pointer is not itself a container, it has
to point to something valid, and large enough.

Clifford

[Joe Creaney]

What I mean is my algorithm works as a function but runs into problems when
put the same information in an object. That is I put the same data is a
class{}; and functions in that class.

I have a constructor where I:
for(int lp = 0; lp != size; lp++){
if(num == 16){num =0;}
bitData.push_back(0);
}

I display like this:
dc = CreateCompatibleDC(NULL);

BYTE * bits = flip();

void *pvBits;

hbitmap= CreateDIBSection(dc,&bmi,
DIB_RGB_COLORS, &pvBits,NULL,0 );

CopyMemory(pvBits, bits, 16*16);

SelectObject(dc, hbitmap);

StretchBlt(hdc, ac, dw,165,165,dc,0,0,16,16,MERGECOPY);

Then I use this function:

This is my header.
class bitmap{

HBITMAP hbitmap;
BITMAPINFO bmi;

HDC dc;

std::vector<BYTE>bitData;

BYTE * Bits;

BYTE size;
int acc;
int dwn;

void create();
BYTE* flip();

public: ...

I wonder why the pointers are not pointing to anything is there a problem of
scope, I have the constructor go out of scope and the data that the pointer
points to go away leaving problems that I might be having?

[cpns]

So when I explain how to post code, you choose to ignore the advice!? Good
start.

Posting fragments may not help, without seeing how those fargments fit into
your code, all information about scope, sequence, and initialisation is lost.
For example, how do I know that size has a valid value in the constructor.
You have not even shown how this class is instantiated (i.e. how the object is
created). Neither have you explained the nature of the failure.

One thing I note is that you have a class variable Bits _and a local variable
_bits. Was that intentional? It is certainly not best coding practice if it
was.

Your approach to describing your problem is not helping you; a better approach
is to use a debugger, step through the code, and fix it yourself. However I
cannot recommend Dev-C++'s integarted debugger at all!

[Joe Creaney]

Thanks for the advice. I am not sure what you mean in posting code.

I try not to post too much code because it gets too long. I often find it
difficult to post code of problems I have in complex problems. Basically I
need to create an array of 255 BYTES or use a vector. I push back 255 0s in
the vector or I can loop in 255 0 into any array. Which that array has to be a
pointer in an object. I can't declare the size in the header of the object and
I might want a different size later and I will have to use a pointer to deal
with that.

Once I get the basics of the bit map working I will add on more and more
functions be able to edit that array of data and the colors. I also plan to
create an editor as I had done with a monochrome bitmap. I am trying to put
data in an array for a graphic I have done everything I am having problems
with past objects and the algorithm works in a function. I always find it
difficult to post pages of code.

[cpns]

I am not sure what you mean in posting code.

Right above where you type your message in this forum is a row of buttons, did
curiosity never cause you to investigate those (or the "BBCode" linke below
the message box)? HTML rendering reduces multiple whitespacce to a single
space, so all code indenting is removed. Your three posted code fragments
should have looked like:

  for(int lp = 0; lp != size; lp++){
    if(num == 16){num =0;}
    bitData.push_back(0); 
     }

  dc = CreateCompatibleDC(NULL);

  BYTE * bits = flip();

  void *pvBits;

  hbitmap= CreateDIBSection(dc,&bmi,
                          DIB_RGB_COLORS, &pvBits,NULL,0 );

  CopyMemory(pvBits, bits, 16*16);

  SelectObject(dc, hbitmap);

  StretchBlt(hdc, ac, dw,16*5,16*5,dc,0,0,16,16,MERGECOPY);

class bitmap{

  HBITMAP hbitmap;
  BITMAPINFO bmi;

  HDC dc;

  std::vector<BYTE>bitData;

  BYTE * Bits;

  BYTE size;
  int acc;
  int dwn;

  void create();
  BYTE* flip();

public:  ...

Though it is probably academic, this is not the best formatted code I've seen
in any event.

I try not to post too much code because it gets too long.

That is sometimes a good idea, but if it is not enough to reproduce your
problem, it is probably not enough to solve your problem. It is best to post
something that is stand-alone compilable unless you know precisely where the
problem lies, and judging from the fragments you posted, you have no idea. You
do not need to worry about length if what you post is all useful and relevant
information, randomly selecting broken fragments is far worse; like asking me
why your car does not work by showing me the steering wheel, the tail light,
and a bolt from somewhere unknown under the hood!

If you do not wish to post complete code, you should at least post complete
functions and a complete class definition, and the functions you post should
include those that are failing. And you should post at the very least an
example of how you are using this class in a way that causes the failure. For
example if you know that the constructor is not correct, post the complete
constructor and an example of the object instantiation. For example:

class  cMyClass
{
    // complete definition here
} ;

cMyClass::cMyClass()
{
    // complete constructor here
}

// other complete member functions needed to reproduce the error here

int main()
{
    cMyClass  object ;
    // Show or explain how object is invalid here
}

Your original statement "works fine in a function but when I create an object
out of the same code I get bugs" makes little sense since a function and a
class are not analogous, there is no direct mapping between a single function
and an equivalent class; to think there might be would be to misunderstand the
object oriented paradigm completely. Without seeing either your original
implementation or the class implementation, the statment is merely a
platitude. The problem is not that classes do not work, but rather that you
wrote code that does not work; how can we tell why without seeing the code?

[cpns]

Damn Sourceforge and their constant mark-up tinkering! Now code tagged text is
rendered in a proprotional font! Why does anyone trust these idiots to host
their projects!? Just when they seem to have debugged the elagentaly simple
Mark-Down compositor, they replace it with the ancient and cumbersome BBCode
and screw it up at the same time.

Maybe they thonk it does not matter because no one uses it; maybe no one uses
it because it sucks.

[Joe Creaney]

Thanks all for the help. I will see if I can find the problems. I have worked
with these issues in the past and I have not had problems, I am trying to find
out how what I am doing is different than I had done in previous projects and
what I got to work in a function but has bugs in an object.