Error setting class attribute with pointer - c++

Question

I am having an issue setting class attributes with a pointer, here is what my code looks like: (this version will compiles, but you need the have minesweeper window open).
WindowObject.h:
#ifndef WINDOWOBJECT_H_INCLUDED
#define WINDOWOBJECT_H_INCLUDED
#include <windows.h>
#include <stdio.h>
class WindowObject
{
public:
WindowObject();
BYTE* getPixelMap() const;
void setPixelMap(BYTE* bitPointer);
void getPixelMap(HWND hwnd);
private:
BYTE* mPixelMap;
};
#endif
WindowObject.cpp:
#include "WindowObject.h"
WindowObject::WindowObject()
{
}
BYTE* WindowObject::getPixelMap() const
{
printf("getPixelMap: %d\n", mPixelMap[0]); // it crashes, bad pointer!!
return mPixelMap;
}
void WindowObject::setPixelMap(BYTE *bitPointer)
{
printf("setPixelMap 1: %d\n", bitPointer[0]); // It works!
mPixelMap = bitPointer;
printf("setPixelMap 2: %d\n", mPixelMap [0]); // Why does this works and getPixelMap doesn't???
}
void WindowObject::getPixelMap(HWND hwnd)
{
HDC hdcWindow, hdcMemDC = NULL;
HBITMAP hbmScreen = NULL;
BITMAP bmpScreen;
// Retrieve the handle to a display device context for the client
// area of the window.
hdcWindow = GetDC(hwnd);
// Create a compatible DC which is used in a BitBlt from the window DC
hdcMemDC = CreateCompatibleDC(hdcWindow);
if(!hdcMemDC)
{
MessageBoxA(hwnd, "CreateCompatibleDC has failed", "Failed", MB_OK);
goto done;
}
// Get the client area for size calculation
RECT rcClient;
GetClientRect(hwnd, &rcClient);
// Create a compatible bitmap from the Window DC
hbmScreen = CreateCompatibleBitmap(hdcWindow, rcClient.right-rcClient.left, rcClient.bottom-rcClient.top);
if(!hbmScreen)
{
MessageBoxA(hwnd, "CreateCompatibleBitmap Failed", "Failed", MB_OK);
goto done;
}
//Get the BITMAP from the HBITMAP
GetObject(hbmScreen,sizeof(BITMAP),&bmpScreen);
BITMAPFILEHEADER bmfHeader;
BITMAPINFOHEADER bi;
BYTE* bitPointer;
bi.biSize = sizeof(BITMAPINFOHEADER);
bi.biWidth = bmpScreen.bmWidth;
bi.biHeight = bmpScreen.bmHeight;
bi.biPlanes = 1;
bi.biBitCount = 32;
bi.biCompression = BI_RGB;
bi.biSizeImage = 0;
bi.biXPelsPerMeter = 0;
bi.biYPelsPerMeter = 0;
bi.biClrUsed = 0;
bi.biClrImportant = 0;
// Create a device independent bitmap that so that we can write to it directly
hbmScreen = CreateDIBSection(hdcMemDC, (BITMAPINFO *)&bi, DIB_RGB_COLORS, (void**)(&bitPointer), NULL, NULL);
// Select the compatible bitmap into the compatible memory DC.
SelectObject(hdcMemDC, hbmScreen);
// Bit block transfer into our compatible memory DC.
if ( !BitBlt(hdcMemDC, 0, 0, rcClient.right-rcClient.left, rcClient.bottom-rcClient.top, hdcWindow, 0, 0, SRCCOPY) )
{
MessageBox(hwnd, "BitBlt has failed", "Failed", MB_OK);
goto done;
}
// Create PixelMap
int width = (rcClient.right-rcClient.left),
height = (rcClient.bottom-rcClient.top);
printf("Inside getPixelMap: %d\n", bitPointer[0]); // it works!
setPixelMap(bitPointer);
done:
DeleteObject(hbmScreen);
DeleteObject(hdcMemDC);
ReleaseDC(hwnd, hdcWindow);
}
main.cpp:
#include "WindowObject.h"
int main()
{
WindowObject wObj;
HWND hwnd;
hwnd = FindWindow(NULL, "Minesweeper");
if (hwnd)
{
wObj.getPixelMap(hwnd);
BYTE* bPointer = wObj.getPixelMap();
printf("After getPixelMap: %d", bPointer[0]);
}
}
The program crashes, and it gives me an "access violation reading location". I'm almost 100% sure the problem comes from my pointer but I find it wierd that it works in every stage of my tests, except when I get it with my getPixelMap method. Any help would be much appreciated!

Answer 1

The problem is that this data member either is not initialized (if an object of the class is default initialized) or has value 0. So using member function getPixelMap() is unsafe. Also as I think the life of an argument of function setPixelMap may be less than the life of an object of the class. So the behaviour of the program is undefined.
I think it would be better to use either a smart pointer or to allocate a memory for using by the data member yourself.

Answer 2

Inside the function getPixelMap(HWND hwnd) you are calling DeleteObject on the returned bitmap, after saving the pointer but before returning. So by the time you access the bitmap later, it is no longer a valid pointer, since according to the CreateDIBSection docs it says that the bitmap will be deallocated when you call DeleteObject.
You will need to keep the bitmap object (hbmScreen) and delete it later, perhaps in the destructor.