CMemDC class issue - c++

Question

Here is the following CMemDC class I am using from code projects that supposedly fixes the flickering:
#ifndef _MEMDC_H_
#define _MEMDC_H_
#include "stdafx.h"
namespace MemoryDC
{
class CMemDC : public CDC {
private:
CBitmap m_bitmap; // Offscreen bitmap
CBitmap* m_oldBitmap; // bitmap originally found in CMemDC
CDC* m_pDC; // Saves CDC passed in constructor
CRect m_rect; // Rectangle of drawing area.
BOOL m_bMemDC; // TRUE if CDC really is a Memory DC.
public:
CMemDC(CDC* pDC, const CRect* pRect = NULL) : CDC()
{
ASSERT(pDC != NULL);
// Some initialization
m_pDC = pDC;
m_oldBitmap = NULL;
m_bMemDC = !pDC->IsPrinting();
// Get the rectangle to draw
if (pRect == NULL) {
pDC->GetClipBox(&m_rect);
}
else {
m_rect = *pRect;
}
if (m_bMemDC) {
// Create a Memory DC
CreateCompatibleDC(pDC);
pDC->LPtoDP(&m_rect);
m_bitmap.CreateCompatibleBitmap(pDC, m_rect.Width(), m_rect.Height());
m_oldBitmap = SelectObject(&m_bitmap);
SetMapMode(pDC->GetMapMode());
SetWindowExt(pDC->GetWindowExt());
SetViewportExt(pDC->GetViewportExt());
pDC->DPtoLP(&m_rect);
SetWindowOrg(m_rect.left, m_rect.top);
}
else {
// Make a copy of the relevent parts of the current DC for printing
m_bPrinting = pDC->m_bPrinting;
m_hDC = pDC->m_hDC;
m_hAttribDC = pDC->m_hAttribDC;
}
// Fill background
FillSolidRect(m_rect, pDC->GetBkColor());
}
~CMemDC()
{
if (m_bMemDC) {
// Copy the offscreen bitmap onto the screen.
m_pDC->BitBlt(m_rect.left, m_rect.top, m_rect.Width(), m_rect.Height(),
this, m_rect.left, m_rect.top, SRCCOPY);
//Swap back the original bitmap.
SelectObject(m_oldBitmap);
}
else {
// All we need to do is replace the DC with an illegal value,
// this keeps us from accidently deleting the handles associated with
// the CDC that was passed to the constructor.
m_hDC = m_hAttribDC = NULL;
}
}
// Allow usage as a pointer
CMemDC* operator->()
{
return this;
}
// Allow usage as a pointer
operator CMemDC*()
{
return this;
}
};
}
#endif
Here is my OnDraw function using above class:
void ViewerView::OnDraw(CDC* pDC)
{
MemoryDC::CMemDC dc(pDC);
ViewerDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
if (!pDoc)
return;
else
RenderPage(dc, 0);
}
And here is the OnEraseBkgnd function:
BOOL ViewerView::OnEraseBkgnd(CDC* pDC)
{
return TRUE;
}
Here is the following video which best describes what is happening to my scrolling since a video is worth a thousand words
I tried double buffering and what not to fix this issue, but I am not having a good time.
If anyone has any suggestions or help much is appreciated.
Salute.

Answer 1

Do not reinvent the wheel.
MFC already has a memory DC class, you can easily use it like this:
void ViewerView::OnDraw(CDC* pDC)
{
CRect rc;
GetClientRect(&rc);
CMemDC memDC(*pDC, rc);
auto& rDC = memDC.GetDC();
RenderPage(&rDC, 0);
}
As a sidenote:
If you don't want the system to erase the background for you every time, you should return TRUE from the OnEraseBkgnd function.