双缓冲类（转，打个tag）

#ifndef _MEMDC_H_
#define _MEMDC_H_

//////////////////////////////////////////////////
// CMemDC - memory DC
//
// Author: Keith Rule
// Email: 
// Copyright 1996-1999, Keith Rule
//
// You may freely use or modify this code provided this
// Copyright is included in all derived versions.
//
// History - 10/3/97 Fixed scrolling bug.
//                   Added print support. - KR
//
//           11/3/99 Fixed most common complaint. Added
//                   background color fill. - KR
//
//           11/3/99 Added support for mapping modes other than
//                   MM_TEXT as suggested by Lee Sang Hun. - KR
//
// This class implements a memory Device Context which allows
// flicker free drawing.

class CMemDC : public CDC {
protected:
   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. in logical coordinates
   BOOL     m_bMemDC;       // TRUE if CDC really is a Memory DC.
   
   void Construct(CDC* pDC)
   {
        ASSERT(pDC != NULL);

        // Some initialization
        m_pDC = pDC;
        m_oldBitmap = NULL;
        m_bMemDC = TRUE; // 原文是m_bMemDC = pDC->IsPrinting();就是如果是打印机的CDC就不用双缓冲。现在我要做一个打印机项目，所以直接跳过这个。

        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());
            pDC->DPtoLP(&m_rect);
            SetWindowOrg(m_rect.left, m_rect.top);
        } else {
            // Make a copy of the relavent 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());
    }

// TRK begin
public:
   CMemDC(CDC* pDC                  ) : CDC() { pDC->GetClipBox(&m_rect); Construct(pDC); }
   CMemDC(CDC* pDC, const RECT& rect) : CDC() { m_rect = rect           ; Construct(pDC); }
// TRK end
   
   virtual ~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

用法

void CHSEPView::OnPrint(CDC* pDC, CPrintInfo* pInfo)
{
 {
  printf("new\nleft : %4d  ,right %4d , top %4d, bottom %4d\n",
   pInfo->m_rectDraw.left, pInfo->m_rectDraw.right, pInfo->m_rectDraw.top, pInfo->m_rectDraw.bottom);

  RECT rect;
  rect.left = pInfo->m_rectDraw.left;
  rect.top  = pInfo->m_rectDraw.top;
  rect.right = pInfo->m_rectDraw.right;
  rect.bottom = pInfo->m_rectDraw.bottom;

  CMemDC memDC(pDC, rect);
  if(m_pPrinter)
   m_pPrinter->Print(&memDC, pInfo);
 }

 CView::OnPrint(pDC, pInfo);
}