CMemDC类只赏不析

#ifndef _MEMDC_H_
#define _MEMDC_H_

class CMemDC: public CDC {
private:
	CBitmap m_bitmap;
	CBitmap* m_oldBitmap;
	CDC* m_pDC;
	CRect m_rect;
	BOOL m_bMemDC;
public:
	CMemDC(CDC* pDC, const CRect* pRect = NULL, bool boolToMemory = TRUE): CDC() {
		ASSERT(pDC != NULL);
		m_pDC = pDC;
		m_oldBitmap = NULL;
		if (boolToMemory) m_bMemDC = !pDC->IsPrinting();
		else m_bMemDC = FALSE;

		if (pRect == NULL) pDC->GetClipBox(&m_rect);
		else m_rect = *pRect;

		if (m_bMemDC) {
			CreateCompatibleDC(pDC);
			pDC->LPtoDP(&m_rect);

			m_bitmap.CreateCompatibleBitmap(pDC, m_rect.Width(), m_rect.Height());
			m_oldBitmap = SelectObject(&m_bitmap);

			SetMapMode(pDC->GetMapMode());
			SetWindowOrg(m_rect.left, m_rect.top);
		} else {
			if(pDC->IsPrinting()) m_bPrinting = pDC->m_bPrinting;
			m_hDC = pDC->m_hDC;
			m_hAttribDC = pDC->m_hAttribDC;
		}
		FillSolidRect(m_rect, pDC->GetBkColor());
	}

	~CMemDC() {
		if(m_bMemDC) {
			m_pDC->BitBlt(m_rect.left, m_rect.top, m_rect.Width(), m_rect.Height(), this, m_rect.left, m_rect.top, SRCCOPY);
			SelectObject(m_oldBitmap);
		} else {
			m_hDC = m_hAttribDC = NULL;
		}
	}

	CMemDC* operator->() {
		return this;
	}

	operator CMemDC* () {
		return this;
	}
};
#endif