VC++ Example: SaveDC and RestoreDC, IsPrinting and GetSysColor

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VC++ Example: SaveDC and RestoreDC, IsPrinting and GetSysColor

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Source Code

SaveDC.h

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class CSaveDC {
    public:
       CSaveDC(CDC & dc) { sdc = &dc; saved = dc.SaveDC(); }
       CSaveDC(CDC * dc) { sdc = dc; saved = dc->SaveDC(); }
       virtual ~CSaveDC() { sdc->RestoreDC(saved); }
    protected:
       CDC * sdc;
       int saved;
};

That's all there is to it! Note that it has two constructors, one if you have a CDC * and one if you have a CDC or CDC &. All you do is declare a dummy variable. But there's an important trick, illustrated below.

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void CMyView::OnPaint()
   {
    CPaintDC dc(this);
    CFont f;
    f.CreateFont(...); // parameters not shown
    { /* save context */
     CSaveDC sdc(dc);
     dc.SelectObject(&f);
     dc.TextOut(...); // whatever...
    } /* save context */
   } // destructors called here...

Note that the save context you create by using the CSaveDC class must be in a scope that is smaller than the objects selected into the DC. Thus the /* save context */ block guarantees that the CSaveDC destructor is guaranteed to be called before the destructor for the font. All you have to do is declare your fonts, pens, brushes, and regions outside the /* save context */ block and you can be guaranteed that their destructors will be called in a context where they are not selected into the active DC.

Note that CSaveDCs can nest (because the ::SaveDC can nest). The nesting can be static or dynamic. For example

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void CMyView::OnPaint()
   {
    CPaintDC dc(this);
    CFont f;
    f.CreateFont(12,...); // most parameters not shown
    { /* save context */
     CSaveDC sdc(dc);
     dc.SelectObject(&f);
     drawboxes(dc);
     dc.TextOut(...); // whatever...
   } // destructors called here

void CMyView::drawboxes(CDC & dc)
   {
    CPen RedPen(PS_SOLID, 0, RGB(255, 0, 0));
    CBrush GreenBrush(RGB(0, 255, 0);
    CFont f;
    f.CreateFont(6, ...); // most parameters not shown
    { /* save context */
     dc.SelectObject(&RedPen);
     dc.SelectObject(&GreenBrush);
     dc.SetBkMode(TRANSPARENT);
     dc.SetTextColor(::GetSysColor(COLOR_GRAYTEXT);
     ...
    } /* save context */
   }

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#ifndef _MEMDC_H_
#define _MEMDC_H_
 
//////////////////////////////////////////////////
// CMemDC - memory DC
//
// Author: Keith Rule
// Email:  keithr@europa.com
// Copyright 1996-2002, 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
//
//       02/11/02 Added support for CScrollView as supplied
//             by Gary Kirkham. - KR
//
// This class implements a memory Device Context which allows
// flicker free drawing.
 
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 accidentally 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

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Code Snippet

Source Code

SaveDC.h