Problems with applying Strategy Pattern in C++

"Mitesh" <>
Wed, 14 Feb 2007 13:40:43 CST
Hi I am applying Strategy pattern in C++. Please ingnore some classes
here which are MFC(Microsoft Foundation Class) specific.

Following is my class:

class CChart {
    CChart(CRenderingPropertyPage *pPage, CGraphPropertySheet
*pGraphPropertySheet, CString csTitle, CString csXAxisTitle,
        CString csYAxisTitle, CStrategy *pStrategy, int
nChartCounter, int
    virtual ~CChart();
    void DrawChart(CDC *pDC, CRect & rectClip);
    int m_ChartHorzSize;
    int m_ChartVertSize;
    CString m_csTitle;
    CString m_csXAxisTitle;
    CString m_csYAxisTitle;
    CGraphPropertySheet *m_pGraphPropertySheet;
    CStrategy *m_pStrategy;
    CBitmap m_NormalBitmap;
    CRenderingPropertyPage *m_pParent;
    CRect m_BoundingRect;
    int m_ChartCounter;
    int m_ArrayIndex;
    int GetChartWidth() const
        return m_ChartHorzSize;

    void SetChartWidth(int size)
        m_ChartHorzSize = size;

    int GetChartHeight() const
        return m_ChartVertSize;

    void SetChartHeight(int size)
        m_ChartVertSize = size;

    const CString & GetTitle() const
        return m_csTitle;

    const CString & GetXAxisTitle() const
        return m_csXAxisTitle;

    const CString & GetYAxisTitle() const
        return m_csYAxisTitle;

    const CGraphPropertySheet & GetGraphPropertySheet() const
        return *m_pGraphPropertySheet;

    CStrategy * GetStrategy()
        return m_pStrategy;

    CBitmap * GetNormalBitmap()
        return &m_NormalBitmap;

    const CRenderingPropertyPage & GetParentPage() const
        return *m_pParent;

    const CRect & GetBoundingRect() const
        return m_BoundingRect;

    int GetChartCounter() const
        return m_ChartCounter;

    int GetArrayIndex() const
        return m_ArrayIndex;

class CStrategy
    double NonVirCalculateMax(const CArray<double, double> & pData);
    double NonVirCalculateMin(const CArray<double, double> & pData);
    void BaseDraw(CDC *pDC);
    void BlitToScreen(CDC *pDCSrc, CDC *pDCDest, CRect *rectClip);
    double scaleX;
    double scaleY;
    double max;
    double min;
    double XOrigin;
    double YOrigin;
    double m_BiggerMaxMin;
    double m_YPointsArray[11];
    CStringList m_YValuesList;
    double *m_pXPointsArray;
    CStringList m_XValuesList;
    virtual void CalculateScales() = 0;
    virtual void CalculateMax() = 0;
    virtual void CalculateMin() = 0;

    friend class CChart;
    virtual void OnDraw(CDC *pDC, CRect & rectClip) = 0;
    virtual ~CStrategy(void);
    void CleanUp();
    CChart *m_pChart;

//One of the strategies implemented

class CAccResponseLogSpectraStrategy :
    public CStrategy
    virtual void CalculateScales();
    virtual void CalculateMax();
    virtual void CalculateMin();
    virtual void OnDraw(CDC *pDC, CRect & rectClip);
    CArray<double, double> m_LogPsuAccArray;
    CArray<double, double> m_LogPeriodArray;

I have a CChart class. The charts differ from each other only in how
they are drawn. So I thought Strategy Pattern would hold good here.
Only the drawing algorithm differs.

I use the member variables of the CChart class in the OnDraw function.
I need quite a few members so I thought if the base strategy class
(CStrategy) should hold a pointer (m_pChart) to the CChart object it
would be easier than passing a lot of variables to the OnDraw

Some how I don't feel right. I feel like I have made the chart class
naked. I feel providing the access to the member variables of the
Chart class through the Get/Set function is not better than providing
public variables.

To avoid the above, I thought of using private variables and making
the CStrategy class a friend of CChart class. But this doesn't work.
Only the base strategy class (CStrategy) will become a friend but not
the derived classes like CAccResponseLogSpectraStrategy.

I had posted the exact problem in one of the forums I visit. Luckily
one of my forum colleagues proposed a nice solution.

To apply another abstraction.

class IFriendlyChartInterface
    virtual int GetChartWidth() = 0;
    virtual void SetChartWidth(int) = 0;

Now I derive my CChart class from IFriendlyChartInterface class:

class CChart : public IFriendlyChartInterface
      virtual int GetChartWidth();
      virtual void SetChartWidth(int);

Note this way I am able to make implementation of GetChartWidth() and
SetChartWidth(int) private in the CChart class. I think this is a
better design but still not sure. So here I am asking you how can I
improve my design or if there are other alternatives. Using templates
with policies (compile time polymorphism) isn't my option since I
require run-time polymorphism.

One deep fear that I have is whether this is a case to apply Strategy
at all.

Any suggestions and comments, example code snippets are welcome.


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