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C++

C++ Tricks of the Trade: Friend Functions
By: Kais Dukes
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    2002-06-21

    Table of Contents:
  • C++ Tricks of the Trade: Friend Functions
  • Friendly Friends
  • Commutative Operators
  • Friend Classes
  • Conclusion

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    C++ Tricks of the Trade: Friend Functions - Friend Classes


    (Page 4 of 5 )

    As well as choosing a non-member friend function, a class has two other possibilities for its friends. A class can declare a member function of another class as a friend, or declare another class as a friend class.

    Friend classes are used in cases where one class is tightly coupled to another class. For example, suppose we have a class CPoint that represents a coordinate, and a class CPointCollection that holds a list of points. Since the collection class may need to manipulate point objects, we could declare CPointCollection as a friend of the CPoint class:

    // Forward declaration of friend class.
    class CPointCollection;

    // Point class.
    class CPoint
    {
    friend CPointCollection;

    public:
    CPoint(const double x, const double y) :
    m_x(x),
    m_y(y)
    {
    }

    ~CPoint(void)
    {
    }

    // ...

    private:
    double m_x;
    double m_y;
    };


    Since the collection class is a friend of CPoint, it has access to the internal data of any point object. This is useful when individual elements of the collection need to be manipulated. For example, a set method of the CPointCollection class could set all CPoint elements to a particular value:

    class CPointCollection
    {
    public:
    CPointCollection(const int nSize) :
    m_vecPoints(nSize)
    {

    }

    ~CPointCollection(void);
    void set(const double x, const double y);

    // ...

    private:
    vector<CPoint> m_vecPoints;
    };


    The set member can iterate over the collection and reset each point:

    void CPointCollection::set(const double x, const double y)
    {
    // Get the number of elements in the collection.
    const int nElements = m_vecPoints.size();

    // Set each element.
    for(int i=0; i<nElements; i++)
    {
    m_vecPoints[i].m_x = x;
    m_vecPoints[i].m_y = y;
    }
    }


    One thing worth noting about friend classes is that friendship is not mutual: although CPointCollection can access CPoint, the converse is not true. Friendship is also not something that is passed down a class hierarchy. Derived classes of CPointCollection will not be able to access CPoint. The principle behind this is that friendship is not implicitly granted; each class must explicitly choose its friends.

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