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

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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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