1.7. Friends of classes 1.7.2. Friendly class

A class may be friendly to another class. This means that all the component functions of a class are friendly to another class. A friendly class must be defined outside the body of the class “providing friendship”.

For example,

class X2 {friend class X1; . . .};

class X1

{. . .

void f1 (...);

void f2 (...);

. . .

};

// In this example, functions f1 and f2 of class X1 are friends of class X2, although they

// described without the friend specifier.

Example 1.7.2

Consider the point class — a point in n-dimensional space and its friendly vector class — the radius vector of a point (“vector with the origin at the origin of n-dimensional space”). In the vector class, we define a function for determining the norm of a vector, which is calculated as the sum of the squares of the coordinates of its end.

clas s point

{int N; // dimension

double * x; // pointer to an array of coordinates

friend class vector;

public:

point (int n, double d = 0.0);

};

point:: point (int n, double d)

{N = n;

x = new double [N];

for (int i = 0; i <N; i ++) x [i] = d;

}

class vector

{double * xv;

int N;

public:

vector (point, point);

double norma ();

};

vector:: vector (point begin, point end)

{N = begin.N;

xv = new double [N];

for (int i = 0; i <N; i ++) xv [i] = end.x [i] –begin.x [i];

}

double vector:: norma ()

{double dd = 0.0;

for (int i = 0; i <N; i ++) dd + = xv [i] * xv [i];

return dd;

}

void main (void)

{point A (2,4.0);

point B (2,2.0);

vector V (A, B);

cout << V.norma ();

}

// It will be displayed - 8.

The disadvantage of the proposed point class is that the values ​​of all coordinates of the point x [i] are the same. In order for them to be arbitrary and different, it is necessary to define a constructor as a function with a variable number of parameters, for example:

point:: point (int n, double d, ...)

{

N = n;

x = new double [N];

double * p = & d;

for (int i = 0; i <N; i ++) {x [i] = * p; p ++;}

}